Chin. Phys. B

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Molecular beam epitaxy and superconductivity of stoichiometric FeSe and K_xFe_2-ySe₂ crystalline films

Wang Li-Li, Ma Xu-Cun, Chen Xi, Xue Qi-Kun

Chin. Phys. B, 2013, 22 (8): 086801 doi: 10.1088/1674-1056/22/8/086801
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Our recent progress in the fabrication of FeSe and K_xFe_2-ySe₂ ultra thin films and the understanding of their superconductivity properties is reviewed. The growth of high-quality FeSe and K_xFe_2-ySe₂ films is achieved in a well controlled manner by molecular beam epitaxy. The high-quality stoichiometric and superconducting crystalline thin films allow us to investigate the intrinsic superconductivity properties and the interplay between the superconductivity and the film thickness, the local structure, the substrate, and magnetism. In situ low-temperature scanning tunneling spectra reveal the nodes and the twofold symmetry in FeSe, high-temperature superconductivity at the FeSe/SrTiO₃ interface, phase separation and magnetic order in K_xFe_2-ySe₂, and the suppression of superconductivity by twin boundaries and Fe vacancies. Our findings not only provide fundamental information for understanding the mechanism of unconventional superconductivity, but also demonstrate a powerful way of engineering superconductors and raising the transition temperature.

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Electron-phonon coupling in cuprate and iron-based superconductors revealed by Raman scattering

Zhang An-Min, Zhang Qing-Ming

Chin. Phys. B, 2013, 22 (8): 087103 doi: 10.1088/1674-1056/22/8/087103
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Electron-phonon coupling (EPC) in cuprate and iron-based superconducting systems, as revealed by Raman scattering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape. Then we discuss the strength of EPC in different high-temperature superconductor (HTSC) systems and possible factors affecting the strength. A comparative study between Raman phonon theories and experiments allows us to gain insight into some crucial electronic properties, especially superconductivity. Finally, we summarize and compare EPC in the two existing HTSC systems, and discuss what role it may play in the HTSC.

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Exploration of iron-chalcogenide superconductors

Dong Chi-Heng, Wang Hang-Dong, Fang Ming-Hu

Chin. Phys. B, 2013, 22 (8): 087401 doi: 10.1088/1674-1056/22/8/087401
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Iron-chalcogenide compounds with FeSe(Te, S) layers did not attract much attention until the discovery of high-T_c superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered.

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Spin, charge, and orbital orderings in iron-based superconductors

Jiang Qing, Kang Yao-Tai, Yao Dao-Xin

Chin. Phys. B, 2013, 22 (8): 087402 doi: 10.1088/1674-1056/22/8/087402
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In this article, we briefly review spin, charge, and orbital orderings in iron-based superconductors, as well as the multi-orbital models. The interplay of spin, charge, and orbital orderings is a key to understand the high temperature superconductivity. As an illustration, we use the two-orbital model to show the spin and charge orderings in iron-based superconductors based on the mean-field approximation in real space. The typical spin and charge orderings are shown by choosing appropriate parameters, which are in good agreement with experiments. We also show the effect of Fe vacancies, which can introduce the nematic phase and interesting magnetic ground states. The orbital ordering is also discussed in iron-based superconductors. It is found that disorder may play a role to produce the superconductivity.

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Optical spectroscopy studies on FeTe_1-xSe_x and A_xFe_2-ySe₂ (A=K, Rb, Cs)：A brief overview

Yuan Rui-Hua, Wang Nan-Lin

Chin. Phys. B, 2013, 22 (8): 087404 doi: 10.1088/1674-1056/22/8/087404
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In this short overview, we summarize the optical spectroscopy studies on iron selenide superconducting systems FeTe_1-xSe_x and A_xFe_2-ySe₂. We elaborate that optical spectroscopy measurements yield fruitful information about the band structure evolution across the AFM phase transition temperature, the electronic correlation effect, the superconducting pairing energy gap, the condensed carrier density or penetration depth, the inhomogeneity and the nanoscale phase separation between superconductivity and antiferromagnetism in those systems.

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Physics picture from neutron scattering study on Fe-based superconductors

Bao Wei

Chin. Phys. B, 2013, 22 (8): 087405 doi: 10.1088/1674-1056/22/8/087405
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Neutron scattering, with its ability to measure the crystal structure, the magnetic order, and the structural and magnetic excitations, plays an active role in investigating various families of Fe-based high-T_c superconductors. Three different types of antiferromagnetic orders have been discovered in the Fe plane, but two of them cannot be explained by the spin-densitywave (SDW) mechanism of nesting Fermi surfaces. Noticing the close relation between antiferromagnetic order and lattice distortion in orbital ordering from previous studies on manganites and other oxides, we have advocated orbital ordering as the underlying common mechanism for the structural and antiferromagnetic transitions in the 1111, 122, and 11 parent compounds. We observe the coexistence of antiferromagnetic order and superconductivity in the (Ba,K)Fe₂As₂ system, when its phase separation is generally accepted. Optimal T_c is proposed to be controlled by the local FeAs₄ tetrahedron from our investigation on the 1111 materials. The Bloch phase coherence of the Fermi liquid is found crucial to the occurrence of bulk superconductivity in iron chalcogenides of both the 11 and the 245 families. Iron chalcogenides carry a larger staggered magnetic moment (> 2 μ_B/Fe) than that in iron pnictides (< 1μ_B/Fe) in the antiferromagnetic order. Normal state magnetic excitations in the 11 superconductor are of the itinerant nature while in the 245 superconductor the spin-waves of localized moments. The observation of superconducting resonance peak provides a crucial piece of information in current deliberation of the pairing symmetry in Fe-based superconductors.

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Photoemission study of iron-based superconductor

Liu Zhong-Hao, Cai Yi-Peng, Zhao Yan-Ge, Jia Lei-Lei, Wang Shan-Cai

Chin. Phys. B, 2013, 22 (8): 087406 doi: 10.1088/1674-1056/22/8/087406
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The iron-based superconductivity (IBSC) is a great challenge in correlated system. Angle-resolved photoemission spectroscopy (ARPES) provides electronic structure of the IBSCs, the pairing strength, and the order parameter symmetry. Here, we briefly review the recent progress in IBSCs and focus on the results from ARPES. The ARPES study shows the electronic structure of "122", "111", "11", and "122^*" families of IBSCs. It has been agreed that the IBSCs are unconventional superconductors in strong coupling region. The order parameter symmetry basically follows s_± form with considerable out-of-plane contribution.

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Angle-resolved photoemission spectroscopy study on iron-based superconductors

Ye Zi-Rong, Zhang Yan, Xie Bin-Ping, Feng Dong-Lai

Chin. Phys. B, 2013, 22 (8): 087407 doi: 10.1088/1674-1056/22/8/087407
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Angle-resolved photoemission spectroscopy (ARPES) has played an important role in determining the band structure and the superconducting gap structure of iron-based superconductors. In this paper, from the ARPES perspective, we briefly review the main results from our group in recent years on the iron-based superconductors and their parent compounds, and depict our current understanding on the antiferromagnetism and superconductivity in these materials.

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Microstructure and structural phase transitions in iron-based superconductors

Wang Zhen, Cai Yao, Yang Huai-Xin, Tian Huan-Fang, Wang Zhi-Wei, Ma Chao, Chen Zhen, Li Jian-Qi

Chin. Phys. B, 2013, 22 (8): 087409 doi: 10.1088/1674-1056/22/8/087409
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Crystal structures and microstructural features, such as structural phase transitions, defect structures, and chemical and structural inhomogeneities, are known to have profound effects on the physical properties of superconducting materials. Recently, many studies on the structural properties of Fe-based high-T_c superconductors have been published. This review article will mainly focus on the typical microstructural features in samples that have been well characterized by physical measurements. (i) Certain common structural features are discussed, in particular, the crystal structural features for different superconducting families, the local structural distortions in the Fe₂Pn₂ (Pn=P, As, Sb) or Fe₂Ch₂ (Ch=S, Se, Te) blocks, and the structural transformations in the 122 system. (ii) In FeTe(Se) (11 family), the superconductivity, chemical and structural inhomogeneities are investigated and discussed in correlation with superconductivity. (iii) In the K_0.8Fe_1,6+xSe₂ system, we focus on the typical compounds with emphasis on the Fe-vacancy order and phase separations. The microstructural features in other superconducting materials are also briefly discussed.

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Crystal chemistry and structural design of iron-based superconductors

Jiang Hao, Sun Yun-Lei, Xu Zhu-An, Cao Guang-Han

Chin. Phys. B, 2013, 22 (8): 087410 doi: 10.1088/1674-1056/22/8/087410
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The second class of high-temperature superconductors (HTSCs), iron-based pnictides and chalcogenides, necessarily contain Fe₂X₂ ("X" refers to a pnictogen or a chalcogen element) layers, just like the first class of HTSCs which possess the essential CuO₂ sheets. So far, dozens of iron-based HTSCs, classified into nine groups, have been discovered. In this article, the crystal-chemistry aspects of the known iron-based superconductors are reviewed and summarized by employing "hard and soft acids and bases (HSAB)" concept. Based on these understandings, we propose an alternative route to exploring new iron-based superconductors via rational structural design.

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Spin fluctuations and unconventional superconducting pairing in iron-based superconductors

Yu Shun-Li, Li Jian-Xin

Chin. Phys. B, 2013, 22 (8): 087411 doi: 10.1088/1674-1056/22/8/087411
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In this article, we review the recent theoretical works on the spin fluctuations and superconductivity in iron-based superconductors. Using the fluctuation exchange approximation and multi-orbital tight-binding models, we study the characteristics of the spin fluctuations and the symmetries of the superconducting gaps for different iron-based superconductors. We explore the systems with both electron-like and hole-like Fermi surfaces (FS) and the systems with only the electron-like FS. We argue that the spin-fluctuation theories are successful in explaining at least the essential part of the problems, indicating that the spin fluctuation is the common origin of superconductivity in iron-based superconductors.

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Exploring FeSe-based superconductors by liquid ammonia method

Ying Tian-Ping, Wang Gang, Jin Shi-Feng, Shen Shi-Jie, Zhang Han, Zhou Ting-Ting, Lai Xiao-Fang, Wang Wan-Yan, Chen Xiao-Long

Chin. Phys. B, 2013, 22 (8): 087412 doi: 10.1088/1674-1056/22/8/087412
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Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K～46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, K_xFe₂Se₂(NH₃)_y (x ≈ 0.3 and 0.6), determined mainly by the concentration of potassium. NH₃ has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.

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Electronic phase diagram of NaFe_1-xCo_xAs investigated by scanning tunneling microscopy

Zhou Xiao-Dong, Cai Peng, Wang Ya-Yu

Chin. Phys. B, 2013, 22 (8): 087413 doi: 10.1088/1674-1056/22/8/087413
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Our recent scanning tunneling microscopy (STM) studies of the NaFe_1-xCo_xAs phase diagram over a wide range of dopings and temperatures are reviewed. Similar to the high-T_c cuprates, the iron-based superconductors lie in close proximity to a magnetically ordered phase. Therefore, it is widely believed that magnetic interactions or fluctuations play an important role in triggering their Cooper pairings. Among the key issues regarding the electronic phase diagram are the properties of the parent spin density wave (SDW) phase and the superconducting (SC) phase, as well as the interplay between them. The NaFe_1-xCo_xAs is an ideal system for resolving these issues due to its rich electronic phases and the charge-neutral cleaved surface. In our recent work, we directly observed the SDW gap in the parent state, and it exhibits unconventional features that are incompatible with the simple Fermi surface nesting picture. The optimally doped sample has a single SC gap, but in the underdoped regime we directly viewed the microscopic coexistence of the SDW and SC orders, which compete with each other. In the overdoped regime we observed a novel pseudogap-like feature that coexists with superconductivity in the ground state, persists well into the normal state, and shows great spatial variations.The rich electronic structures across the phase diagram of NaFe_1-xCo_xAs revealed here shed important new light for defining microscopic models of the iron-based superconductors. In particular, we argue that both the itinerant electrons and local moments should be considered on an equal footing in a realistic model.

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Review of nuclear magnetic resonance studies on iron-based superconductors

Ma Long, Yu Wei-Qiang

Chin. Phys. B, 2013, 22 (8): 087414 doi: 10.1088/1674-1056/22/8/087414
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The newly discovered iron-based superconductors have triggered renewed enormous research interest in the condensed matter physics community. Nuclear magnetic resonance (NMR) is a low-energy local probe for studying strongly correlated electrons, and particularly important for high-T_C superconductors. In this paper, we review NMR studies on the structural transition, antiferromagnetic order, spin fluctuations, and superconducting properties of several iron-based high-T_C superconductors, including LaFeAsO_1-xF_x, LaFeAsO_1-x, BaFe₂As₂, Ba_1-xK_xFe₂As₂, Ca_0.23Na_0.67Fe₂As₂, BaFe₂(As_1-xP_x)₂, Ba(Fe_1-xRu_x)₂As₂, Ba(Fe_1-xCo_x)₂As₂, Li_1+xFeAs, LiFe_1-xCo_xAs, NaFeAs, NaFe_1-xCo_xAs, K_yFe_2-xSe₂, and (Tl,Rb)_yFe_2-xSe₂.

