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  • Coherence transfer from 1064 nm to 578 nm using an optically referenced frequency comb

    Fang Su, Jiang Yan-Yi, Chen Hai-Qin, Yao Yuan, Bi Zhi-Yi, Ma Long-Sheng
    Chin. Phys. B 2015, 24 (7): 074202
    A laser at 578 nm is phase-locked to an optical frequency comb (OFC) which is optically referenced to a subhertz-linewidth laser at 1064 nm. Coherence is transferred from 1064 nm to 578 nm via the OFC. By comparing with a cavity-stabilized laser at 578 nm, the absolute linewidth of 1.1 Hz and the fr...

  • Phase diagram and collective modes in Rashba spin–orbit coupled BEC: Effect of in-plane magnetic field

    Dong Dong, Zou Xu-Bo, Guo Guang-Can
    Chin. Phys. B 2015, 24 (7): 076701
    We studied the system of pure Rashba spin–orbit coupled Bose gas with an in-plane magnetic field. Based on the mean field theory, we obtained the zero temperature phase diagram of the system which exhibits three phases, plane wave (PW) phase, striped wave (SW) phase, and zero momentum (ZM) pha...

  • Observation of mode-like features in tunneling spectra of iron-based superconductors

    Gong Jing, Hou Xing-Yuan, Zhu Jun, Jie Yun-Yin, Gu Ya-Dong, Shen Bing, Ren Cong, Li Chun-Hong, Shan Lei
    Chin. Phys. B 2015, 24 (7): 077402
    We report scanning tunneling microscopy/spectroscopy (STM/STS) studies on iron-based superconductors of Ba1-xKxFe2As2 and nearly optimally doped Fe(Te,Se). Mode-like features were observed universally outside the superconducting gaps in the tunneling spectra, which are similar to our previous observ...

Chin. Phys. B  
  Chin. Phys. B--2015, Vol.24, No.7
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TOPICAL REVIEW—Magnetism, magnetic materials, and interdisciplinary research

Manipulating magnetic anisotropy and ultrafast spin dynamics of magnetic nanostructures

Cheng Zhao-Hua, He Wei, Zhang Xiang-Qun, Sun Da-Li, Du Hai-Feng, Wu Qiong, Ye Jun, Fang Ya-Peng, Liu Hao-Liang
Chin. Phys. B, 2015, 24 (7): 077505 doi: 10.1088/1674-1056/24/7/077505
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We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si(111) substrate by a novel method: varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a CoFeB/MgO/CoFeB magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect (TRMOKE) for both the parallel state (P state) and the antiparallel state (AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.


A novel transient rotor current control scheme of a doubly-fed induction generator equipped with superconducting magnetic energy storage for voltage and frequency support

Shen Yang-Wu, Ke De-Ping, Sun Yuan-Zhang, Daniel Kirschen, Wang Yi-Shen, Hu Yuan-Chao
Chin. Phys. B, 2015, 24 (7): 070201 doi: 10.1088/1674-1056/24/7/070201
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A novel transient rotor current control scheme is proposed in this paper for a doubly-fed induction generator (DFIG) equipped with a superconducting magnetic energy storage (SMES) device to enhance its transient voltage and frequency support capacity during grid faults. The SMES connected to the DC-link capacitor of the DFIG is controlled to regulate the transient dc-link voltage so that the whole capacity of the grid side converter (GSC) is dedicated to injecting reactive power to the grid for the transient voltage support. However, the rotor-side converter (RSC) has different control tasks for different periods of the grid fault. Firstly, for Period I, the RSC injects the demagnetizing current to ensure the controllability of the rotor voltage. Then, since the dc stator flux degenerates rapidly in Period II, the required demagnetizing current is low in Period II and the RSC uses the spare capacity to additionally generate the reactive (priority) and active current so that the transient voltage capability is corroborated and the DFIG also positively responds to the system frequency dynamic at the earliest time. Finally, a small amount of demagnetizing current is provided after the fault clearance. Most of the RSC capacity is used to inject the active current to further support the frequency recovery of the system. Simulations are carried out on a simple power system with a wind farm. Comparisons with other commonly used control methods are performed to validate the proposed control method.

Stochastic stability of the derivative unscented Kalman filter

Hu Gao-Ge, Gao She-Sheng, Zhong Yong-Min, Gao Bing-Bing
Chin. Phys. B, 2015, 24 (7): 070202 doi: 10.1088/1674-1056/24/7/070202
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This is the second of two consecutive papers focusing on the filtering algorithm for a nonlinear stochastic discrete-time system with linear system state equation. The first paper established a derivative unscented Kalman filter (DUKF) to eliminate the redundant computational load of the unscented Kalman filter (UKF) due to the use of unscented transformation (UT) in the prediction process. The present paper studies the error behavior of the DUKF using the boundedness property of stochastic processes. It is proved that the estimation error of the DUKF remains bounded if the system satisfies certain conditions. Furthermore, it is shown that the design of the measurement noise covariance matrix plays an important role in improvement of the algorithm stability. The DUKF can be significantly stabilized by adding small quantities to the measurement noise covariance matrix in the presence of large initial error. Simulation results demonstrate the effectiveness of the proposed technique.

An efficient locally one-dimensional finite-difference time-domain method based on the conformal scheme

Wei Xiao-Kun, Shao Wei, Shi Sheng-Bing, Zhang Yong, Wang Bing-Zhong
Chin. Phys. B, 2015, 24 (7): 070203 doi: 10.1088/1674-1056/24/7/070203
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An efficient conformal locally one-dimensional finite-difference time-domain (LOD-CFDTD) method is presented for solving two-dimensional (2D) electromagnetic (EM) scattering problems. The formulation for the 2D transverse-electric (TE) case is presented and its stability property and numerical dispersion relationship are theoretically investigated. It is shown that the introduction of irregular grids will not damage the numerical stability. Instead of the staircasing approximation, the conformal scheme is only employed to model the curve boundaries, whereas the standard Yee grids are used for the remaining regions. As the irregular grids account for a very small percentage of the total space grids, the conformal scheme has little effect on the numerical dispersion. Moreover, the proposed method, which requires fewer arithmetic operations than the alternating-direction-implicit (ADI) CFDTD method, leads to a further reduction of the CPU time. With the total-field/scattered-field (TF/SF) boundary and the perfectly matched layer (PML), the radar cross section (RCS) of two 2D structures is calculated. The numerical examples verify the accuracy and efficiency of the proposed method.

Quantum nonlocality of generic family of four-qubit entangled pure states

Ding Dong, He Ying-Qiu, Yan Feng-Li, Gao Ting
Chin. Phys. B, 2015, 24 (7): 070301 doi: 10.1088/1674-1056/24/7/070301
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We directly introduce a Bell-type inequality for four-qubit systems. Using the inequality we investigate quantum nonlocality of a generic family of states |Gabcd> [Phys. Rev. A 65 052112 (2002)] and several canonical four-qubit entangled states. It has been demonstrated that the inequality is maximally violated by the so called “four-qubit the maximally entangled state |Gm>” and it is also violated by four-qubit W state and a special family of states |Gab00>. Moreover, a useful entanglement-nonlocality relationship for the family of states |Gab00> is derived. Finally, we present a scheme of preparation of the state |Gm ightangle with linear optics and cross-Kerr nonlinearities.

Detection of the ideal resource for multiqubit teleportation

Zhao Ming-Jing, Chen Bin, Fei Shao-Ming
Chin. Phys. B, 2015, 24 (7): 070302 doi: 10.1088/1674-1056/24/7/070302
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We give a sufficient condition for detecting the entanglement resource for perfect multiqubit teleportation. The criterion involves only local measurements on some complementary observables and can be experimentally implemented. It is also a necessary condition for full separability of multiqubit states. Moreover, by proving the optimality of teleportation witnesses, we solve the open problem in Phys. Rev. A 86, 032315 (2012).

A compact Einstein–Podolsky–Rosen entangled light source

Wang Ya-Jun, Yang Wen-Hai, Zheng Yao-Hui, Peng Kun-Chi
Chin. Phys. B, 2015, 24 (7): 070303 doi: 10.1088/1674-1056/24/7/070303
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We present a stable entangled light source that integrates the pump laser, entanglement generator, detectors, and electronic control systems. By optimizing the design of the mechanical elements and the optical path, the size of the source is minimized, and the quantum correlations over 6 dB can be directly provided by the entangled source. The compact and stable entangled light source is suitable for practical applications in quantum information science and technology. The presented protocol provides a useful reference for manufacturing products of bright entangled light sources.

Explicit solution of diffusion master equation under the action of linear resonance force via the thermal entangled state representation

Yao Fei, Wang Ji-Suo, Xu Tian-Niu
Chin. Phys. B, 2015, 24 (7): 070304 doi: 10.1088/1674-1056/24/7/070304
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Using the well-behaved features of the thermal entangled state representation, we solve the diffusion master equation under the action of a linear resonance force, and then obtain the infinitive operator-sum representation of the density operator. This approach may also be effective for treating other master equations. Moreover, we find that the initial pure coherent state evolves into a mixed thermal state after passing through the diffusion process under the action of the linear resonance force.

Quantum mechanical operator realization of the Stirling numbers theory studied by virtue of the operator Hermite polynomials method

Fan Hong-Yi, Lou Sen-Yue
Chin. Phys. B, 2015, 24 (7): 070305 doi: 10.1088/1674-1056/24/7/070305
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Based on the operator Hermite polynomials method (OHPM), we study Stirling numbers in the context of quantum mechanics, i.e., we present operator realization of generating function formulas of Stirling numbers with some applications. As a by-product, we derive a summation formula involving both Stirling number and Hermite polynomials.

Time evolution of a squeezed chaotic field in an amplitude damping channel when used as a generating field for a squeezed number state

Xu Xing-Lei, Li Hong-Qi, Fan Hong-Yi
Chin. Phys. B, 2015, 24 (7): 070306 doi: 10.1088/1674-1056/24/7/070306
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We investigate how an optical squeezed chaotic field (SCF) evolves in an amplitude dissipation channel. We have used the integration within ordered product of operators technique to derive its evolution law. We also show that the density operator of SCF can be viewed as a generating field of the squeezed number state.

Nonlocal multi-target controlled—controlled gate using Greenberger–Horne–Zeilinger channel and qutrit catalysis

Chen Li-Bing, Lu Hong
Chin. Phys. B, 2015, 24 (7): 070307 doi: 10.1088/1674-1056/24/7/070307
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We present a scheme for implementing locally a nonlocal N-target controlled–controlled gate with unit probability of success by harnessing two (N+1)-qubit Greenberger–Horne–Zeilinger (GHZ) states as quantum channel and N qutrits as catalyser. The quantum network that implements this nonlocal (N+2)-body gate is built entirely of local single-body and two-body gates, and has only (3N+2) two-body gates. This result suggests that both the computational depth of quantum network and the quantum resources required to perform this nonlocal gate might be significantly reduced. This scheme can be generalized straightforwardly to implement a nonlocal N-target and M-control qubits gate.

Multi-user quantum key distribution with collective eavesdropping detection over collective-noise channels

Huang Wei, Wen Qiao-Yan, Liu Bin, Gao Fei
Chin. Phys. B, 2015, 24 (7): 070308 doi: 10.1088/1674-1056/24/7/070308
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A multi-user quantum key distribution protocol is proposed with single particles and the collective eavesdropping detection strategy on a star network. By utilizing this protocol, any two users of the network can accomplish quantum key distribution with the help of a serving center. Due to the utilization of the collective eavesdropping detection strategy, the users of the protocol just need to have the ability of performing certain unitary operations. Furthermore, we present three fault-tolerant versions of the proposed protocol, which can combat with the errors over different collective-noise channels. The security of all the proposed protocols is guaranteed by the theorems on quantum operation discrimination.

Direct measurement of the concurrence for two-qubit electron spin entangled pure state based on charge detection

Liu Jiong, Zhou Lan, Sheng Yu-Bo
Chin. Phys. B, 2015, 24 (7): 070309 doi: 10.1088/1674-1056/24/7/070309
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We propose a protocol for directly measuring the concurrence of a two-qubit electronic pure entangled state. To complete this task, we first design a parity-check measurement (PCM) which is constructed by two polarization beam splitters (PBSs) and a charge detector. By using the PCM for three rounds, we can achieve the concurrence by calculating the total probability of picking up the odd parity states from the initial states. Since the conduction electron may be a good candidate for the realization of quantum computation, this protocol may be useful in future solid quantum computation.