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Interplay of superconductivity and d-f correlation in CeFeAs_1-xP_xO_1-yF_y

Luo Yong-Kang, Li Yu-Ke, Wang Cao, Lin Xiao, Dai Jian-Hui, Cao Guang-Han, Xu Zhu-An

Chin. Phys. B, 2013, 22 (8): 087415 doi: 10.1088/1674-1056/22/8/087415
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The recent discovery of high-temperature superconductivity in iron-based pnictides (chalcogenides) not only triggers tremendous enthusiasm in searching for new superconducting materials, but also opens a new avenue to the study of the Kondo physics. CeFeAsO is a parent compound of the 1111-type iron-based superconductors. It shows 3d-antiferromagnetic (AFM) ordering below ～139 K and 4f-AFM ordering below ～4 K. On the other hand, the phosphide CeFePO is a ferromagnetically correlated heavy-fermion (HF) metal with Kondo scale T_K～10 K. These properties set up a new platform for research of the interplay among magnetism, Kondo effect, and superconductivity (SC). In this review, we present the recent progress in the study of chemical pressure effect in CeFeAsO_1-yF_y (y=0 and 0.05). This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, x_c1≈0.4 and x_c2≈0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce³⁺ moment at x_c3≈0.37. The SC is absent in the phase diagram, which is attributed to the destruction to Cooper pair by Ce-FM fluctuations in the vicinity of x_c1. We continue to investigate CeFeAs_1-xP_xO_0.95F_0.05. With the separation of x_c1 and x_c3, this chemical pressure results in a broad SC region 0≤x≤q 0.53, while the original HF behavior is driven away by 5% F^- doping. Different roles of P and F dopings are addressed, and the interplay between SC and Ce-4f magnetism is also discussed.

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An efficient block variant of robust structured multifrontal factorization method

Zuo Xian-Yu, Mo Ze-Yao, Gu Tong-Xiang

Chin. Phys. B, 2013, 22 (8): 080201 doi: 10.1088/1674-1056/22/8/080201
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Based on the two-dimensional three-temperature (2D3T) radiation diffusion equations and its discrete system, using the block diagonal structure of the three-temperature matrix, the reordering and symbolic decomposition parts of the RSMF method are replaced with corresponding block operation in order to improve the solution efficiency. We call this block form method block RSMF (in brief, BRSMF) method. The new BRSMF method not only makes the reordering and symbolic decomposition become more effective, but also keeps the cost of numerical factorization from increasing and ensures the precision of solution very well. The theoretical analysis of the computation complexity about the new BRSMF method shows that the solution efficiency about the BRSMF method is higher than the original RSMF method. The numerical experiments also show that the new BRSMF method is more effective than the original RSMF method.

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The dynamics of a symmetric coupling of three modified quadratic maps Suggested by editors

Paulo C. Rech

Chin. Phys. B, 2013, 22 (8): 080202 doi: 10.1088/1674-1056/22/8/080202
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We investigate the dynamical behavior of a symmetric linear coupling of three quadratic maps with exponential terms, and identify various interesting features as a function of two control parameters. In particular, we investigate the emergence of quasiperiodic states arising from Naimark-Sacker bifurcations of stable period-1, period-2, and period-3 orbits. We also investigate the multistability in the same coupling. Lyapunov exponents, parameter planes, phase space portraits, and bifurcation diagrams are used to investigate transitions from periodic to quasiperiodic states, from quasiperiodic to mode-locked states and to chaotic states, and from chaotic to hyperchaotic states.

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A complex variable meshless local Petrov-Galerkin method for transient heat conduction problems

Wang Qi-Fang, Dai Bao-Dong, Li Zhen-Feng

Chin. Phys. B, 2013, 22 (8): 080203 doi: 10.1088/1674-1056/22/8/080203
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On the basis of the complex variable moving least-square (CVMLS) approximation, a complex variable meshless local Petrov-Galerkin (CVMLPG) method is presented for transient heat conduction problems. The method is developed based on the CVMLS approximation for constructing shape functions at scattered points, and the Heaviside step function is used as a test function in each sub-domain to avoid the need for a domain integral in symmetric weak form. In the construction of the well-performed shape function, the trial function of a two-dimensional (2D) problem is formed with a one-dimensional (1D) basis function, thus improving computational efficiency. The numerical results are compared with the exact solutions of the problems and the finite element method (FEM). This comparison illustrates the accuracy as well as the capability of the CVMLPG method.

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A meshless Galerkin method with moving least square approximations for infinite elastic solids

Li Xiao-Lin, Li Shu-Ling

Chin. Phys. B, 2013, 22 (8): 080204 doi: 10.1088/1674-1056/22/8/080204
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Combining moving least square approximations and boundary integral equations, a meshless Galerkin method, which is the Galerkin boundary node method (GBNM), for two-and three-dimensional infinite elastic solid mechanics problems with traction boundary conditions is discussed. In this numerical method, the resulting formulation inherits the symmetry and positive definiteness of variational problems, and boundary conditions can be applied directly and easily. A rigorous error analysis and convergence study for both displacement and stress is presented in Sobolev spaces. The capability of this method is illustrated and assessed by some numerical examples.

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Optical field’s quadrature excitation studied by new Hermite-polynomial operator identity Suggested by editors

Fan Hong-Yi, He Rui, Da Cheng, Liang Zu-Feng

Chin. Phys. B, 2013, 22 (8): 080301 doi: 10.1088/1674-1056/22/8/080301
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We study the optical field's quadrature excitation state X^m|0>, where X=(a+a)√2 is the quadrature operator. We find it is ascribed to the Hermite-polynomial excitation state. For the first time, we determine this state's normalization constant which turns out to be a Laguerre polynomial. This is due to the integration method within the ordered product of operators (IWOP). The normalization for the two-mode quadrature excitation state is also completed by virtue of the entangled state representation.

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Relativistic symmetries in the Hulthén scalar–vector–tensor interactions

Majid Hamzavi, Ali Akbar Rajabi

Chin. Phys. B, 2013, 22 (8): 080302 doi: 10.1088/1674-1056/22/8/080302
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In the presence of spin and pseudospin (p-spin) symmetries, the approximate analytical bound states of the Dirac equation for scalar-vector-tensor Hulthén potentials are obtained with any arbitrary spin-orbit coupling number κ using the Pekeris approximation. The Hulthén tensor interaction is studied instead of the commonly used Coulomb or linear terms. The generalized parametric Nikiforov-Uvarov (NU) method is used to obtain energy eigenvalues and corresponding wave functions in their closed forms. It is shown that tensor interaction removes degeneracy between spin and p-spin doublets. Some numerical results are also given.

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Relation between initial conditions and entanglement sudden death for two-qubit extended Werner-like states Suggested by editors

Yang Bai-Yuan, Fang Mao-Fa, Huang Jiang

Chin. Phys. B, 2013, 22 (8): 080303 doi: 10.1088/1674-1056/22/8/080303
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In this paper, the dynamical behavior of entanglement of an uncoupled two-qubit system, which interacts with independent identical amplitude damping environments and is initially prepared in the extended Werner-like (EWL) states, is investigated. The results show that whether entanglement sudden death (ESD) of an EWL state will occur or not depends on initial purity and concurrence. The boundaries between ESD states and ESD-free states for two kinds of EWL states are found to be different. Furthermore, some regions are shown where ESD states can be transformed into ESD-free states by local unitary operations.

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Effect of excess noise on continuous variable entanglement sudden death and Gaussian quantum discord

Su Xiao-Long

Chin. Phys. B, 2013, 22 (8): 080304 doi: 10.1088/1674-1056/22/8/080304
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A symmetric two-mode Gaussian entangled state is used to investigate the effect of excess noise on entanglement sudden death and Gaussian quantum discord with continuous variables. The results show that the excess noise in the channel can lead to entanglement sudden death of a symmetric two-mode Gaussian entangled state, while Gaussian quantum discord never vanishes. As a practical application, the security of a quantum key distribution (QKD) scheme based on a symmetric two-mode Gaussian entangled state against collective Gaussian attacks is analyzed. The calculation results show that the secret key cannot be distilled when entanglement vanishes and only quantum discord exists in such a QKD scheme.

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Control of sudden transition between classical and quantum correlations of two strongly driven atoms in dissipative cavities

Mu Qing-Xia

Chin. Phys. B, 2013, 22 (8): 080305 doi: 10.1088/1674-1056/22/8/080305
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We investigate analytically the dynamics of classical and quantum correlations between two strongly driven atoms, each of which is trapped inside a dissipative cavity. It is found that there exists a finite time interval during which the quantum discord initially prepared in the X-type states is not destroyed by the decay of the cavities. The sudden transition between classical correlation and quantum discord is sensitive to the initial-state parameter, the cavity decay rate, and the cavity mode-driving field detuning. Interestingly, we show that the transition time can be prolonged significantly by increasing the degree of the detuning.

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Dynamics of measurement-induced non-locality and geometric discord with initial system–environment correlations Suggested by editors

Xiao Rui-Lin, Xiao Xing, Zhong Wo-Jun

Chin. Phys. B, 2013, 22 (8): 080306 doi: 10.1088/1674-1056/22/8/080306
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We analyze the dynamics of geometric measure of discord (GMOD) and measurement-induced non-locality (MIN) in the presence of initial system-reservoir correlations without Born and Markov approximation. Although the initial system-environment states have the same reduced density matrices for both the system and environment, the effects of different initial system-environment correlations have been shown to fundamentally alter the time evolution of GMOD and MIN between two quantum systems in both Markovian and non-Markovian regimes. In general, both GMOD and MIN experience a sudden increase for initially quantum-correlated states, and a sudden decrease for classical-correlated states before they reach the same stationary values with initially factorized states.

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Tunneling dynamics of superfluid Fermi gas in a triple-well potential

Gou Xue-Qiang, Meng Hong-Juan, Wang Wen-Yuan, Duan Wen-Shan

Chin. Phys. B, 2013, 22 (8): 080307 doi: 10.1088/1674-1056/22/8/080307
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The tunneling dynamics of superfluid Fermi gas in a triple-well potential in the unitarity regime is investigated in the present paper. The fixed points of the (0,0) mode and the (π,π) mode are given. We find that the interaction parameter U and the coupling strength k could have an extreme effect on the quantum tunneling dynamics. We also find that, in the zero mode, only Josophson oscillation appears. However, for the π mode, the trapping phenomena take place. An irregular oscillation of the particle number in each well could appear by adjusting the scanning period T. It is noted that if the scanning period is less than a critical point T^*, the particle number will come back to the fixed point with small oscillation, while if T >T^* the particle number cannot come back to the fixed point, but with irregular oscillations. The dependence of the critical point T^* on the system parameter of coupling strength k is numerically given.

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Excitation spectrum and structure factor of a two-component Bose-Einstein condensate in different hyperfine states

Han Yu-Feng, Chen Liang, Liu Hou-Tong, Huang Xian-Shan

Chin. Phys. B, 2013, 22 (8): 080308 doi: 10.1088/1674-1056/22/8/080308
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The elementary excitation spectrum of a two-component Bose-Einstein condensate in different hyperfine states is obtained by Green's function method. It is found to have two branches. In the long wave-length limit, the two branches of the excitation spectrum are reduced to one phonon excitation and one single-particle excitation. The single-particle one has an energy gap. When the energy gap exists, we study the Landau critical velocity and the depletion of the condensate. With the obtained Green's functions, we calculate the structure factor of a two-component condensate. It is found that the static structure factor comprises only the branch of the phonon excitation and the single-particle excitation makes no contribution to the structure factor.

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Effects of the symmetry energy slope on the axial oscillations of neutron stars

Wen De-Hua, Zhou Ying

Chin. Phys. B, 2013, 22 (8): 080401 doi: 10.1088/1674-1056/22/8/080401
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The impact of symmetry energy slope L on the axial w-mode oscillations is explored, where the range of the constrained slope L of symmetry energy at saturation density is adopted from 25 MeV to 115 MeV while keeping the equation of state (EOS) of symmetric nuclear matter fixed. Based on the range of the symmetry energy slope, a constraint on the frequency and damping time of the w_I-mode of the neutron star is given. It is found that there is a perfect linear relation between the frequency and the stellar mass for a fixed slope L, and the softer symmetry energy corresponds to a higher frequency. Moreover, it is confirmed that both the frequencies and damping times have a perfect universal scaling behavior for the EOSs with different symmetry energy slopes at saturation density.

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The fractional coupled KdV equations：Exact solutions and white noise functional approach

Hossam A. Ghany, A. S. Okb El Bab, A. M. Zabel, Abd-Allah Hyder

Chin. Phys. B, 2013, 22 (8): 080501 doi: 10.1088/1674-1056/22/8/080501
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Variable coefficients and Wick-type stochastic fractional coupled KdV equations are investigated. By using the modified fractional sub-equation method, Hermite transform, and white noise theory the exact travelling wave solutions and white noise functional solutions are obtained, including the generalized exponential, hyperbolic, and trigonometric types.

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Directed transport of coupled Brownian ratchets with time-delayed feedback Suggested by editors

Gao Tian-Fu, Zheng Zhi-Gang, Chen Jin-Can

Chin. Phys. B, 2013, 22 (8): 080502 doi: 10.1088/1674-1056/22/8/080502
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A time-delayed feedback ratchet consisting of two Brownian particles interacting through the elastic spring is considered. The model describes the directed transport of coupled Brownian particles in an asymmetric two-well ratchet potential which can be calculated theoretically and implemented experimentally. We explore how the centre-of-mass velocity is affected by the time delay, natural length of the spring, amplitude strength, angular frequency, external force, and the structure of the potential. It is found that the enhancement of the current can be obtained by varying the coupling strength of the delayed feedback system. When the thermal fluctuation and the harmonic potential match appropriately, directed current evolves periodically with the natural length of the spring and can achieve a higher transport coherence. Moreover, the external force and the amplitude strength can enhance the directed transport of coupled Brownian particles under certain conditions. It is expected that the polymer of large biological molecules may demonstrate a variety of novel cooperative effects in real propelling devices.

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A self-adapting image encryption algorithm based on spatiotemporal chaos and ergodic matrix

Luo Yu-Ling, Du Ming-Hui

Chin. Phys. B, 2013, 22 (8): 080503 doi: 10.1088/1674-1056/22/8/080503
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To ensure the security of a digital image, a new self-adapting encryption algorithm based on the spatiotemporal chaos and ergodic matrix is proposed in this paper. First, the plain-image is divided into different blocks of the same size, and each block is sorted in ascending order to obtain the corresponding standard ergodic matrix. Then each block is encrypted by the spatiotemporal chaotic system and shuffled according to the standard ergodic matrix. Finally, all modules are rearranged to acquire the final encrypted image. In particular, the plain-image information is used in the initial conditions of the spatiotemporal chaos and the ergodic matrices, so different plain-images will be encrypted to obtain different cipher-images. Theoretical analysis and simulation results indicate that the performance and security of the proposed encryption scheme can encrypt the image effectively and resist various typical attacks.

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Hopf bifurcation analysis and circuit implementation for a novel four-wing hyper-chaotic system

Xue Wei, Qi Guo-Yuan, Mu Jing-Jing, Jia Hong-Yan, Guo Yan-Ling

Chin. Phys. B, 2013, 22 (8): 080504 doi: 10.1088/1674-1056/22/8/080504
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In the paper, a novel four-wing hyper-chaotic system is proposed and analyzed. A rare dynamic phenomenon is found that this new system with one equilibrium generates a four-wing-hyper-chaotic attractor as parameter varies. The system has rich and complex dynamical behaviors, and it is investigated in terms of Lyapunov exponents, bifurcation diagrams, Poincaré maps, frequency spectrum, and numerical simulations. In addition, the theoretical analysis shows that the system undergoes a Hopf bifurcation as one parameter varies, which is illustrated by the numerical simulation. Finally, an analog circuit is designed to implement this hyper-chaotic system.

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Continuous-time chaotic systems：Arbitrary full-state hybrid projective synchronization via a scalar signal

Giuseppe Grassi

Chin. Phys. B, 2013, 22 (8): 080505 doi: 10.1088/1674-1056/22/8/080505
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Referring to continuous-time chaotic systems, this paper presents a new projective synchronization scheme, which enables each drive system state to be synchronized with a linear combination of response system states for any arbitrary scaling matrix. The proposed method, based on a structural condition related to the uncontrollable eigenvalues of the error system, can be applied to a wide class of continuous-time chaotic (hyperchaotic) systems and represents a general framework that includes any type of synchronization defined to date. An example involving a hyperchaotic oscillator is reported, with the aim of showing how a response system attractor is arbitrarily shaped using a scalar synchronizing signal only. Finally, it is shown that the recently introduced dislocated synchronization can be readily achieved using the conceived scheme.

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Studying the intermittent stable theorem and the synchronization of a delayed fractional nonlinear system

Hu Jian-Bing, Zhao Ling-Dong, Xie Zheng-Guang

Chin. Phys. B, 2013, 22 (8): 080506 doi: 10.1088/1674-1056/22/8/080506
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[PDF 5564 KB] (Downloads:366)

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In this paper, an intermittent synchronizing delayed fractional nonlinear system is studied. We propose a novel intermittent stable theorem for the delayed fractional system and derive a new synchronization criterion for delayed fractional systems by means of fractional stable theorem and the differential inequality method. Intermittent synchronizing fractional delayed Newton-Leipnik system is taken as an illustrative example and numerical simulation of this example is presented to show the feasibility and effectiveness of the proposed theorem.

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Adaptive lag synchronization of uncertain dynamical systems with time delays via simple transmission lag feedback

Gu Wei-Dong, Sun Zhi-Yong, Wu Xiao-Ming, Yu Chang-Bin

Chin. Phys. B, 2013, 22 (8): 080507 doi: 10.1088/1674-1056/22/8/080507
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In this paper we present an adaptive scheme to achieve lag synchronization for uncertain dynamical systems with time delays and unknown parameters. In contrast to the nonlinear feedback scheme reported in the previous literature, the proposed controller is a linear one which only involves simple feedback information from the drive system with signal propagation lags. Besides, the unknown parameters can also be identified via the proposed updating laws in spite of the existence of model delays and transmission lags, as long as the linear independence condition between the related function elements is satisfied. Two examples, i.e., the Mackey-Glass model with single delay and the Lorenz system with multiple delays, are employed to show the effectiveness of this approach. Some robustness issues are also discussed, which shows that the proposed scheme is quite robust in switching and noisy environment.

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Comment on ‘Mathematical structure of the three-dimensional (3D)Ising model’

Jacques H. H. Perk

Chin. Phys. B, 2013, 22 (8): 080508 doi: 10.1088/1674-1056/22/8/080508
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The review paper by Zhang Zhi-Dong (Zhang Z D 2013 Chin. Phys. B 22 030513, arXiv:1305.2956) contains many errors and is based on several earlier works that are equally wrong.

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Entropy resistance analyses of a two-stream parallel flow heat exchanger with viscous heating Suggested by editors

Cheng Xue-Tao, Liang Xin-Gang

Chin. Phys. B, 2013, 22 (8): 080509 doi: 10.1088/1674-1056/22/8/080509
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Heat exchangers are widely used in industry, and analyses and optimizations of the performance of heat exchangers are important topics. In this paper, we define the concept of entropy resistance based on the entropy generation analyses of a one-dimensional heat transfer process. With this concept, a two-stream parallel flow heat exchanger with viscous heating is analyzed and discussed. It is found that the minimization of entropy resistance always leads to the maximum heat transfer rate for the discussed two-stream parallel flow heat exchanger, while the minimizations of entropy generation rate, entropy generation numbers, and revised entropy generation number do not always.

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Synthesis of nitrogen-doped single-walled carbon nanotubes and monitoring of doping by Raman spectroscopy

Wu Mu-Hong, Li Xiao, Pan Ding, Liu Lei, Yang Xiao-Xia, Xu Zhi, Wang Wen-Long, Sui Yu, Bai Xue-Dong

Chin. Phys. B, 2013, 22 (8): 086101 doi: 10.1088/1674-1056/22/8/086101
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Nitrogen-doped single-walled carbon nanotubes (CN_x-SWNTs) with tunable dopant concentrations were synthesized by chemical vapor deposition (CVD), and their structure and elemental composition were characterized by using transmission electron microscopy (TEM) in combination with electron energy loss spectroscopy (EELS). By comparing the Raman spectra of pristine and doped nanotubes, we observed the doping-induced Raman G band phonon stiffening and 2D band phonon softening, both of which reflect doping-induced renormalization of the electron and phonon energies in the nanotubes and behave as expected in accord with the n-type doping effect. On the basis of first principles calculations of the distribution of delocalized carrier density in both the pristine and doped nanotubes, we show how the n-type doping occurs when nitrogen heteroatoms are substitutionally incorporated into the honeycomb tube-shell carbon lattice.

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Abnormal magnetic behaviors induced by the antisite phase boundary in La₂NiMnO₆ ^Hot!

Zhao Yue-Lei, Chai Yi-Sheng, Pan Li-Qing, Sun Young

Chin. Phys. B, 2013, 22 (8): 087601 doi: 10.1088/1674-1056/22/8/087601
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The ferromagnetic semiconductor La₂NiMnO₆ (LNMO) has recently received much attention due to its high Curie temperature (T_C ～ 280 K), which is close to room temperature. We prepared single-phase LNMO polycrystalline samples and investigated the temperature-and field-dependent magnetic behaviors of bulk LNMO. Between T_C and T^*=300 K, we observed upward and downward deviations from the Curie-Weiss law for high and low magnetic fields, respectively. From the electron spin resonance results, we can exclude the existence of the Griffiths phase. On the contrary, our results indicate that the abnormal magnetic behaviors might be induced by antisite phase boundaries with antiferromagnetic interaction.

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Molecular dynamics simulation of self-diffusion coefficients for liquid metals

Ju Yuan-Yuan, Zhang Qing-Ming, Gong Zi-Zheng, Ji Guang-Fu

Chin. Phys. B, 2013, 22 (8): 083101 doi: 10.1088/1674-1056/22/8/083101
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The temperature-dependent coefficients of self-diffusion for liquid metals are simulated by molecular dynamics methods based on the embedded-atom-method (EAM) potential function. The simulated results show that a good inverse linear relation exists between the natural logarithm of self-diffusion coefficients and temperature, though the results in the literature vary somewhat, due to the employment of different potential functions. The estimated activation energy of liquid metals obtained by fitting the Arrhenius formula is close to the experimental data. The temperature-dependent shear-viscosities obtained from the Stokes-Einstein relation in conjunction with the results of molecular dynamics simulation are generally consistent with other values in the literature.

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A polyene chain of canthaxanthin investigated by temperature-dependent resonance Raman spectra and density functional theory (DFT) calculations Suggested by editors

Chen Yuan-Zheng, Li Shuo, Zhou Mi, Li Zuo-Wei, Sun Cheng-Lin

Chin. Phys. B, 2013, 22 (8): 083301 doi: 10.1088/1674-1056/22/8/083301
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We report on a temperature-dependent resonance Raman spectral characterization of the polyene chain of canthaxanthin. It is observed that all vibrational intensities of the polyene chain are inversely proportional to temperature, which is analyzed by the resonance Raman effect and the coherent weakly damped electron/lattice vibrations. The increase in intensity of the CC overtone/combination relative to the fundamental with temperature decreasing is detected and discussed in terms of electron/phonon coupling and the activation energy U_op. Moreover, the polyene chain studies using the density functional theory B3LYP/6-31G^* level reveal a prominent peak at 1525 cm^-1 consisting of two closely spaced modes that are both dominated by C=C stretching coordinates of the polyene chain.

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Reanalysis of the photoassociation spectrum of ¹³³Cs₂ (6P_3/2) 1_g state ^Hot!

Ma Jie, Li Yu-Qing, Wu Ji-Zhou, Fan Qun-Chao, Feng Hao, Sun Wei-Guo, Xiao Lian-Tuan, Jia Suo-Tang

Chin. Phys. B, 2013, 22 (8): 083302 doi: 10.1088/1674-1056/22/8/083302
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Reanalysis of the photoassociation spectrum of the weakly binding (6S_1/2+6P_3/2) 1_g ¹³³Cs₂ levels, reported in the previous study [J. Mol. Spectro. 255 (2009) 106], is performed by using a Lu-Fano graph coupled to the improved LeRoy-Bernstein formula including two additional modified terms. A more accurate coefficient (c₃) is obtained for the leading long-range potential (-c₃/R³) of a diatomic molecule.

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Mössbauer studies on the shape effect of Fe_84.94Si_9.68Al_5.38 particles on their microwave permeability

Han Man-Gui, Deng Long-Jiang

Chin. Phys. B, 2013, 22 (8): 083303 doi: 10.1088/1674-1056/22/8/083303
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Ball milling for long time (such as 10, 20, and 30 h) can transform Fe_84.94Si_9.68Al_5.38 alloy powders with irregular shapes into flakes. X-ray diffraction (XRD) and Mössbauer measurements have proven that the unmilled particles and the flakes obtained by milling for 10 h have the same D0₃-type superlattice structure. The flakes obtained by milling for 20 h and 30 h have the same disorder α-Fe(Si, Al) structure. There are more than 6 absorption peaks in the transmission Mössbauer spectra (TMSs) for the particles with D0₃-type superlattice structure, which can be fitted with 5 sextets representing 5 different Fe-site environments. However, only 6 TMS absorption peaks have been found for particles with a disorder α-Fe(Si, Al) structure, which can be fitted with the distributions of Mössbauer parameters (Bhf, isomer shift). The TMS results show that the flaky particles have a stronger tendency to possess the planar magnetic anisotropy. As the result, the flakes have larger microwave permeability values than particles with irregular shapes. The conversion electron Mössbauer spectra (CEMSs) also show the significantly different Fe-sites environments between the alloy surface and the inside.

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The influence of 3d-metal alloy additions on the elastic and thermodynamic properties of CuPd₃

Huang Shuo, Zhang Chuan-Hui, Sun Jing, Shen Jiang

Chin. Phys. B, 2013, 22 (8): 083401 doi: 10.1088/1674-1056/22/8/083401
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[PDF 2138 KB] (Downloads:481)

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Embedded-atom method (EAM) potentials are used to investigate the effects of alloying (e.g. 3d-metals) on the trends of elastic and thermodynamic properties for CuPd₃ alloy. Our calculated lattice parameter, cohesive energy, and elastic constants of CuPd₃ are consistent with the available experimental and theoretical data. The results of elastic constants indicate that all these alloys are mechanically stable. Further mechanical behavior analysis shows that the additions of Cr, Fe, Co, and Ni could improve the hardness of CuPd₃ while V could well increase its ductility. Moreover, in order to evaluate the thermodynamic contribution of 3d-metals, the Debye temperature, phonon density of states, and vibrational entropy for CuMPd₆ alloy are also investigated.

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The reagent vibrational excitation effect on the stereodynamics of the reaction O(¹D)+HBr→OH+Br

Zhang Ying-Ying, Shi Ying, Xie Ting-Xian, Jin Ming-Xing, Hu Zhan

Chin. Phys. B, 2013, 22 (8): 083402 doi: 10.1088/1674-1056/22/8/083402
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[PDF 813 KB] (Downloads:380)

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Calculations on the dynamics of the reaction O(¹D)+HBr→OH+Br are performed on the ab initio potential energy surfaces (PESs) of the ground state given by Peterson [Peterson K A J. Chem. Phys. 113 4598 (2000)] using the quasiclassical trajectory (QCT) method. The product distribution of the dihedral angle, P(φ_r), and that of the angle between k and j', P(θ_r), are presented in three dimensions. Moreover, we also investigate the reagent vibrational excitation effects on the two polarization-dependent generalized differential cross sections (PDDCS), PDDCS₀₀ and PDDCS₂₀, in the centerof-mass frame. The results indicate that the vector properties are sensitive to the reagent vibrational quantum number.