Quantum state transfer between atomic ensembles trapped in separate cavities via adiabatic passage

Zhang Chun-Ling, Chen Mei-Feng
Chin. Phys. B, 2015, 24 (7): 070310 doi: 10.1088/1674-1056/24/7/070310
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We propose a new approach for quantum state transfer (QST) between atomic ensembles separately trapped in two distant cavities connected by an optical fiber via adiabatic passage. The three-level Λ-type atoms in each ensemble dispersively interact with the nonresonant classical field and cavity mode. By choosing appropriate parameters of the system, the effective Hamiltonian describes two atomic ensembles interacting with “the same cavity mode” and has a dark state. Consequently, the QST between atomic ensembles can be implemented via adiabatic passage. Numerical calculations show that the scheme is robust against moderate fluctuations of the experimental parameters. In addition, the effect of decoherence can be suppressed effectively. The idea provides a scalable way to an atomic-ensemble-based quantum network, which may be reachable with currently available technology.

Nonlinear tunneling through a strong rectangular barrier

Zhang Xiu-Rong, Li Wei-Dong
Chin. Phys. B, 2015, 24 (7): 070311 doi: 10.1088/1674-1056/24/7/070311
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Nonlinear tunneling is investigated by analytically solving the one-dimensional Gross–Pitaevskii equation (GPE) with a strong rectangular potential barrier. With the help of analytical solutions of the GPE, which can be reduced to the solution of the linear case, we find that only the supersonic solution in the downstream has a linear counterpart. A critical nonlinearity is explored as an up limit, above which no nonlinear tunneling solution exists. Furthermore, the density solution of the critical nonlinearity as a function of the position has a step-like structure.

Hawking radiation of stationary and non-stationary Kerr–de Sitter black holes

T. Ibungochouba Singh
Chin. Phys. B, 2015, 24 (7): 070401 doi: 10.1088/1674-1056/24/7/070401
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Hawking radiation of the stationary Kerr–de Sitter black hole is investigated using the relativistic Hamilton–Jacobi method. Meanwhile, extending this work to a non-stationary black hole using Dirac equations and generalized tortoise coordinate transformation, we derived the locations, the temperature of the thermal radiation as well as the maximum energy of the non-thermal radiation. It is found that the surface gravity and the Hawking temperature depend on both time and different angles. An extra coupling effect is obtained in the thermal radiation spectrum of Dirac particles which is absent from thermal radiation of scalar particles. Further, the chemical potential derived from the thermal radiation spectrum of scalar particle has been found to be equal to the highest energy of the negative energy state of the scalar particle in the non-thermal radiation for the Kerr–de Sitter black hole. It is also shown that for stationary black hole space time, these two different methods give the same Hawking radiation temperature.

Rotational stretched exponential relaxation in random trap-barrier model

Ekrem Aydıner
Chin. Phys. B, 2015, 24 (7): 070501 doi: 10.1088/1674-1056/24/7/070501
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The relaxation behavior of complex-disordered systems, such as spin glasses, polymers, colloidal suspensions, structural glasses,and granular media, has not been clarified. Theoretical studies show that relaxation in these systems has a topological origin. In this paper, we focus on the rotational stretched exponential relaxation behavior in complex-disordered systems and introduce a simple phase space model to understand the mechanism of the non-exponential relaxation of these systems. By employing the Monte Carlo simulation method to the model, we obtain the rotational relaxation function as a function of temperature. We show that the relaxation function has a stretched exponential form under the critical temperature while it obeys the Debye law above the critical temperature.

Multifractal analysis of white matter structural changes on 3D magnetic resonance imaging between normal aging and early Alzheimer's disease

Ni Huang-Jing, Zhou Lu-Ping, Zeng Peng, Huang Xiao-Lin, Liu Hong-Xing, Ning Xin-Bao, the Alzheimer's Disease Neuroimaging Initiative
Chin. Phys. B, 2015, 24 (7): 070502 doi: 10.1088/1674-1056/24/7/070502
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Applications of multifractal analysis to white matter structure changes on magnetic resonance imaging (MRI) have recently received increasing attentions. Although some progresses have been made, there is no evident study on applying multifractal analysis to evaluate the white matter structural changes on MRI for Alzheimer's disease (AD) research. In this paper, to explore multifractal analysis of white matter structural changes on 3D MRI volumes between normal aging and early AD, we not only extend the traditional box-counting multifractal analysis (BCMA) into the 3D case, but also propose a modified integer ratio based BCMA (IRBCMA) algorithm to compensate for the rigid division rule in BCMA. We verify multifractal characteristics in 3D white matter MRI volumes. In addition to the previously well studied multifractal feature, Δα, we also demonstrated Δf as an alternative and effective multifractal feature to distinguish NC from AD subjects. Both Δα and Δf are found to have strong positive correlation with the clinical MMSE scores with statistical significance. Moreover, the proposed IRBCMA can be an alternative and more accurate algorithm for 3D volume analysis. Our findings highlight the potential usefulness of multifractal analysis, which may contribute to clarify some aspects of the etiology of AD through detection of structural changes in white matter.

Compression and stretching of ring-vortex solitons in a bulk nonlinear medium

Lai Xian-Jing, Cai Xiao-Ou, Zhang Jie-Fang
Chin. Phys. B, 2015, 24 (7): 070503 doi: 10.1088/1674-1056/24/7/070503
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Compression and stretching of ring-vortex solitons, which is a novel self-similar solution of (2+1)-dimensional diffraction decreasing waveguide, is investigated analytically and numerically. We obtain the ring-vortex solitons via the similarity transformation method. The distance modulation for the width, the diffraction, and the nonlinear response, strongly affects the form and the behavior of the self-similar vortex, and facilitates the efficient compression of optical waves. This approximate ring-vortex solitons can reflect the real properties of self-similar optical vortex beams during propagation under certain parameter window selection. Specific examples and figures are given to illustrate discussed features. The results obtained in this paper may have potential values for all-optical data-processing schemes and the design of beam compressors and amplifiers.