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Quasiclassical trajectory theoretical study on the chemical stereodynamics of the O(¹D)+H₂→OH+H reaction and its isotopic variants (HD, D₂)

Yao Cui-Xia, Zhao Guang-Jiu

Chin. Phys. B, 2013, 22 (8): 083403 doi: 10.1088/1674-1056/22/8/083403
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The quasiclassical trajectory (QCT) method is used to study stereodynamic information about the reaction O (¹D)+H₂→OH+H on the DK (Dobbyn and Knowles) (1¹A') ab initio potential energy surface (PES). A wide scale of collision energy (E_c) from 0.05 eV to 0.5 eV is considered in the dynamic calculations. To reveal the rovibrational excitation effect, calculations at a collision energy of 0.52 eV are carried out for the v=0~5, j=0 and v=0, j=0~15 initial states. The two popularly used polarization-dependent differential cross sections (PDDCSs), σ₀₀/dw_t (0, 0) and dσ₂₀/dw_t(2, 0), and two angular distributions, P(θ_r) and P(φ_r) are calculated to obtain an insight into the alignment and the orientation of the product molecules. From the calculations, we can obtain that the alignment of the OH product is weaker at high collision energy and becomes stronger with the increase of initial vibrational level, and it is almost insensitive to the initially rotational excitation. Influences of the mass values of isotopes (HD, D₂) on the stereodynamics are also shown and discussed. Comparisons between available theoretical results and experimental results are made and discussed.

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Resonance suppression and electromagnetic shielding effectiveness improvement of an apertured rectangular cavity by using wall losses

Jiao Chong-Qing, Zhu Hong-Zhao

Chin. Phys. B, 2013, 22 (8): 084101 doi: 10.1088/1674-1056/22/8/084101
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[PDF 403 KB] (Downloads:1173)

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The cavity-mode resonance effect could result in significant degradation of the shielding effectiveness (SE) of a shielding enclosure around its resonance frequencies. In this paper, the influence of coated wall loss on the suppression of the resonance effect is investigated. For this purpose, an equivalent circuit model is employed to analyze the SE of an apertured rectangular cavity coated with an inside layer of resistive material. The model is developed by extending Robinson's equivalent circuit model through incorporating the effect of the wall loss into both the propagation constant and the characteristic impedance of the waveguide. Calculation results show that the wall loss could lead to great improvement on the SE for frequencies near the resonance but almost no effect on the SE for frequencies far away from the resonance.

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Ablation and geometry change study of solid armature in a railgun

Zhang Ya-Dong, Ruan Jiang-Jun, Hu Yuan-Chao, Gong Ruo-Han, Wen Wu

Chin. Phys. B, 2013, 22 (8): 084102 doi: 10.1088/1674-1056/22/8/084102
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[PDF 1049 KB] (Downloads:585)

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Armature plays an important role in the electromagnetic launch process. Due to the skin effect, the current density distribution is neither uniform on the rail, nor on the armature. High current density centralization in one part could lead to a partial high temperature and make the armature material melt down and be ablated, especially at low velocity. In this paper we try to change the geometry of a Cshaped armature to improve the current density distribution and reduce the ablation. Four variants of C-shaped armatures are designed to study the specific features, including a conventional C-shaped armature (CCA), a rounded leading edge C-shaped armature (LCA), a rounded trailing edge C-shaped armature (TCA), and a rounded incorporate edge C-shaped armature (ICA). A novel low-speed experiment is constructed and tested. The armatures are ablated and recovered to compare the improved effects. Then finite element simulations according to the experimental results are performed to further analyze the experimental results. It is proved that the current density and hence the temperature distribution can be immensely improved by simply changing the armature geometry. LCA and ICA show that the erosion is more uniform on the contact surface due to the rounded leading edge. The curved trailing edge could improve the uniformity of the current on the interface. ICA which combines the effects of LCA and TCA is the best option in the four armatures. How much the leading edge and the trailing edge should be curved involves the geometry of CCA and the posture of the interface on the rail. A saddle shape is a good option to improve the current density and temperature distribution in the throat. Erosion mechanism is analyzed finally. The experiments and simulations support the erosion and transition mechanism. A detailed description of the experiments and simulations is also presented in this paper.

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An all-optical buffer based on polarization rotation in an EAM

Wang Kui-Ru, Kuang Hai, Wang Yong-Jun, Yuan Jin-Hui, Yan Bin-Bin

Chin. Phys. B, 2013, 22 (8): 084201 doi: 10.1088/1674-1056/22/8/084201
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[PDF 292 KB] (Downloads:388)

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A theoretical model of the refractive index changes of the TE and TM modes in an electro-absorption modulator (EAM) is deduced. The photon absorption and refractive index changes are analyzed numerically. The influence of pump intensity on the phase difference between the TE and TM modes is studied. The polarization rotation effect is obtained in the EAM, and a novel all-optical fiber loop buffer is designed.

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Barut–Girardello and Gilmore–Perelomov coherent states for pseudoharmonic oscillators and their nonclassical properties：Factorization method

M K Tavassoly, H R Jalali

Chin. Phys. B, 2013, 22 (8): 084202 doi: 10.1088/1674-1056/22/8/084202
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[PDF 238 KB] (Downloads:516)

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In this paper we try to introduce the ladder operators associated with the pseudoharmonic oscillator, after solving the corresponding Schrödinger equation by using the factorization method. The obtained generalized raising and lowering operators naturally lead us to the Dirac representation space of the system which is much easier to work with, in comparison to the functional Hilbert space. The SU(1,1) dynamical symmetry group associated with the considered system is exactly established through investigating the fact that the deduced operators satisfy appropriate commutation relations. This result enables us to construct two important and distinct classes of Barut-Girardello and Gilmore-Perelomov coherent states associated with the system. Finally, their identities as the most important task are exactly resolved and some of their nonclassical properties are illustrated, numerically.

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Simple formula for the ionization rate of diatomic molecules in an intense laser field using the velocity gauge Suggested by editors

Tang Zeng-Hua, Li Yong-Qing, Ma Feng-Cai

Chin. Phys. B, 2013, 22 (8): 084203 doi: 10.1088/1674-1056/22/8/084203
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[PDF 254 KB] (Downloads:357)

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We derive a general ionization rate formula for the system of diatomic molecules in the velocity gauge. A more concise expression of the photoionization rate in the tunnel region is obtained for the first time. Comparisons are made among the different versions of strong-field approximation. The numerical study shows that the ionization rate in the velocity gauge is underestimated by a few orders compared with that in the length gauge. Our simple formula of ionization rate may provide an insight into the ionization mechanism for the system of diatomic molecules.

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A power and wavelength detuning-dependent hysteresis loop in a single mode Fabry-Pérot laser diode

Wu Jian-Wei, Bikash Nakarmi

Chin. Phys. B, 2013, 22 (8): 084204 doi: 10.1088/1674-1056/22/8/084204
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[PDF 231 KB] (Downloads:394)

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In this paper, we observe experimentally the optical bistability induced by the side-mode injection power and wavelength detuning in a single mode Fabry-Pérot laser diode (SMFP-LD). Results show that the bistability characteristics of the dominant and injected modes are strongly dependent on the injected input optical power and wavelength detuning in an SMFP-LD. We observe three types of hysteresis loops: counterclockwise, clockwise, and butterfly hysteresis with various loop widths. In the case of a bistability loop caused by injection power, the transition from counterclockwise to clockwise in the hysteresis direction with the wavelength detuning from 0.028 nm to 0.112 nm is observed in a way of butterfly hysteresis for the dominant mode by increasing the wavelength detuning. The width of hysteresis loop, induced by wavelength detuning is also changed while the injection power is enhanced from-7 dBm to-5 dBm.

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Different supercontinuum generation processes in photonic crystal fibers pumped with a 1064-nm picosecond pulse

Chen Hong-Wei, Jin Ai-Jun, Chen Sheng-Ping, Hou Jing, Lu Qi-Sheng

Chin. Phys. B, 2013, 22 (8): 084205 doi: 10.1088/1674-1056/22/8/084205
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[PDF 680 KB] (Downloads:558)

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Picosecond pulse pumped supercontinuum generation in photonic crystal fiber is investigated by performing a series of comparative experiments. The main purpose is to investigate the supercontinuum generation processes excited by a given pump source through the experimental study of some specific fibers. A 20-W all-fiber picosecond master oscillator-power amplifier (MOPA) laser is used to pump three different kinds of photonic crystal fibers for supercontinuum generation. Three diverse supercontinuum formation processes are observed to correspond to photonic crystal fibers with distinct dispersion properties. The experimental results are consistent with the relevant theoretical results. Based on the above analyses, a watt-level broadband white light supercontinuum source spanning from 500 nm to beyond 1700 nm is demonstrated by using a picosecond fiber laser in combination with the matched photonic crystal fiber. The limitation of the group velocity matching curve of the photonic crystal fiber is also discussed in the paper.

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Effect of initial chirp on near-infrared supercontinuum generation by a nanosecond pulse in a nonlinear fiber amplifier

Song Rui, Hou Jing, Wang Ze-Feng, Xiao Rui, Lu Qi-Sheng

Chin. Phys. B, 2013, 22 (8): 084206 doi: 10.1088/1674-1056/22/8/084206
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[PDF 512 KB] (Downloads:503)

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Theoretical and experimental research on the effect of initial chirp on near-infrared supercontinuum generation by a nanosecond pulse in a nonlinear fiber amplifier is carried out. The complex Ginzburg-Landau equation is used to simulate the propagation of the pulse in the fiber amplifier and the results show that pulses with negative initial chirp produce the widest supercontinuum and pulses with positive initial chirp produce the narrowest supercontinuum when the central wavelength of the pump lies in the normal dispersion region of the gain fiber. A self-made line width narrowing system is utilized to control the initial chirp of the nanosecond pump pulse and a four-stage master oscillator power amplifier configuration is adopted to produce a high power near-infrared suppercontinuum. The experimental results are in good agreement with simulations which can provide some guidance on further optimization of the system in future work.

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Suppressing the preferential σ-polarization oscillation in a high power Nd：YVO₄ laser with wedge laser crystal

Zheng Yao-Hui, Zhou Hai-Jun, Wang Ya-Jun, Wu Zhi-Qiang

Chin. Phys. B, 2013, 22 (8): 084207 doi: 10.1088/1674-1056/22/8/084207
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[PDF 427 KB] (Downloads:442)

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We observe the phenomenon of priority oscillation of the unexpected σ-polarization in high-power Nd:YVO₄ ring laser. The severe thermal lens of the σ-polarized lasing, compared with the π-polarized lasing, is the only reason for the phenomenon. By designing a wedge Nd:YVO₄ crystal as the gain medium, the unexpected σ-polarization is completely suppressed in the entire range of pump powers, and the polarization stability of the expected π-polarized output is enhanced. With the output power increasing from threshold to the maximum power, no σ-polarization lasing is observed. As a result, 25.3 W of stable single-frequency laser output at 532 nm is experimentally demonstrated.

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Thermal modeling optimization and experimental validation for a single concentrator solar cell system with a heat sink

Cui Min, Chen Nuo-Fu, Deng Jin-Xiang, Liu Li-Ying

Chin. Phys. B, 2013, 22 (8): 084208 doi: 10.1088/1674-1056/22/8/084208
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[PDF 778 KB] (Downloads:747)

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A single concentrator solar cell model with a heat sink is established to simulate the thermal performance of the system by varying the number, height, and thickness of fins, the base thickness and thermal resistance of the thermal conductive adhesive. Influence disciplines of those parameters on temperatures of the solar cell and heat sink are obtained. With optimized number, height and thickness of fins, and the thickness values of base of 8, 1.4 cm, 1.5 mm, and 2 mm, the lowest temperatures of the solar cell and heat sink are 41.7 ℃ and 36.3 ℃ respectively. A concentrator solar cell prototype with a heat sink fabricated based on the simulation optimized structure is built. Outdoor temperatures of the prototype are tested. Temperatures of the solar cell and heat sink are stabilized with time continuing at about 37 ℃-38 ℃ and 35 ℃-36 ℃ respectively, slightly lower than the simulation results because of effects of the wind and cloud. Thus the simulation model enables to predict the thermal performance of the system, and the simulation results can be a reference for designing heat sinks in the field of single concentrator solar cells.

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The propagation of shape changing soliton in a nonuniform nonlocal media

L. Kavitha, C. Lavanya, S. Dhamayanthi, N. Akila, D. Gopi

Chin. Phys. B, 2013, 22 (8): 084209 doi: 10.1088/1674-1056/22/8/084209
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[PDF 2788 KB] (Downloads:679)

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Magnetization dynamics in uniformly magnetized ferromagnetic media is studied by using Landau-Lifshitz-Gilbert equation. The nonlinear evolution equation is integrable with site-dependent and biquadratic exchange interaction by means of Landau-Lifshitz (LL) equation which is well understood. In the present work, we construct the exact solitary solutions of the nonlinear evolution equation, particularly, we employ the modified extended tangent hyperbolic function method. We show the shape changing property of solitons for the given integrable system in the presence of damping as well as inhomogeneities.

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A comparison of different entransy flow definitions and entropy generation in thermal radiation optimization

Zhou Bing, Cheng Xue-Tao, Liang Xin-Gang

Chin. Phys. B, 2013, 22 (8): 084401 doi: 10.1088/1674-1056/22/8/084401
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[PDF 345 KB] (Downloads:507)

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In thermal radiation, taking heat flow as an extensive quantity and defining the potential as temperature T or the blackbody emissive power U will lead to two different definitions of radiation entransy flow and the corresponding principles for thermal radiation optimization. The two definitions of radiation entransy flow and the corresponding optimization principles are compared in this paper. When the total heat flow is given, the optimization objectives of the extremum entransy dissipation principles (EEDPs) developed based on potentials T and U correspond to the minimum equivalent temperature difference and the minimum equivalent blackbody emissive power difference respectively. The physical meaning of the definition based on potential U is clearer than that based on potential T, but the latter one can be used for the coupled heat transfer optimization problem while the former one cannot. The extremum entropy generation principle (EEGP) for thermal radiation is also derived, which includes the minimum entropy generation principle for thermal radiation. When the radiation heat flow is prescribed, the EEGP reveals that the minimum entropy generation leads to the minimum equivalent thermodynamic potential difference, which is not the expected objective in heat transfer. Therefore, the minimum entropy generation is not always appropriate for thermal radiation optimization. Finally, three thermal radiation optimization examples are discussed, and the results show that the difference in optimization objective between the EEDPs and the EEGP leads to the difference between the optimization results. The EEDP based on potential T is more useful in practical application since its optimization objective is usually consistent with the expected one.

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Effects of material properties on the competition mechanism of heat transfer of a granular bed in rotary cylinders

Xie Zhi-Yin, Feng Jun-Xiao

Chin. Phys. B, 2013, 22 (8): 084501 doi: 10.1088/1674-1056/22/8/084501
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[PDF 557 KB] (Downloads:400)

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Mixing and heat transfer processes of the granular materials within rotary cylinders play a key role in industrial processes. The numerical simulation is carried out by using the discrete element method (DEM) to investigate the influences of material properties on the bed mixing and heat transfer process, including heat conductivity, heat capacity, and shear modulus. Moreover, a new Péclet number is derived to determine the dominant mechanism of the heating rate within the particle bed, which is directly related to thermal and mechanical properties. The system exhibits a faster heating rate with the increase of ratio of thermal conductivity and heat capacity, or the decrease of shear modulus when inter-particle conduction dominates the heating rate; conversely, it shows a fast-mixing bed when particle convection governs the heating rate. The simulation results show good agreement with the theoretical predictions.

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Instantaneous frequency and wave mode identification in a magnetosheath using few spatial points

Nathaniel E U, Beloff N, George N J

Chin. Phys. B, 2013, 22 (8): 084701 doi: 10.1088/1674-1056/22/8/084701
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[PDF 1360 KB] (Downloads:311)

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Observational data such as those obtained from the magnetosheath in the downstream of Earth's bow shock have waveforms that differ from those of sinusoidal signals. In practice, they are not even aggregates of sinusoidal signals. Therefore, the frequency decomposition for the data requires technique that will account for the time-varying features of the data that will lead to deduction of physical meaning of the observations. The combination of empirical mode decomposition (EMD) and Hilbert transform has been used for extracting the various contributing oscillatory modes (EMDs) and the instantaneous frequency determination (Hilbert transform) of every physically meaningful mode called intrinsic mode function (IMF). The resulting instantaneous frequencies are used to determine instantaneous wave vectors. The combination of the instantaneous frequencies and wave vectors is useful in the identification of wave modes based on the characteristics of the waves. The results show that EMD-Hilbert can be more reliable than simple Hilbert transform alone.

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Droplet impact on regular micro-grooved surfaces Suggested by editors

Hu Hai-Bao, Huang Su-He, Chen Li-Bin

Chin. Phys. B, 2013, 22 (8): 084702 doi: 10.1088/1674-1056/22/8/084702
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[PDF 1344 KB] (Downloads:519)

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We have investigated experimentally the process of a droplet impact on a regular micro-grooved surface. The target surfaces are patterned such that micro-scale spokes radiate from the center, concentric circles, and parallel lines on the polishing copper plate, using Quasi-LIGA molding technology. The dynamic behavior of water droplets impacting on these structured surfaces is examined using a high-speed camera, including the drop impact processes, the maximum spreading diameters, and the lengths and numbers of fingers at different values of Weber number. Experimental results validate that the spreading processes are arrested on all target surfaces at low velocity. Also, the experimental results at higher impact velocity demonstrate that the spreading process is conducted on the surface parallel to the micro-grooves, but is arrested in the direction perpendicular to the micro-grooves. Besides, the lengths of fingers increase observably, even when they are ejected out as tiny droplets along the groove direction, at the same time the drop recoil velocity is reduced by micro-grooves which are parallel to the spreading direction, but not by micro-grooves which are vertical to the spreading direction.

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Electric field distribution around the chain of composite nanoparticles in ferrofluids

Fan Chun-Zhen, Wang Jun-Qiao, Cheng Yong-Guang, Ding Pei, Liang Er-Jun, Huang Ji-Ping

Chin. Phys. B, 2013, 22 (8): 084703 doi: 10.1088/1674-1056/22/8/084703
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[PDF 1514 KB] (Downloads:571)

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Composite nanoparticles (NPs) have the ability of combining materials with different properties together, thus receiving extensive attention in many fields. Here we theoretically investigate the electric field distribution around core/shell NPs (a type of composite NPs) in ferrofluids under the influence of an external magnetic field. The NPs are made of cobalt (ferromagnetic) coated with gold (metallic). Under the influence of the external magnetic field, these NPs will align along the direction of this field, thus forming a chain of NPs. According to Laplace's equations, we obtain electric fields inside and outside the NPs as a function of the incident wavelength by taking into account the mutual interaction between the polarized NPs. Our calculation results show that the electric field distribution is closely related to the resonant incident wavelength, the metallic shell thickness, and the inter-particle distance. These analytical calculations agree well with our numerical simulation results. This kind of field-induced anisotropic soft-matter systems offers the possibility of obtaining an enhanced Raman scattering substrate due to enhanced electric fields.

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X-ray spectra of high temperature tungsten plasma calculated with collisional radiative model

Wang Jun, Zhang Hong, Cheng Xin-Lu

Chin. Phys. B, 2013, 22 (8): 085201 doi: 10.1088/1674-1056/22/8/085201
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[PDF 261 KB] (Downloads:386)

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Tungsten is regarded as an important candidate of plasma facing material in international thermonuclear experimental reactor (ITER), so the determination and modeling of spectra of tungsten plasma, especially the spectra at high temperature were intensely focused on recently. In this work, using the atomic structure code of Cowan, a collisional radiative model (CRM) based on the spin-orbit-split-arrays is developed. Based on this model, the charge state distribution of tungsten ions is determined and the soft X-ray spectra from high charged ions of tungsten at different temperatures are calculated. The results show that both the average ionization charge and line positions are well agreed with others calculations and measurements with discrepancies of less than 0.63% and 1.26%, respectively. The spectra at higher temperatures are also reported and the relationship between ion abundance and temperature is predicted in this work.

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Pressure ranges of velocity splitting of ablated particles produced by pulsed laser deposition in different inert gases

Ding Xue-Cheng, Wang Ying-Long, Chu Li-Zhi, Deng Ze-Chao, Liang Wei-Hua, Fu Guang-Sheng

Chin. Phys. B, 2013, 22 (8): 085202 doi: 10.1088/1674-1056/22/8/085202
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[PDF 282 KB] (Downloads:258)

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The transport of ablated particles produced by single pulsed-laser ablation is simulated via Monte Carlo method. The pressure ranges of velocity splitting of ablated particles in different inert gases are investigated. The result shows that the range of velocity splitting decreases with the atomic mass of the ambient gas increasing. The ambient gas whose atomic mass is more than that of Kr cannot induce the velocity splitting of ablated particles. The results are explained by the underdamping model and the inertia flow model.

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Numerical study on the electron-wall interaction in a Hall thruster with segmented electrodes placed at the channel exit

Qing Shao-Wei, E Peng, Duan Ping, Xu Dian-Guo

Chin. Phys. B, 2013, 22 (8): 085203 doi: 10.1088/1674-1056/22/8/085203
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[PDF 374 KB] (Downloads:550)

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Electron-wall interaction is always recognized as an important physical problem because of its remarkable influences on thruster discharge and performance. Based on existing theories, an electrode is predicted to weaken electron-wall interaction due to its low secondary electron emission characteristic. In this paper, the electron-wall interaction in an Aton-type Hall thruster with low-emissive electrodes placed near the exit of discharge channel is studied by a fully kinetic particle-in-cell method. The results show that the electron-wall interaction in the region of segmented electrode is indeed weakened, but it is significantly enhanced in the remaining region of discharge channel. It is mainly caused by electrode conductive property which makes equipotential lines convex toward channel exit and even parallel to wall surface in near-wall region; this convex equipotential configuration results in significant physical effects such as repelling electrons, which causes the electrons to move toward the channel center, and the electrons emitted from electrodes to be remarkably accelerated, thereby increasing electron temperature in the discharge channel, etc. Furthermore, the results also indicate that the discharge current in the segmented electrode case is larger than in the non-segmented electrode case, which is qualitatively in accordance with previous experimental results.

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Angular dependence of multiple-bit upset response in static random access memories under heavy ion irradiation

Zhang Zhan-Gang, Liu Jie, Hou Ming-Dong, Sun You-Mei, Su Hong, Duan Jing-Lai, Mo Dan, Yao Hui-Jun, Luo Jie, Gu Song, Geng Chao, Xi Kai

Chin. Phys. B, 2013, 22 (8): 086102 doi: 10.1088/1674-1056/22/8/086102
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[PDF 363 KB] (Downloads:527)

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Experimental evidence is presented relevant to the angular dependences of multiple-bit upset (MBU) rates and patterns in static random access memories (SRAMs) under heavy ion irradiation. The single event upset (SEU) cross sections under tilted ion strikes are overestimated by 23.9%-84.6%, compared with under normally incident ion with the equivalent linear energy transfer (LET) value of ～ 41 MeV/(mg/cm²), which can be partially explained by the fact that the MBU rate for tilted ions of 30° is 8.5%-9.8% higher than for normally incident ions. While at a lower LET of ～ 9.5 MeV/(mg/cm²), no clear discrepancy is observed. Moreover, since the ion trajectories at normal and tilted incidences are different, the predominant double-bit upset (DBU) patterns measured are different in both conditions. Those differences depend on the LET values of heavy ions and devices under test. Thus, effective LET method should be used carefully in ground-based testing of single event effects (SEE) sensitivity, especially in MBU-sensitive devices.

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Vibrational properties of cagelike diamondoid nitrogen at high pressure Suggested by editors

Wang Hui

Chin. Phys. B, 2013, 22 (8): 086301 doi: 10.1088/1674-1056/22/8/086301
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[PDF 1137 KB] (Downloads:351)

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Under high pressure, a cagelike diamondoid nitrogen structure was lately discovered by first-principles structure researches. This newly proposed structure is very unique and has not been observed in any other element. Using density-functional calculations, we study the pressure effect on its vibrational properties. The Born effective charges are calculated, and the resulting LO-TO splittings of certain infrared active modes are beyond 20 cm^-1. We depict the Γ-point vibrational modes and find the breathing mode, rotational mode, and shearing mode. Frequencies of all the optical modes increase with pressure increasing. Moreover, the relation between the breathing mode frequency and the nitrogen cage diameter is discussed in detail. Our calculation results give a deeper insight into the vibrational properties of the cagelike diamondoid nitrogen.

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Periodic, quasiperiodic, and chaotic breathers in two-dimensional discrete β-Fermi-Pasta-Ulam lattice

Xu Quan, Tian Qiang

Chin. Phys. B, 2013, 22 (8): 086302 doi: 10.1088/1674-1056/22/8/086302
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[PDF 307 KB] (Downloads:321)

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Using numerical method, we investigate whether periodic, quasiperiodic, and chaotic breathers are supported by the two-dimensional discrete Fermi-Pasta-Ulam (FPU) lattice with linear dispersion term. The spatial profile and time evolution of the two-dimensional discrete β-FPU lattice are segregated by the method of separation of variables, and the numerical simulations suggest that the discrete breathers (DBs) are supported by the system. By introducing a periodic interaction into the linear interaction between the atoms, we achieve the coupling of two incommensurate frequencies for a single DB, and the numerical simulations suggest that the quasiperiodic and chaotic breathers are supported by the system, too.

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Surface saturation control on the formation of wurtzite polytypes in zinc blende SiC nanofilms grown on Si-(100) substrates

Liu Xing-Fang, Sun Guo-Sheng, Liu Bin, Yan Guo-Guo, Guan Min, Zhang Yang, Zhang Feng, Dong Lin, Zheng Liu, Liu Sheng-Bei, Tian Li-Xin, Wang Lei, Zhao Wan-Shun, Zeng Yi-Ping

Chin. Phys. B, 2013, 22 (8): 086802 doi: 10.1088/1674-1056/22/8/086802
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[PDF 795 KB] (Downloads:430)

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We investigate the formations of wurtzite (WZ) SiC nano polytypes in zinc blende (ZB) SiC nanofilms hetero-grown on Si-(100) substrates via low pressure chemical vapor deposition (LPCVD) by adjusting the Si/C ratio of the introduced precursors. Through SEM, TEM, and Raman characterizations, we find that the nanofilms consist of discrete WZ SiC nano polytypes and ZB SiC polytypes composed of WZ polytypes (WZ+ZB) and disordered ZB SiC polytypes, respectively, according to Si/C ratios of 0.5, 1.5, and 3. We attribute the WZ polytype formation to being due to a kinetic mechanism based on the Si/C surface saturation control.