Effects of evacuation assistant's leading behavior on the evacuation efficiency: Information transmission approach

Wang Xiao-Lu, Guo Wei, Zheng Xiao-Ping
Chin. Phys. B, 2015, 24 (7): 070504 doi: 10.1088/1674-1056/24/7/070504
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Evacuation assistants are expected to spread the escape route information and lead evacuees toward the exit as quickly as possible. Their leading behavior influences the evacuees' movement directly, which is confirmed to be a decisive factor of the evacuation efficiency. The transmission process of escape information and its function on the evacuees' movement are accurately presented by the proposed extended dynamic communication field model. For evacuation assistants and evacuees, their sensitivity parameter of static floor field (SFF), kSL, and kSe, are fully discussed. The simulation results indicate that the appropriate kSL is associated with the maximum kSe of evacuees. The optimal combinations of kSL and kSe were found to reach the highest evacuation efficiency. There also exists an optimal value for evacuation assistants' information transmission radius.

Synthesis mechanism of heterovalent Sn2O3 nanosheets in oxidation annealing process

Zhao Jun-Hua, Tan Rui-Qin, Yang Ye, Xu Wei, Li Jia, Shen Wen-Feng, Wu Guo-Qiang, Yang Xu-Feng, Song Wei-Jie
Chin. Phys. B, 2015, 24 (7): 070505 doi: 10.1088/1674-1056/24/7/070505
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Heterovalent Sn2O3 nanosheets were fabricated via an oxidation annealing process and the formation mechanism was investigated. The temperature required to complete the phase transformation from Sn3O4 to Sn2O3 was considered. Two contrasting experiments showed that both oxygen and heating were not necessary conditions for the phase transition. Sn2O3 was formed under an argon protective atmosphere by annealing and could also be obtained at room temperature by exposing Sn3O4 in atmosphere or dispersing in ethanol. The synthesis mechanism was proposed and discussed. This fundamental research is important for the technological applications of intermediate tin oxide materials.

Output power analyses of an endoreversible Carnot heat engine with irreversible heat transfer processes based on generalized heat transfer law

Wu Yan-Qiu
Chin. Phys. B, 2015, 24 (7): 070506 doi: 10.1088/1674-1056/24/7/070506
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In this paper, an endoreversible Carnot heat engine with irreversible heat transfer processes is analyzed based on generalized heat transfer law. The applicability of the entropy generation minimization, exergy analyses method, and entransy theory to the analyses is discussed. Three numerical cases are presented. It is shown that the results obtained from the entransy theory are different from those from the entropy generation minimization, which is equivalent to the exergy analyses method. For the first case in which the application preconditions of the entropy generation minimization and entransy loss maximization are satisfied, both smaller entropy generation rate and larger entransy loss rate lead to larger output power. For the second and third cases in which the preconditions are not satisfied, the entropy generation minimization does not lead to the maximum output power, while larger entransy loss rate still leads to larger output power in the third case. For the discussed cases, the concept of entransy dissipation is not applicable for the analyses of output power. The problems in the negative comments on the entransy theory are pointed out and discussed. The related researchers are advised to focus on some new specific application cases to show if the entransy theory is the same as some other theories.

Recent improvements on the atomic fountain clock at SIOM

Du Yuan-Bo, Wei Rong, Dong Ri-Chang, Zou Fan, Wang Yu-Zhu
Chin. Phys. B, 2015, 24 (7): 070601 doi: 10.1088/1674-1056/24/7/070601
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We report the recent advance in our rubidium atomic fountain clock (AFC). The parameters of the Ramsey cavity are optimized by balancing the coupling from the two ports. The temperature control system of the Ramsey interaction region is renovated, and the resonant temperature of the Ramsey cavity is regulated to be slightly above the room temperature. The quality of magnetic environment in the Ramsey interaction region is also improved. A new digital-to-analog converter (DAC) circuit that controls the local oscillator is adopted to decrease the noise of the oven-controlled crystal oscillator output. As a result, the short-term fractional frequency stability of 2.7 ×10-13τ-1/2 and the long-term fractional frequency stability of 1.6 ×10-15 at the average time of 32800 s are achieved.

Adsorption behavior of Fe atoms on a naphthalocyanine monolayer on Ag(111) surface

Yan Ling-Hao, Wu Rong-Ting, Bao De-Liang, Ren Jun-Hai, Zhang Yan-Fang, Zhang Hai-Gang, Huang Li, Wang Ye-Liang, Du Shi-Xuan, Huan Qing, Gao Hong-Jun
Chin. Phys. B, 2015, 24 (7): 076802 doi: 10.1088/1674-1056/24/7/076802
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Adsorption behavior of Fe atoms on a metal-free naphthalocyanine (H2Nc) monolayer on Ag(111) surface at room temperature has been investigated using scanning tunneling microscopy combined with density functional theory (DFT) based calculations. We found that the Fe atoms were adsorbed on the centers of H2Nc molecules and formed Fe–H2Nc complexes at low coverage. DFT calculations show that Fe sited in the center of the molecule is the most stable configuration, in good agreement with the experimental observations. After an Fe–H2Nc complex monolayer was formed, the extra Fe atoms self-assembled to Fe clusters of uniform size and adsorbed dispersively at the interstitial positions of Fe–H2Nc complex monolayer. Therefore, the H2Nc monolayer grown on Ag(111) could be a good template to grow dispersed magnetic metal atoms and clusters at room temperature for further investigation of their magnetism-related properties.

Observation of mode-like features in tunneling spectra of iron-based superconductors Hot!

Gong Jing, Hou Xing-Yuan, Zhu Jun, Jie Yun-Yin, Gu Ya-Dong, Shen Bing, Ren Cong, Li Chun-Hong, Shan Lei
Chin. Phys. B, 2015, 24 (7): 077402 doi: 10.1088/1674-1056/24/7/077402
Full Text: [PDF 495 KB] (Downloads:359) RICH HTML
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We report scanning tunneling microscopy/spectroscopy (STM/STS) studies on iron-based superconductors of Ba1-xKxFe2As2 and nearly optimally doped Fe(Te,Se). Mode-like features were observed universally outside the superconducting gaps in the tunneling spectra, which are similar to our previous observations in other samples and can be ascribed to the interaction between electrons and spin excitations. Furthermore, an almost linear relationship between the superconducting gaps and the superconducting transition temperatures was noted and should also be taken into account in understanding the mechanism of iron-based superconductors.

Temperature dependence of multi-jump magnetic switching process in epitaxial Fe/MgO (001) films

Hu Bo, He Wei, Ye Jun, Tang Jin, Zhang Yong-Sheng, Syed Sheraz Ahmad, Zhang Xiang-Qun, Cheng Zhao-Hua
Chin. Phys. B, 2015, 24 (7): 077502 doi: 10.1088/1674-1056/24/7/077502
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Temperature dependence of magnetic switching processes with multiple jumps in Fe/MgO (001) films is investigated by magnetoresistance measurements. When the temperature decreases from 300 K to 80 K, the measured three-jump hysteresis loops turn into two-jump loops. The temperature dependence of the fourfold in-plane magnetic anisotropy constant K1, domain wall pinning energy, and an additional uniaxial magnetic anisotropy constant KU are responsible for this transformation. The strengths of K1 and domain wall pinning energy increase with decreasing temperature, but KU remains unchanged. Moreover, magnetization reversal mechanisms, with either two successive or two separate 90° domain wall propagation, are introduced to explain the multi-jump magnetic switching process in epitaxial Fe/MgO(001) films at different temperatures.

Characterizing silicon intercalated graphene grown epitaxially on Ir films by atomic force microscopy

Zhang Yong, Wang Ye-Liang, Que Yan-De, Gao Hong-Jun
Chin. Phys. B, 2015, 24 (7): 078104 doi: 10.1088/1674-1056/24/7/078104
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An efficient method based on atomic force microscopy (AFM) has been developed to characterize silicon intercalated graphene grown on single crystalline Ir(111) thin films. By combining analyses of the phase image, force curves, and friction–force mapping, acquired by AFM, the locations and coverages of graphene and silicon oxide can be well distinguished. We can also demonstrate that silicon atoms have been successfully intercalated between graphene and the substrate. Our method gives an efficient and simple way to characterize graphene samples with interacted atoms and is very helpful for future applications of graphene-based devices in the modern microelectronic industry, where AFM is already widely used.

Improved performance of microcrystalline silicon solar cell with graded-band-gap silicon oxide buffer layer

Shi Zhen-Liang, Ji Yun, Yu Wei, Yang Yan-Bin, Cong Ri-Dong, Chen Ying-Juan, Li Xiao-Wei, Fu Guang-Sheng
Chin. Phys. B, 2015, 24 (7): 078105 doi: 10.1088/1674-1056/24/7/078105
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Microcrystalline silicon (μc-Si:H) solar cell with graded band gap microcrystalline silicon oxide (μc-SiOx:H) buffer layer is prepared by plasma enhanced chemical vapor deposition and exhibits improved performance compared with the cell without it. The buffer layer moderates the band gap mismatch by reducing the barrier of the p/i interface, which promotes the nucleation of the i-layer and effectively eliminates the incubation layer, and then enhances the collection efficiency of the cell in the short wavelength region of the spectrum. The p/i interface defect density also decreases from 2.2× 1012 cm-2 to 5.0× 1011 cm-2. This graded buffer layer allows to simplify the deposition process for the μc-Si:H solar cell application.

Low frequency noise in asymmetric double barrier magnetic tunnel junctions with a top thin MgO layer

Guo Hui-Qiang, Tang Wei-Yue, Liu Liang, Wei Jian, Li Da-Lai, Feng Jia-Feng, Han Xiu-Feng
Chin. Phys. B, 2015, 24 (7): 078504 doi: 10.1088/1674-1056/24/7/078504
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Low frequency noise has been investigated at room temperature for asymmetric double barrier magnetic tunnel junctions (DBMTJs), where the coupling between the top and middle CoFeB layers is antiferromagnetic with a 0.8-nm thin top MgO barrier of the CoFeB/MgO/CoFe/CoFeB/MgO/CoFeB DBMTJ. At enough large bias, 1/f noise dominates the voltage noise power spectra in the low frequency region, and is conventionally characterized by the Hooge parameter αmag. With increasing external field, the top and bottom ferromagnetic layers are aligned by the field, and then the middle free layer rotates from antiparallel state (antiferromagnetic coupling between top and middle ferromagnetic layers) to parallel state. In this rotation process αmag and magnetoresistance-sensitivity-product show a linear dependence, consistent with the fluctuation dissipation relation. With the magnetic field applied at different angles (θ) to the easy axis of the free layer, the linear dependence persists while the intercept of the linear fit satisfies a cos(θ) dependence, similar to that for the magnetoresistance, suggesting intrinsic relation between magnetic losses and magnetoresistance.

Optimal migration path of Ag in HfO2: A first-principles study

Dai Yue-Hua, Chen Zhen, Jin Bo, Li Ning, Li Xiao-Feng
Chin. Phys. B, 2015, 24 (7): 073101 doi: 10.1088/1674-1056/24/7/073101
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First-principles calculations are used to investigate the migration path of Ag in the HfO2-based resistive random access memory (ReRAM). The formation energy calculation suggests that there are two different sites (site 1 and site 3) for the incorporation of Ag atoms into the HfO2 unit cell. Thermodynamic analysis shows that the motion of Ag atom in the HfO2 supercell appears to be anisotropic, which is due to the fact that the Ag atom at site 3 moves along the [111] orientation, but the Ag atom at site 1 moves along the [001] orientation. The migration barriers of the Ag atoms hopping between neighboring unit cells are calculated along five different orientations. Difficulty in producing motion of the Ag atom varies with the migration barrier: this motion is minimized along [111] orientation. Furthermore, The optimal circulation path for Ag migration within the HfO2 supercells is obtained, and is found to be approximately along the [111] orientation. Therefore, it is proposed that the positive voltage should be applied along this orientation, the conduction filament may form more easily, which could improve the response time and reduce the power consumption in ReRAM applications.