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Light-extraction efficiency and forward voltage in GaN-based light-emitting diodes with different patterns of V-shaped pits Suggested by editors

Wang Min-Shuai, Huang Xiao-Jing

Chin. Phys. B, 2013, 22 (8): 086803 doi: 10.1088/1674-1056/22/8/086803
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[PDF 367 KB] (Downloads:1217)

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We present a new method of making a textured V-pit surface for improving the light extraction efficiency in GaN-based light-emitting diodes and compare it with the usual low-temperature method for p-GaN V-pits. Three types of GaN-based light-emitting diodes (LEDs) with surface V-pits in different densities and regions were grown by metal-organic chemical vapor deposition. We achieved the highest output power and lowest forward voltage values with the p-InGaN V-pit LED. The V-pits enhanced the light output power values by 1.45 times the values of the conventional LED owing to an enhancement of the light scattering probability and an effective reduction of Mg-acceptor activation energy. Moreover, this new technique effectively solved the higher forward voltage problem of the usual V-pit LED.

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Some corrections to the Thomas–Fermi theory

Janusz Chrzanowski

Chin. Phys. B, 2013, 22 (8): 087101 doi: 10.1088/1674-1056/22/8/087101
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[PDF 11289 KB] (Downloads:4397)

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In the presented model the wave function describing the electron is a superposition of contributions from individual components of the system, in the case of metals–lattice ions and in this sense refers not to a single electron, but rather to the system as a whole. An unconventional approach to the Schrödinger equation can provide a simple analytical relationship between the total energy of the electron and the wave number. This expression can directly determine the basic parameters such as Fermi radius, the screening radius or work function and also produce a graphical interpretation of the Fermi surface.

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First-principles investigation on the structural and elastic properties of cubic-Fe₂ TiAl under high pressures

Liu Xian-Kun, Liu Cong, Zheng Zhou, Lan Xiao-Hua

Chin. Phys. B, 2013, 22 (8): 087102 doi: 10.1088/1674-1056/22/8/087102
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[PDF 304 KB] (Downloads:467)

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The structural, elastic, and thermodynamic properties of cubic-Fe₂TiAl under high temperatures and pressures are investigated by performing ab initio calculation and using the quasi-harmonic Debye model. Some ground state properties such as lattice constant, bulk modulus, pressure derivative of the bulk modulus, and elastic constants are in good agreement with the available experimental results and theoretical data. The thermodynamic properties of Fe₂TiAl such as thermal expansion coefficient, Debye temperature, and heat capacity in ranges of 0 K-1200 K and 0 GPa-250 GPa are also obtained. The calculation results indicate that the heat capacities at different pressures all increase with temperature increasing and are close to the Dulong-Petit limit at higher temperatures, Debye temperature decreases with temperature increasing, and increases with pressure rising. The cubic-Fe₂TiAl is stable mechanically under 250 GPa. Moreover, under lower pressure, thermal expansion coefficient rises rapidly with temperature increasing, and the increasing rate becomes slow at higher pressure.

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Point defect determination by photoluminescence and capacitance-voltage characterization in a GaN terahertz Gunn diode

Li Liang, Yang Lin-An, Zhou Xiao-Wei, Zhang Jin-Cheng, Hao Yue

Chin. Phys. B, 2013, 22 (8): 087104 doi: 10.1088/1674-1056/22/8/087104
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[PDF 560 KB] (Downloads:1063)

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Photoluminescence (PL) measurement is used to study the point defect distribution in a GaN terahertz Gunn diode, which is able to the degrade high-field transport characteristic during further device operation. PL, secondary ion mass spectroscopy (SIMS), transmission electron microscope (TEM), and capacitance-voltage (C-V) measurements are used to discuss the origin of point defects responsible for the yellow luminescence in structures. The point defect densities of about 10¹¹ cm^-2 in structures are extracted by analysis of C-V characterization. After thermal annealing treatment, diminishments of point defect densities in structures are efficiently demonstrated by PL and C-V results.

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Oxygen ion conductivity of La_0.8Sr_0.2Ga_0.83Mg_0.17-xCo_xO_3-δ synthesized by laser rapid solidification

Zhang Jie, Yuan Chao, Wang Jun-Qiao, Liang Er-Jun, Chao Ming-Ju

Chin. Phys. B, 2013, 22 (8): 087201 doi: 10.1088/1674-1056/22/8/087201
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[PDF 1623 KB] (Downloads:258)

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Materials La_0.8Sr_0.2Ga_0.83Mg_0.17-xCo_xO_3-δ with x=0, 0.05, 0.085, 0.10, and 0.15 are synthesized by laser rapid solidification. It is shown that the samples prepared by laser rapid solidification give rise to unique spear-like or leaf-like microstructures which are orderly arranged and densely packed. Their electrical properties each show a general dependence of the Co content and the total conductivities of La_0.8Sr_0.2Ga_0.83Mg_0.085Co_0.085O_3-δ prepared by laser rapid solidification are measured to be 0.067, 0.124, and 0.202 S·cm^-1 at 600, 700, and 800 ℃, respectively, which are much higher than by conventional solid state reactions. Moreover, the electrical conductivities each as a function of the oxygen partial pressure are also measured. It is shown that the samples with the Co content values ≤ 8.5 mol% each exhibit basically ionic conduction while those for Co content values ≥ 10 mol % each show ionic mixed electronic conduction under oxygen partial pressures from 10^-16 atm (1 atm=1.01325×10⁵ Pa) to 0.98 atm. The improved ionic conductivity of La_0.8Sr_0.2Ga_0.83Mg_0.085Co_0.085O_3-δ prepared by laser rapid solidification compared with by solid state reactions is attributed to the unique microstructure of the sample generated during laser rapid solidification.

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Investigation of chlorine-based etchants in wet and dry etching technology for an InP planar Gunn diode

Bai Yang, Jia Rui, Wu De-Qi, Jin Zhi, Liu Xin-Yu, Lin Mei-Yu

Chin. Phys. B, 2013, 22 (8): 087202 doi: 10.1088/1674-1056/22/8/087202
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[PDF 1022 KB] (Downloads:612)

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Mesa etching technology is considerably important in the Gunn diode fabrication process. In this paper we fabricate InP Gunn diodes with two different kinds of chlorine-based etchants for the mesa etching for comparative study. We use two chlorine-based etchants, one is HCl-based solution (HCl/H₃PO₄), and the other is Cl₂-based gas mixture by utilizing inductively coupled plasma system (ICP). The results show that the wet etching (HCl-based) offers low cost and approximately vertical sidewall, whilst ICP system (Cl₂-based) offers an excellent and uniform vertical sidewall, and the over-etching is tiny on the top and the bottom of mesa. And the fabricated mesas of Gunn diodes have average etching rates of ～ 0.6 μ/min and ～ 1.2 μ/min, respectively. The measured data show that the current of Gunn diode by wet etching is lower than that by ICP, and the former has a higher threshold voltage. It provides a low-cost and reliable method which is potentially applied to the fabrication of chip terahertz sources.

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Structural, electronic, and optical properties of ZnO_1-xSe_x alloys using first-principles calculations

Muhammad Rashid, Fayyaz Hussain, Muhammad Imran, S A Ahmad, N A Noor M U Sohaib, S M Alay-e-Abbas

Chin. Phys. B, 2013, 22 (8): 087301 doi: 10.1088/1674-1056/22/8/087301
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[PDF 488 KB] (Downloads:1011)

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The structural, electronic, and optical properties of binary ZnO, ZnSe compounds, and their ternary ZnO_1-xSe_x alloys are computed using the accurate full potential linearized augmented plane wave plus local orbital (FP-LAPW+lo) method in the rocksalt (B1) and zincblende (B3) crystallographic phases. The electronic band structures, fundamental energy band gaps, and densities of states for ZnO_1-xSe_x are evaluated in the range 0≤ x ≤ 1 using Wu-Cohen (WC) generalized gradient approximation (GGA) for the exchange-correlation potential. Our calculated results of lattice parameters and bulk modulus reveal a nonlinear variation for pseudo-binary and their ternary alloys in both phases and show a considerable deviation from Vegard's law. It is observed that the predicted lattice parameter and bulk modulus are in good agreement with the available experimental and theoretical data. We establish that the composition dependence of band gap is semi-metallic in B1 phase, while a direct band gap is observed in B3 phase. The calculated density of states is described by taking into account the contribution of Zn 3d, O 2p, and Se 4s, and the optical properties are studied in terms of dielectric functions, refractive index, reflectivity, and energy loss function for the B3 phase and are compared with the available experimental data.

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Lowering plasma frequency by enhancing the effective mass of electrons：A route to deep sub-wavelength metamaterials Suggested by editors

Qin Gang, Wang Jia-Fu, Yan Ming-Bao, Chen Wei, Chen Hong-Ya, Li Yong-Feng

Chin. Phys. B, 2013, 22 (8): 087302 doi: 10.1088/1674-1056/22/8/087302
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[PDF 425 KB] (Downloads:620)

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Deep sub-wavelength metamaterials are the key to the further development of practical metamaterials with small volumes and broadband properties. We propose to reduce the electrical sizes of metamaterials down to more sub-wavelength scales by lowering the plasma frequencies of metallic wires. The theoretical model is firstly established by analyzing the plasma frequency of continuous thin wires. By introducing more inductance elements, the effective electron mass can be enhanced drastically, leading to significantly lowered plasma frequencies. Based on this theory, we demonstrate that both the electric and the magnetic plasma frequencies of metamaterials can be lowered significantly and thus the electrical sizes of metamaterials can be reduced to more sub-wavelength scales. This provides an efficient route to deep sub-wavelength metamaterials and will give rigorous impetus for the further development of practical metamaterials.

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Spin gapless armchair graphene nanoribbons under magnetic field and uniaxial strain

Hou Hai-Ping, Xie Yue-E, Chen Yuan-Ping, Ouyang Tao, Ge Qing-Xia, Zhong Jian-Xin

Chin. Phys. B, 2013, 22 (8): 087303 doi: 10.1088/1674-1056/22/8/087303
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[PDF 654 KB] (Downloads:586)

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Using Green's function method, we investigate the spin transport properties of armchair graphene nanoribbons (AGNRs) under magnetic field and uniaxial strain. Our results show that it is very difficult to transform narrow AGNRs directly from semiconductor to spin gapless semiconductors (SGS) by applying magnetic fields. However, as a uniaxial strain is exerted on the nanoribbons, the AGNRs can transform to SGS by a small magnetic field. The combination mode between magnetic field and uniaxial strain displays a nonmonotonic arch-pattern relationship. In addition, we find that the combination mode is associated with the widths of nanoribbons, which exhibits group behaviors.

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Superconducting properties of barium in three phases under high pressure from first principles

Zhou Da-Wei, Pu Chun-Ying, Song Hai-Zhen, Li Gen-Quan, Song Jin-Fan, Lu Cheng, Bao Gang

Chin. Phys. B, 2013, 22 (8): 087403 doi: 10.1088/1674-1056/22/8/087403
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[PDF 523 KB] (Downloads:326)

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Electron-phonon coupling (EPC) in the three high-pressure phases of Ba is investigated using a pseudopotential plane-wave method based on density functional perturbation theory. The calculated values of superconducting critical temperature T_c of Ba-I and Ba-II under pressure are consistent well with the trends observed experimentally. Moreover, Ba-V is found to be superconducting with a maximum T_c exceeding 7.8 K at 45 GPa. With the increase of pressure, the values of T_c increase in Ba I and Ba-II but the value of T_c decreases in Ba-V. For Ba-I at pressures below 2 GPa, the increases of logarithmic average frequency ω_log and electron-phonon coupling parameters λ both contribute to the enhancement of T_c. For all the three phases at pressures above 2 GPa, T_c is found to be primarily determined by λ. Further investigation reveals that for all the three phases, the change in λ with pressure can be explained mainly by change in the phonon frequency. Thus for Ba-II and Ba-V, although they exhibit completely different superconducting behaviors, their superconductivities have the same origin; the pressure dependence of T_c is determined finally by the pressure dependence of phonon frequency.

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Properties of pseudospin polarization on a graphene-based spin singlet superconducting junction ^Hot!

Jia Shuan-Wen, Wang Jun-Tao, Yang Yan-Ling, Bai Chun-Xu

Chin. Phys. B, 2013, 22 (8): 087408 doi: 10.1088/1674-1056/22/8/087408
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[PDF 325 KB] (Downloads:351)

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We investigate theoretically transport characteristics in a graphene-based pseudospinmagnet/superconductor junction, including the s-wave and the d-wave pairing symmetry potential in the superconducting region. It is found that the pseudospin polarization, in sharp contrast to spin polarization in the graphene-based ferromagnet/superconductor junction, holds no influence on the specular Andreev reflection for a negligible Fermi energy. Furthermore, the Fano factor is crucially affected by the zero bias state. Therefore, we suggest here that the findings could shed light on the realization of graphene-based pseudospintronics devices and provide a new way to detect the specular Andreev reflection and the zero bias state in the actual experiments.

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Ecological optimization for an irreversible magnetic Ericsson refrigeration cycle

Wang Hao, Wu Guo-Xing

Chin. Phys. B, 2013, 22 (8): 087501 doi: 10.1088/1674-1056/22/8/087501
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[PDF 402 KB] (Downloads:493)

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An irreversible Ericsson refrigeration cycle model is established, in which multi-irreversibilities such as finite-rate heat transfer, regenerative loss, heat leakage, and the efficiency of the regenerator are taken into account. Expressions for several important performance parameters, such as the cooling rate, coefficient of performance (COP), power input, exergy output rate, entropy generation rate, and ecological function are derived. The influences of the heat leakage and the time of the regenerative processes on the ecological performance of the refrigerator are analyzed. The optimal regions of the ecological function, cooling rate, and COP are determined and evaluated. Furthermore, some important parameter relations of the refrigerator are revealed and discussed in detail. The results obtained here have general significance and will be helpful in gaining a deep understanding of the magnetic Ericsson refrigeration cycle.