Dependence of above-threshold ionization spectrum on polarization directions of two-color laser fields

Liu Min, Guo Ying-Chun, Wang Bing-Bing
Chin. Phys. B, 2015, 24 (7): 073201 doi: 10.1088/1674-1056/24/7/073201
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Using the frequency-domain theory, we investigate the above-threshold ionization (ATI) process of an atom in two-color laser fields. When both photon energies of the two-color laser fields are much smaller than the atomic ionization threshold, the ATI spectrum depends on the angle between the two lasers' polarization directions. While when the photon energy of one laser is comparable with or larger than the atomic ionization threshold, the ATI spectrum is independent of the angle, and only several dips appear at certain angles. By analyzing the contributions of different quantum channels, we find that, for the case that both frequencies of the two color lasers are low, the quantum interferences between the channels are strong, and hence the spectrum changes with the angle between the two lasers' polarization directions. While for the case that the frequency of one of the two color lasers is high, the contributions of the channels to the ATI spectrum decrease dramatically with increasing channel order, hence the interferences between the channels disappear, and the ATI spectrum has a step-like structure, which is independent of the angle between the two lasers' polarizations. These results can shed light on the study of the corresponding relation between classical and quantum mechanisms of the matter–laser interaction in high-frequency laser fields.

Quantum path control and isolated attosecond pulse generation in the combination of near-infrared and terahertz pulses

Zhong Hui-Ying, Guo Jing, Zhang Hong-Dan, Du Hui, Liu Xue-Shen
Chin. Phys. B, 2015, 24 (7): 073202 doi: 10.1088/1674-1056/24/7/073202
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We present an efficient and realizable scheme for the generation of an ultrashort single attosecond (as) pulse from H atom with a 800-nm fundamental laser field combined with a terahertz (THz) field. The high-order harmonic generation (HHG) can be obtained by solving the time-dependent Schrödinger equation accurately and efficiently with time-dependent generalized pseudo-spectral (TDGPS) method. The result shows that the plateau of high-order harmonics is extended and the broadband spectra can be produced by the combined laser pulse, which can be explained by the corresponding ionization probability. The time–frequency analysis and semi-classical three-step model are also presented to further investigate this mechanism. Besides, by the superposition of the harmonics near the cutoff region, an isolated 133-as pulse can be obtained.

Assignment of terahertz vibrational modes of L-glutamine using density functional theory within generalized-gradient approximation

Zhang Han, Zhang Zhao-Hui, Zhao Xiao-Yan, Zhang Tian-Yao, Yan Fang, Shen Jiang
Chin. Phys. B, 2015, 24 (7): 073301 doi: 10.1088/1674-1056/24/7/073301
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The crystal structure of L-glutamine is stabilized by a three-dimensional network of intermolecular hydrogen bonds. We utilize plane-wave density functional theory lattice-dynamics calculations within the generalized-gradient approximation (GGA), Perdew–Burke–Ernzerhof (PBE), PBE for solids (PBEsol), PBE with Wu–Cohen exchange (WC), and dispersion-corrected PBE, to investigate the effect of these intermolecular contacts on the absorption spectra of glutamine in the terahertz frequency range. Among these calculations, the solid-state simulated results obtained using the WC method exhibit a good agreement with the measured absorption spectra, and the absorption features are assigned with the help of WC. This indicates that the vibrational modes of glutamine were related to the combination of intramolecular and intermolecular motions, the intramolecular modes were dominated by rocking or torsion involving functional groups; the intermolecular modes mainly result from the translational motions of individual molecules, and the rocking of the hydrogen-bonded functional groups.

Influence of electron correlations on double-capture process in proton helium collisions

Hoda Ghavaminia, Ebrahim Ghanbari-Adivi
Chin. Phys. B, 2015, 24 (7): 073401 doi: 10.1088/1674-1056/24/7/073401
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The first-order correct-boundary Coulomb–Born distorted-wave approximation is used to study the double-electron capture by protons from the ground-state helium atoms at intermediate and high impact energies. The differential double capture cross sections are obtained as a function of the projectile scattering angle and the total cross sections as a function of the impact energy. In the considered range of impact energy, our calculation shows that although the results are not so sensitive to the static inter-electronic correlations in the initial channel, the strong final-state correlations have a large effect on the magnitudes of the double capture cross sections. The calculated differential and integral cross sections are compared with their available experimental values. The comparison shows a good agreement between the present calculations and the measurements. The comparison of the integral cross sections shows that the present approach is compatible with other theories.

Theoretical study on photorecombination of C V ion

Ma Kun, Xie Lu-You, Zhang Deng-Hong, Dong Chen-Zhong
Chin. Phys. B, 2015, 24 (7): 073402 doi: 10.1088/1674-1056/24/7/073402
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The partial and total photorecombination cross sections of the ground-state C V ion in the KLL and KLM resonant energy regions were calculated in detail by using the Dirac atomic R-matrix code based on a fully relativistic R-matrix method. Meanwhile, the principal resonant lines in each photorecombination channel have been classified according to the calculated transition energies and probabilities from the KLL and KLM resonant states to the 1s2nl (n=2, 3 and l=s, p, d) final states. The validity of these calculations is assessed by comparison with previously published experimental and theoretical data. The good agreement between the present calculated results and those obtained using different approaches confirms the accuracy of the present results. In addition, it is found that the damping effect can be neglected for the KLL resonant, but not for the KLM resonant.

Giant transmission Goos–Hänchen shift in surface plasmon polaritons excitation and its physical origin

Yang Yang, Liu Ju, Li Zhi-Yuan
Chin. Phys. B, 2015, 24 (7): 074201 doi: 10.1088/1674-1056/24/7/074201
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Excitation of surface plasmon polaritons (SPPs) propagating at the interface between a dielectric medium and a silver thin film by a focused Gaussian beam in a classical Kretschmann prism setup is studied theoretically. We find that the center of the transmitted Gaussian evanescent wave has a giant lateral shift relative to the incident Gaussian beam center for a wide range of incident angle and Gaussian beam wavelength to excite SPPs, which can be more than two orders of magnitude larger than the silver film thickness. The phenomenon is closely related with the conventional Goos–Hänchen effect for total internal reflection of light beam, and it is called the transmission Goos–Hänchen shift. We find that this lateral shift depends heavily on the excitation wavelength, incident angle, and the silver layer thickness. Finite-difference time-domain simulations show that this transmission Goos–Hänchen shift is induced by a unique dynamical process of excitation, transport, and leakage of SPPs.

Coherence transfer from 1064 nm to 578 nm using an optically referenced frequency comb Hot!

Fang Su, Jiang Yan-Yi, Chen Hai-Qin, Yao Yuan, Bi Zhi-Yi, Ma Long-Sheng
Chin. Phys. B, 2015, 24 (7): 074202 doi: 10.1088/1674-1056/24/7/074202
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A laser at 578 nm is phase-locked to an optical frequency comb (OFC) which is optically referenced to a subhertz-linewidth laser at 1064 nm. Coherence is transferred from 1064 nm to 578 nm via the OFC. By comparing with a cavity-stabilized laser at 578 nm, the absolute linewidth of 1.1 Hz and the fractional frequency instability of 1.3× 10-15 at an averaging time of 1 s for each laser at 578 nm have been determined, which is limited by the performance of the reference laser for the OFC.

Relationship between electromagnetically-induced transparency and Autler–Townes splitting in a Doppler-broadened system

Pei Li-Ya, Niu Jin-Yan, Wang Ru-Quan, Qu Yi-Zhi, Zuo Zhan-Chun, Wu Ling-An, Fu Pan-Ming
Chin. Phys. B, 2015, 24 (7): 074203 doi: 10.1088/1674-1056/24/7/074203
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We study the relationship between electromagnetically-induced transparency (EIT) and Autler–Townes (AT) splitting in a cascade three-level Doppler-broadened system. By comparing the absorption spectrum with the fluorescence excitation spectrum, it is found that for a Doppler-broadened system, EIT resonance cannot be explained as the result of quantum interference, unlike the case of a homogeneously broadened system. Instead, the macroscopic polarization interference plays an important role in determining the spectra of EIT and AT splitting, which can be explained within the same framework when being detected by the absorption spectra.

Image information transfer via electromagnetically induced transparency-based slow light

Wang Xiao-Xiao, Sun Jia-Xiang, Sun Yuan-Hang, Li Ai-Jun, Chen Yi, Zhang Xiao-Jun, Kang Zhi-Hui, Wang Lei, Wang Hai-Hua, Gao Jin-Yue
Chin. Phys. B, 2015, 24 (7): 074204 doi: 10.1088/1674-1056/24/7/074204
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In this work, we experimentally demonstrate an image information transfer between two channels by using slow light based on electromagnetically induced transparency (EIT) in a solid. The probe optical image is slowed due to steep dispersion induced by EIT. By applying an additional control field to an EIT-driven medium, the slowed image is transferred into two information channels. Image intensities between two information channels can be controlled by adjusting the intensities of the control fields. The similarity of output images is further analyzed. This image information transfer allows for manipulating images in a controlled fashion, and will be important in further information processing.

Multi-level effects in the high-order harmonic generation driven by intense frequency-comb laser fields

Zhao Di, Jiang Chen-Wei, Li Fu-Li
Chin. Phys. B, 2015, 24 (7): 074205 doi: 10.1088/1674-1056/24/7/074205
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High harmonic generation (HHG) driven by intense frequency-comb laser fields can be dramatically enhanced via multiphoton resonance by tuning the carrier-envelope phase (CEP) shift, without increasing the driving intensity. However, the multiphoton-resonant enhancement (MRE) factor in the realistic atomic hydrogen is much smaller than that in a two-level system. To study the deviation, we present a theoretical investigation of the multiphoton resonance dynamics of three-level systems driven by intense frequency-comb laser fields. The many-mode Floquet theorem (MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between the quantum system and the laser fields. The investigations show that the dipole interaction of a two-level system with the third level affects the multiphoton resonance dynamics and enhances the HHG spectra. It is the dipole interaction of the excited level of the two-level system with other levels that results in the smaller MRE factor in the realistic atomic system.

Absorption enhancement and sensing properties of Ag diamond nanoantenna arrays

Yuan Yu-Yang, Yuan Zong-Heng, Li Xiao-Nan, Wu Jun, Zhang Wen-Tao, Ye Song
Chin. Phys. B, 2015, 24 (7): 074206 doi: 10.1088/1674-1056/24/7/074206
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Noble metal nanoantenna could effectively enhance light absorption and increase detection sensitivity. In this paper, we propose a periodic Ag diamond nanoantenna array to increase the absorption of thin-film solar cells and to improve the detection sensitivity via localized surface plasmon resonance. The effect of nanoantenna arrays on the absorption enhancement is theoretically investigated using the finite difference time domain (FDTD) method with manipulating the spectral response by geometrical parameters of nanoantennas. A maximum absorption enhancement factor of 1.51 has been achieved in this study. In addition, the relation between resonant wavelength (intensity reflectivity) and refractive index is discussed in detail. When detecting the environmental index using resonant wavelengths, a maximum detection sensitivity of about 837 nm/RIU (refractive index unit) and a resolution of about 10-3 RIU can be achieved. Moreover, when using the reflectivity, the sensitivity can be as high as 0.93 AU/RIU. Furthermore, we also have theoretically studied the effectiveness of nanoantennas in distinguishing chemical reagents, solution concentrations, and solution allocation ratios by detecting refractive index. From the results presented in this paper, we conclude that this work might be useful for biosensor detection and other types of detections.

Influences of cavity dispersion distribution on the output pulse properties of an all-normal-dispersion fiber laser

Li Pan, Shi Lei, Sun Qing, Feng Su-Juan, Mao Qing-He
Chin. Phys. B, 2015, 24 (7): 074207 doi: 10.1088/1674-1056/24/7/074207
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In this paper, the influences of the dispersion distribution in the cavity on the output pulse properties of the all-normal-dispersion (ANDi) fiber laser are investigated. Our simulations show that, as the relative length of the dispersion fiber increases, the temporal width and the spectral bandwidth of the output pulse for an ANDi fiber laser with fixed total cavity dispersion or fiber length are decreased, while the pulse energy is enhanced and the compressed pulse width is increased. These simulation predictions have been proved by our experimental results. The reason may be that the nonlinear phase shift accumulated in the nonlinear fiber is more than that in the dispersion fiber if they have the same length.