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Field-induced insulator-metal-insulator transitions in low-energy H₂⁺ ion-irradiated epitaxial La_2/3Ca_1/3MnO₃ thin films Suggested by editors

Wu Zhan-Wen, Li Jie, Li Song-Lin, Zheng Dong-Ning

Chin. Phys. B, 2013, 22 (8): 087503 doi: 10.1088/1674-1056/22/8/087503
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[PDF 1270 KB] (Downloads:354)

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Epitaxial La_2/3Ca_1/3MnO₃ thin films grown on LaAlO₃ (001) substrates were irradiated with low-energy 120-keV H₂⁺ ions over doses ranging from 10¹² ions/cm² to 10¹⁷ ions/cm². The irradiation suppresses the intrinsic insulator-metal (I-M) transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 × 10¹⁵ ions/cm² is in a threshold state of the electric insulator where the I-M transition is absent. In an external field of 4 T or higher, the I-M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field.

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L1₀ FePt thin films with [001] crystalline growth fabricated by ZnO addition and rapid thermal annealing

Liu Xi, Ishio Shunji

Chin. Phys. B, 2013, 22 (8): 087504 doi: 10.1088/1674-1056/22/8/087504
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[PDF 904 KB] (Downloads:528)

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FePt films with a high degree of order S of the L1₀ structure (S > 0.90) and well defined [001] crystalline growth perpendicular to the film plane are fabricated on thermally oxidized Si substrates by the addition of ZnO and a successive rapid thermal annealing (RTA) process. The optimum condition to prepare high-ordering L1₀ FePtZnO films is 20 vol% ZnO addition and 450 ℃ annealing. The effect of the ZnO additive on the ordering process of the L1₀ FePtZnO films is discussed. In the annealing process, Zn atoms move to the film surface and evaporate. The motion of the Zn atoms accelerates the intergrain exchange and decreases the ordering temperature.

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Automatic microcircuit formation based on gold-coated SU-8 microrods via dielectrophoresis

Ren Yu-Kun, Tao Ye, Hou Li-Kai, Jiang Hong-Yuan

Chin. Phys. B, 2013, 22 (8): 087701 doi: 10.1088/1674-1056/22/8/087701
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[PDF 986 KB] (Downloads:339)

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To explore the application of the characteristics of metallic microparticles, alternating current electric trapping of the SU-8 microrods coated with a thin gold layer by the chemical approach is investigated. Positive dielectrophoresis is used to absorb the gold-coated SU-8 microrods at the edge of the parallel electrodes, thereby forming chains to connect the electrodes. This is a fast automatic microcircuit formation process. Moreover, a non-charged molecule is modified on the surface of the gold-coated SU-8 microrod, and the modified microrods are controlled by the alternating electric field to form a number of chains. The different chains between the parallel electrodes consist of various parallel circuits. In order to compare these chains with different electric surfaces, the impedances of the metallic and modified microrods are measured and compared, and the results show that the gold-coated microrods act as pure resistors, while the microrods functionalized by a non-charged molecule behave as good capacitors.

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Dielectric loss of half-doped manganite La_0.5Ca_0.5MnO₃

Cao Xian-Sheng, Ji Gao-Feng, Luo Bing-Cheng, Li Feng

Chin. Phys. B, 2013, 22 (8): 087702 doi: 10.1088/1674-1056/22/8/087702
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[PDF 259 KB] (Downloads:379)

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The dielectric loss tanδ of half-doped manganite La_0.5Ca_0.5MnO₃ is investigated using Green's function technique. The La_0.5Ca_0.5MnO₃ is described by the Kondo-lattice model in the double exchange limit, taking into account the Jahn-Teller distortion and the super-exchange interaction between the localized electrons. It is found that the intensity of tanδ decreases with increasing |ε_JT|, V, and U. It is also observed that the transition temperature T_P rises as |ε_JT| and U increase. It is worth noting that T_P remains unchanged and the strength of tanδ increases with increasing g. The calculated dielectric loss results are explained theoretically, and these behaviors are in qualitative agreement with the experimental results.

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The magnetoelectric properties of A-or B-site-doped PbVO₃ films：A first-principles study

Chen Xing-Yuan, Chen Li-Juan, Zhao Yu-Jun

Chin. Phys. B, 2013, 22 (8): 087703 doi: 10.1088/1674-1056/22/8/087703
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[PDF 549 KB] (Downloads:917)

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We employ first-principles calculations to study the magnetic and ferroelectric properties of PbVO₃ with A (X_A=Ca, Sr, Bi, Ba, and La) or B (X_B=Ti, Cr, Mn, Fe, Co, Ni, and Cu) site dopants, with the aim of ascertaining a large ferroelectric polarization and a long magnetic order, or even a macro ferri/ferromagnetism, which is critical to their potential applications in magnetoelectronic devices. It is found that Pb₇X_AV₈O₂₄ (X_A=Ca, Sr, and Ba,) are inclined to maintain the spin glass and large ferroelectric polarization. The degenerated G-and C-antiferromagnetic (AFM) couplings in the ideal PbVO₃ are broken up, accompanied by the loss of ferroelectric properties, when La or Bi is doped at the A site. In contrast, the above-mentioned 3d transition elements doped at the B site of PbVO₃ could induce remnant magnetic moments and preserve the large ferroelectric polarization, except for Ni and Cu. The Fe or Cr at the B site clearly remove the degenerated G-and C-AFM coupling, but the nonmagnetic Ti cannot do so. For the Mn, Co, Ni, or Cu doped at the B sites, even the two-dimensional AFM ordering in PbVO₃ is destabilized. The various doping effects are further discussed with inner strain and charge transfer.

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Analysis of the electrically forced vibrations of piezoelectric mesa resonators

He Hui-Jing, Nie Guo-Quan, Liu Jin-Xi, Yang Jia-Shi

Chin. Phys. B, 2013, 22 (8): 087704 doi: 10.1088/1674-1056/22/8/087704
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[PDF 361 KB] (Downloads:696)

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We study the electrically forced thickness-shear and thickness-twist vibrations of stepped thickness piezoelectric plate mesa resonators made of polarized ceramics or 6-mm class crystals. A theoretical analysis based on the theory of piezoelectricity is performed, and an analytical solution is obtained using the trigonometric series. The electrical admittance, resonant frequencies, and mode shapes are calculated, and strong energy trapping of the modes is observed. Their dependence on the geometric parameters of the resonator is also examined.

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The low-temperature sintering and microwave dielectric properties of (Zn_0.7Mg_0.3)TiO₃ ceramics with H₃BO₃

Shen Guo-Ce, Su Hua, Zhang Huai-Wu, Jing Yu-Lan, Tang Xiao-Li

Chin. Phys. B, 2013, 22 (8): 087801 doi: 10.1088/1674-1056/22/8/087801
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[PDF 2408 KB] (Downloads:597)

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The effects of the addition of H₃BO₃ on the microstructure, phase formation, and microwave dielectric properties of (Zn_0.7Mg_0.3)TiO₃ ceramics sintered at temperatures ranging from 890 ℃ to 950 ℃ are investigated. H₃BO₃ as a sintering agent can effectively lower the sintering temperature of ZMT ceramics below 950 ℃ due to the liquid-phase effect. The microwave dielectric properties are found to strongly correlate with the amount of H₃BO₃. With the increase in H₃BO₃ content, the dielectric constant (ε_r) monotonically increases, but the quality factor (Q×f) reaches a maximum at 1 wt% H₃BO₃, and the apparent density of ZMT ceramics with H₃BO₃ ≥ 1 wt% gradually decreases. At 950 ℃, the ZMT ceramics with 1% H₃BO₃ exhibit excellent microwave dielectric properties: ε_r=19.8, and Q×f=43800 GHz (8.94 GHz).

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Improvement on the magneto-optical Kerr effect of cobalt film with a quadrilayer structure

Zhang Shao-Yin, Tang Shao-Long, Gao Jin-Long, Luo Xiao-Jing, Xia Wen-Bin, Du You-Wei

Chin. Phys. B, 2013, 22 (8): 087802 doi: 10.1088/1674-1056/22/8/087802
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[PDF 373 KB] (Downloads:1129)

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The magneto-optical Kerr effect of the HfO₂/Co/HfO₂/Al multilayer structure is investigated in this work, and an obvious cavity enhancement of the Kerr response for the HfO₂ semiconductor is found both theoretically and experimentally. Surprisingly, a maximum value of about-3° of the polar Kerr rotation for s-polarized incident light is observed in our experiment. We propose that this improvement on the Kerr effect can be attributed to the multiple reflection and optical interference in the cavity, which can also be proved by simulation using the finite element method.

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Enhancement of green emission by the codoping A⁺ (A=Li, Na, K) in Ca₂BO₃Cl：Tb³⁺ phosphor

Li Pan-Lai, Xu Zheng, Zhao Su-Ling, Zhang Fu-Jun, Wang Yong-Sheng

Chin. Phys. B, 2013, 22 (8): 087803 doi: 10.1088/1674-1056/22/8/087803
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[PDF 355 KB] (Downloads:376)

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Tb³⁺-doped Ca₂BO₃Cl compounds with different charge compensation approaches are synthesized by a high-temperature solid-state reaction method, and the luminescent properties and Commission Internationale de l'Eclairage (CIE) chromaticity coordinates are systematically characterized. Ca₂BO₃Cl:Tb³⁺ can produce green emission under 376 nm radiation excitation. With codoped A⁺ (A=Li, Na, K) as charge compensators, the relative emission intensities of Ca₂BO₃Cl:Tb³⁺ are enhanced by about 1.61, 1.97, and 1.81 times compared with those of the direct charge balance, which is considered to be due to the effect of the difference in ion radius on the crystal field. The CIE chromaticity coordinates of Ca₂BO₃Cl:Tb³⁺, A⁺ (A=Li, Na, K) are (0.335, 0.584), (0.335, 0.585), and (0.335, 0.585), corresponding to the hues of green. Therefore, A⁺ (A=Li, Na, K) may be the optimal charge compensator for Ca₂BO₃Cl:Tb³⁺.

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Luminescence properties of ZnS：Cu, Eu semiconductor nanocrystals synthesized by a hydrothermal process

Xin Mei, Hu Li-Zhong

Chin. Phys. B, 2013, 22 (8): 087804 doi: 10.1088/1674-1056/22/8/087804
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[PDF 961 KB] (Downloads:2650)

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ZnS:Cu, Eu nanocrystals with an average diameter of ～ 80 nm are synthesized using a hydrothermal approach at 200 ℃. The photoluminescence (PL) properties of the ZnS:Cu, Eu nanocrystals before and after annealing, as well as the doping form of Eu, are studied. The as-synthesized samples are characterized by X-ray diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry, and the excitation and emission spectra (PL). The results show that both Cu and Eu are indeed incorporated into the ZnS matrix. Compared with the PL spectrum of the Cu mono-doped sample, the PL emission intensity of the Cu and Eu-codoped sample increases and a peak appears at 516 nm, indicating that Eu³⁺ ions, which act as an impurity compensator and activator, are incorporated into the ZnS matrix, forming a donor level. Compared with the unannealed sample, the annealed one has an increased PL emission intensity and the peak position has a blue shift of 56 nm from 516 nm to 460 nm, which means that Eu³⁺ ions reduce to Eu²⁺ ions, thereby leading to the appearance of Eu²⁺ characteristic emission and generating effective host-to-Eu²⁺ energy transfer. The results indicate the potential applications of ZnS:Cu, Eu nanoparticles in optoelectronic devices.

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Optical and magnetic properties of porous anodic alumina films embedded with Co nanowires

Zhang Jing-Jing, Li Zi-Yue, Zhang Hui-Min, Hou Xue, Sun Hui-Yuan

Chin. Phys. B, 2013, 22 (8): 087805 doi: 10.1088/1674-1056/22/8/087805
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[PDF 1481 KB] (Downloads:805)

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A simple method to tune the optical properties of porous anodic alumina (PAA) films embedded with Co nanowires (PAA@Co nanocomposite films) is reported in this paper. The films exhibit vivid structural colors and magnetic properties. The optical properties of the films can be effectively tuned by adjusting the thickness of the PAA template. The deposition of Co nanowires greatly increases the color saturation of the PAA films. The theoretical results of the changes in structural color according to the Bragg-Snell formula are consistent with the experimental results. PAA@Co films can be used in many areas, including decoration, display, and multifunctional anti-counterfeiting applications.

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White electroluminescence of n-ZnO：Al/p-diamond heterostructure devices

Yang Can, Wang Xiao-Ping, Wang Li-Jun, Pan Xiu-Fang, Li Song-Kun, Jing Long-Wei

Chin. Phys. B, 2013, 22 (8): 088101 doi: 10.1088/1674-1056/22/8/088101
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[PDF 5682 KB] (Downloads:592)

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An n-ZnO:Al/p-boron-doped diamond heterostructure electroluminescent device is produced, and a rectifying behavior can be observed. The electroluminescence spectrum at room temperature exhibits two visible bands centred at 450 nm-485 nm (blue emission) and 570 nm-640 nm (yellow emission). Light emission with a luminance of 15 cd/m² is observed from the electroluminescent device at a forward applied voltage of 85 V, which is distinguished from white light by the naked eye.

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The enhancement of light-emitting efficiency using GaN-based multiple quantum well light-emitting diodes with nanopillar arrays

Wan Tu-Tu, Ye Zhan-Qi, Tao Tao, Xie Zi-Li, Zhang Rong, Liu Bin, Xiu Xiang-Qian, Li Yi, Han Ping, Shi Yi, Zheng You-Dou

Chin. Phys. B, 2013, 22 (8): 088102 doi: 10.1088/1674-1056/22/8/088102
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[PDF 521 KB] (Downloads:633)

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The quest for higher modulation speed and lower energy consumption has inevitably promoted the rapid development of semiconductor-based solid lighting devices in recent years. GaN-based light-emitting diodes (LEDs) have emerged as promising candidates for achieving high efficiency and high intensity, and have received increasing attention among many researchers in this field. In this paper, we use a self-assembled array-patterned mask to fabricate InGaN/GaN multi-quantum well (MQW) LEDs with the intention of enhancing the light-emitting efficiency. By utilizing inductively coupled plasma etching with a self-assembled Ni cluster as the mask, nanopillar arrays are formed on the surface of the InGaN/GaN MQWs. We then observe the structure of the nanopillars and find that the V-defects on the surface of the conventional structure and the negative effects of threading dislocation are effectively reduced. Simultaneously, we make a comparison of the photoluminescence (PL) spectrum between the conventional structure and the nanopillar arrays, achieved under an experimental set-up with an excitation wavelength of 325 mm. The analysis demonstrates that MQW-LEDs with nanopillar arrays achieve a PL intensity 2.7 times that of conventional LEDs. In response to the PL spectrum, some reasons are proposed for the enhancement in the light-emitting efficiency as follows: 1) the improvement in crystal quality, namely the reduction in V-defects; 2) the roughened surface effect on the expansion of the critical angle and the attenuated total reflection; and 3) the enhancement of the light-extraction efficiency due to forward scattering by surface plasmon polariton modes in Ni particles deposited above the p-type GaN layer at the top of the nanopillars.

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Growth and characterization of ZnO multipods on functional surfaces with different sizes and shapes of Ag particles

A Kamalianfar, S A Halim, Mahmoud Godarz Naseri, M Navasery, Fasih Ud Din, J A M Zahedi, Kasra Behzad, K P Lim, A Lavari Monghadam, S K Chen

Chin. Phys. B, 2013, 22 (8): 088103 doi: 10.1088/1674-1056/22/8/088103
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[PDF 1785 KB] (Downloads:1024)

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Three-dimensional ZnO multipods are successfully synthesized on functional substrates using the vapor transport method in a quartz tube. The functional surfaces, which include two different distributions of Ag nanoparticles and a layer of commercial Ag nanowires, are coated onto silicon substrates before the growth of ZnO nanostructures. The structures and morphologies of the ZnO/Ag heterostructures are investigated using X-ray diffraction and field emission scanning electron microscopy. The sizes and shapes of the Ag particles affect the growth rates and initial nucleations of the ZnO structures, resulting in different numbers and shapes of multipods. They also influence the orientation and growth quality of the rods. The optical properties are studied by photoluminescence, UV-vis, and Raman spectroscopy. The results indicate that the surface plasmon resonance strongly depends on the sizes and shapes of the Ag particles.

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Electrical properties of MOCVD-grown GaN on Si (111) substrates with low-temperature AlN interlayers

Ni Yi-Qiang, He Zhi-Yuan, Zhong Jian, Yao Yao, Yang Fan, Xiang Peng, Zhang Bai-Jun, Liu Yang

Chin. Phys. B, 2013, 22 (8): 088104 doi: 10.1088/1674-1056/22/8/088104
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[PDF 397 KB] (Downloads:3730)

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The electrical properties of the structure of GaN grown on an Si (111) substrate with low-temperature (LT) AlN interlayers by metal-organic chemical-vapour deposition are investigated. An abnormal P-type conduction is observed in our GaN-on-Si structure by Hall effect measurement, which is mainly due to the Al atom diffusing into the Si substrate and acting as an acceptor dopant. Meanwhile, a constant n-type conduction channel is observed in LT-AlN, which causes a conduction-type conversion at low temperature (50 K) and may further influence the electrical behavior of this structure.

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Magnetostructural transition in NiCuCoMnGa alloys

Li Pan-Pan, Wang Jing-Min, Jiang Cheng-Bao

Chin. Phys. B, 2013, 22 (8): 088105 doi: 10.1088/1674-1056/22/8/088105
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[PDF 314 KB] (Downloads:331)

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The effects of the addition of Co on the martensitic transformation and Curie transition temperatures of polycrystalline Ni_46-xCu₄Co_xMn_33.5Ga_16.5 (x=0, 1, 3, 5) alloys are investigated. An abrupt decrease in the martensitic transformation temperature and an obvious increase in the Curie transition temperature of austenite (T_C^A) are observed when Co is doped in the NiCuMnGa alloy. As a result, the composition range for obtaining the magnetostructural transition is extended. Furthermore, the effect of a strong magnetic field on the magnetostructural transition is analyzed. This study offers a possible method to extend the composition range for obtaining magnetostructural transition in Heusler alloys.

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Enhanced performance of InGaN/GaN multiple quantum well solar cells with double indium content

Zhao Bi-Jun, Chen Xin, Ren Zhi-Wei, Tong Jin-Hui, Wang Xing-Fu, Li Dan-Wei, Zhuo Xiang-Jing, Zhang Jun, Yi Han-Xiang, Li Shu-Ti

Chin. Phys. B, 2013, 22 (8): 088401 doi: 10.1088/1674-1056/22/8/088401
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[PDF 421 KB] (Downloads:613)

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The performance of a multiple quantum well (MQW) InGaN solar cell with double indium content is investigated. It is found that the adoption of a double indium structure can effectively broaden the spectral response of the external quantum efficiencies and optimize the overall performance of the solar cell. Under AM1.5G illumination, the short-circuit current density (J_sc) and conversion efficiency of the solar cell are enhanced by 65% and 13% compared with those of a normal single-indium-content MQW solar cell. These improvements are mainly attributed to the expansion of the absorption spectrum and better extraction efficiency of the photon-generated carriers induced by higher polarization.

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The design and simulation of a titanium oxide memristor-based programmable analog filter in a simulation program with integrated circuit emphasis

Tian Xiao-Bo, Xu Hui

Chin. Phys. B, 2013, 22 (8): 088501 doi: 10.1088/1674-1056/22/8/088501
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[PDF 929 KB] (Downloads:1334)

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In many communication and signal routing applications, it is desirable to have a programmable analog filter. According to this practical demand, we consider the titanium oxide memristor, which is a kind of nano-scale electron device with low power dissipation and nonvolatile memory. Such characteristics could be suitable for designing the desired filter. However, both the non-analytical relation between the memristance and the charges that pass through it, and the changeable V-I characteristics in physical tests make it difficult to accurately set the memristance to the target value. In this paper, the conductive mechanism of the memristor is analyzed, a method of continuously programming the memristance is proposed and simulated in a simulation program with integrated circuit emphasis, and its feasibility and compatibility, both in simulations and physical realizations, are demonstrated. This method is then utilized in a first-order active filter as an example to show its applications in programmable filters. This work also provides a practical tool for utilizing memristors as resistance programmable devices.

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The conductive mechanisms of a titanium oxide memristor with dopant drift and a tunnel barrier

Tian Xiao-Bo, Xu Hui, Li Qing-Jiang

Chin. Phys. B, 2013, 22 (8): 088502 doi: 10.1088/1674-1056/22/8/088502
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[PDF 907 KB] (Downloads:747)

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Nano-scale titanium oxide memristors exhibit complex conductive characteristics, which have already been proved by existing research. One possible reason for this is that more than one mechanism exists, and together they codetermine the conductive behaviors of the memristor. In this paper, we first analyze the theoretical base and conductive process of a memristor, and then propose a compatible circuit model to discuss and simulate the coexistence of the dopant drift and tunnel barrier-based mechanisms. Simulation results are given and compared with the published experimental data to prove the possibility of the coexistence. This work provides a practical model and some suggestions for studying the conductive mechanisms of memristors.

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Performance improvement of blue InGaN light-emitting diodes with a specially designed n-AlGaN hole blocking layer

Ding Bin-Bin, Zhao Fang, Song Jing-Jing, Xiong Jian-Yong, Zheng Shu-Wen, Zhang Yun-Yan, Xu Yi-Qin, Zhou De-Tao, Yu Xiao-Peng, Zhang Han-Xiang, Zhang Tao, Fan Guang-Han

Chin. Phys. B, 2013, 22 (8): 088503 doi: 10.1088/1674-1056/22/8/088503
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[PDF 432 KB] (Downloads:1315)

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Blue InGaN light-emitting diodes (LEDs) with a conventional electron blocking layer (EBL), a common n-AlGaN hole blocking layer (HBL), and an n-AlGaN HBL with gradual Al composition are investigated numerically, which involves analyses of the carrier concentration in the active region, energy band diagram, electrostatic field, and internal quantum efficiency (IQE). The results indicate that LEDs with an n-AlGaN HBL with gradual Al composition exhibit better hole injection efficiency, lower electron leakage, and a smaller electrostatic field in the active region than LEDs with a conventional p-AlGaN EBL or a common n-AlGaN HBL. Meanwhile, the efficiency droop is alleviated when an n-AlGaN HBL with gradual Al composition is used.

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Improved performance of InGaN light-emitting diodes with a novel sawtooth-shaped electron blocking layer Suggested by editors

Wang Tian-Hu, Xu Jin-Liang

Chin. Phys. B, 2013, 22 (8): 088504 doi: 10.1088/1674-1056/22/8/088504
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[PDF 2681 KB] (Downloads:979)

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A sawtooth-shaped electron blocking layer is proposed to improve the performance of light-emitting diodes (LEDs). The energy band diagram, the electrostatic field in the quantum well, the carrier concentration, the electron leakage, and the internal quantum efficiency are systematically studied. The simulation results show that the LED with a sawtooth-shaped electron blocking layer possesses higher output power and a smaller efficiency droop than the LED with a conventional AlGaN electron blocking layer, which is because the electron confinement is enhanced and the hole injection efficiency is improved by the appropriately modified electron blocking layer energy band.

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The laser-intensity dependence of the photoassociation spectrum of the ultracold Cs₂(6S_1/2+6P_1/2) 0_u⁺ long-range molecular state Suggested by editors

Jin Li, Feng Guo-Sheng, Wu Ji-Zhou, Ma Jie, Wang Li-Rong, Xiao Lian-Tuan, Jia Suo-Tang

Chin. Phys. B, 2013, 22 (8): 088701 doi: 10.1088/1674-1056/22/8/088701
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[PDF 516 KB] (Downloads:395)

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The high-resolution photoassociation spectrum of the ultracold cesium molecular 0_u⁺ state below the 6S_1/2+6P_1/2 limit is presented in this paper. The saturation of the photoassociation scattering probability is observed from the dependence of the trap-loss probability on the photoassociation laser intensity. The corresponding resonant line width is also demonstrated to increase linearly with increasing photoassociation laser intensity. Our experimental data have good consistency with the theoretical saturation model of Bohn and Julienne [Bohn J L and Julienne P S 1999 Phys. Rev. A 60 1].

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Experimental investigations of the functional morphology of dragonfly wings

H. Rajabi, A. Darvizeh

Chin. Phys. B, 2013, 22 (8): 088702 doi: 10.1088/1674-1056/22/8/088702
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[PDF 1501 KB] (Downloads:2273)

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Nowadays, the importance of identifying the flight mechanisms of the dragonfly, as an inspiration for designing flapping wing vehicles, is well known. An experimental approach to understanding the complexities of insect wings as organs of flight could provide significant outcomes for design purposes. In this paper, a comprehensive investigation is carried out on the morphological and microstructural features of dragonfly wings. Scanning electron microscopy (SEM) and tensile testing are used to experimentally verify the functional roles of different parts of the wings. A number of SEM images of the elements of the wings, such as the nodus, leading edge, trailing edge, and vein sections, which play dominant roles in strengthening the whole structure, are presented. The results from the tensile tests indicate that the nodus might be the critical region of the wing that is subjected to high tensile stresses. Considering the patterns of the longitudinal corrugations of the wings obtained in this paper, it can be supposed that they increase the load-bearing capacity, giving the wings an ability to tolerate dynamic loading conditions. In addition, it is suggested that the longitudinal veins, along with the leading and trailing edges, are structural mechanisms that further improve fatigue resistance by providing higher fracture toughness, preventing crack propagation, and allowing the wings to sustain a significant amount of damage without loss of strength.

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Analysis on the capacity degradation mechanism of a series lithium-ion power battery pack based on inconsistency of capacity

Wang Zhen-Po, Liu Peng, Wang Li-Fang

Chin. Phys. B, 2013, 22 (8): 088801 doi: 10.1088/1674-1056/22/8/088801
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[PDF 370 KB] (Downloads:2501)

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The lithium-ion battery has been widely used as an energy source. Charge rate, discharge rate, and operating temperature are very important factors for the capacity degradations of power batteries and battery packs. Firstly, in this paper we make use of an accelerated life test and a statistical analysis method to establish the capacity accelerated degradation model under three constant stress parameters according to the degradation data, which are charge rate, discharge rate, and operating temperature, and then we propose a capacity degradation model according to the current residual capacity of a Li-ion cell under dynamic stress parameters. Secondly, we analyze the charge and discharge process of a series power battery pack and interpret the correlation between the capacity degradations of the battery pack and its charge/discharge rate. According to this cycling condition, we establish a capacity degradation model of a series power battery pack under inconsistent capacity of cells, and analyze the degradation mechanism with capacity variance and operating temperature difference. The comparative analysis of test results shows that the inconsistent operating temperatures of cells in the series power battery pack are the main cause of its degradation; when the difference between inconsistent temperatures is narrowed by 5 ℃, the cycle life can be improved by more than 50%. Therefore, it effectively improves the cycle life of the series battery pack to reasonably assemble the batteries according to their capacities and to narrow the differences in operating temperature among cells.