An accurate and stable method of array element tiling for high-power laser facilities

Mu Jie, Wang Xiao, Jing Feng, Li Zhi-Lin, Cheng Ning-Bo, Zhu Qi-Hua, Su Jing-Qin, Zhang Jun-Wei, Zhou Kai-Nan, Zeng Xiao-Ming
Chin. Phys. B, 2015, 24 (7): 074208 doi: 10.1088/1674-1056/24/7/074208
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Due to laser-induced damage, the aperture of optics is one of the main factors limiting the output capability of high-power laser facilities. Because of the general difficulty in achieving large-aperture optics, an alternative solution is to tile some small-aperture ones together. We propose an accurate, stable, and automatic method of array element tiling and verify it on a double-pass 1 × 2 tiled-grating compressor in the XG-III laser facility. The test results show the accuracy and stability of the method. This research provides an efficient way to obtain large-aperture optics for high-power laser facilities.

Grazing bifurcation analysis of a relative rotation system with backlash non-smooth characteristic

Liu Shuang, Wang Zhao-Long, Zhao Shuang-Shuang, Li Hai-Bin, Li Jian-Xiong
Chin. Phys. B, 2015, 24 (7): 074501 doi: 10.1088/1674-1056/24/7/074501
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Grazing bifurcation of a relative rotation system with backlash non-smooth characteristic is studied along with the change of the external excitation in this paper. Considering the oil film, backlash, time-varying stiffness and time-varying error, the dynamical equation of a relative rotation system with a backlash non-smooth characteristic is deduced by applying the elastic hydrodynamic lubrication (EHL) and the Grubin theories. In the process of relative rotation, the occurrence of backlash will lead to the change of dynamic behaviors of the system, and the system will transform from the meshing state to the impact state. Thus, the zero-time discontinuous mapping (ZDM) and the Poincare mapping are deduced to analyze the local dynamic characteristics of the system before as well as after the moment that the backlash appears (i.e., the grazing state). Meanwhile, the grazing bifurcation mechanism is analyzed theoretically by applying the impact and Floquet theories. Numerical simulations are also given, which confirm the analytical results.

Stabilizing effect of plasma discharge on bubbling fluidized granular bed

Hu Mao-Bin, Dang Sai-Chao, Ma Qiang, Xia Wei-Dong
Chin. Phys. B, 2015, 24 (7): 074502 doi: 10.1088/1674-1056/24/7/074502
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Fluidized beds have been widely used for processing granular materials. In this paper, we study the effect of plasma on the fluidization behavior of a bubbling fluidized bed with an atmospheric pressure plasma discharger. Experiment results show that the bubbling fluidized bed is stabilized with the discharge of plasma. When the discharge current reaches a minimum stabilization current Cms, air bubbles in the bed will disappear and the surface fluctuation is completely suppressed. A simplified model is proposed to consider the effect of electric Coulomb force generated by the plasma. It is found that the Coulomb force will propel the particles to move towards the void area, so that the bubbling fluidized bed is stabilized with a high enough plasma discharge.

A new mixed subgrid-scale model for large eddy simulation of turbulent drag-reducing flows of viscoelastic fluids

Li Feng-Chen, Wang Lu, Cai Wei-Hua
Chin. Phys. B, 2015, 24 (7): 074701 doi: 10.1088/1674-1056/24/7/074701
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A mixed subgrid-scale (SGS) model based on coherent structures and temporal approximate deconvolution (MCT) is proposed for turbulent drag-reducing flows of viscoelastic fluids. The main idea of the MCT SGS model is to perform spatial filtering for the momentum equation and temporal filtering for the conformation tensor transport equation of turbulent flow of viscoelastic fluid, respectively. The MCT model is suitable for large eddy simulation (LES) of turbulent drag-reducing flows of viscoelastic fluids in engineering applications since the model parameters can be easily obtained. The LES of forced homogeneous isotropic turbulence (FHIT) with polymer additives and turbulent channel flow with surfactant additives based on MCT SGS model shows excellent agreements with direct numerical simulation (DNS) results. Compared with the LES results using the temporal approximate deconvolution model (TADM) for FHIT with polymer additives, this mixed SGS model MCT behaves better, regarding the enhancement of calculating parameters such as the Reynolds number. For scientific and engineering research, turbulent flows at high Reynolds numbers are expected, so the MCT model can be a more suitable model for the LES of turbulent drag-reducing flows of viscoelastic fluid with polymer or surfactant additives.

Nano watermill driven by revolving charge

Zhou Xiao-Yan, Kou Jian-Long, Nie Xue-Chuan, Wu Feng-Min, Liu Yang, Lu Hang-Jun
Chin. Phys. B, 2015, 24 (7): 074702 doi: 10.1088/1674-1056/24/7/074702
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A novel nanoscale watermill for the unidirectional transport of water molecules through a curved single-walled carbon nanotube (SWNT) is proposed and explored by molecular dynamics simulations. In this nanoscale system, a revolving charge is introduced to drive a water chain confined inside the SWNT, the charge and the tube together serving as a nano waterwheel and nano engine. A resonance-like phenomenon is found, and the revolving frequency of the charge plays a key role in pumping the water chain. The water flux across the SWNT increases with respect to the revolving frequency of the external charge and it reaches its maximum when the frequency is 4 THz. Correspondingly, the number of hydrogen bonds in the water chain inside the SWNT decreases dramatically as the frequency increases from 4 THz to 25 THz. The mechanism behind the resonance phenomenon has been investigated systematically. Our findings are helpful for the design of nanoscale fluidic devices and energy converters.


Dynamics of laser beams in inhomogeneous electron—positron—ion plasmas

Cheng Li-Hong, Tang Rong-An, Du Hong-E, Xue Ju-Kui
Chin. Phys. B, 2015, 24 (7): 075201 doi: 10.1088/1674-1056/24/7/075201
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Nonlinear interaction of laser and electron–positron–ion plasmas is investigated by invoking the variational principle and numerical simulation, in terms of a nonlinear Schrödinger equation with inhomogeneities effect. It is shown that the plasma inhomogeneity has great influence on the laser beam dynamics. The laser beam can be self-trapped, focused, or defocused depending on the inhomogeneity character. The linearly decreasing axial plasma density makes the laser beam defocus, while the linearly increasing axial plasma density results in self-trapping of the beam. The self-focusing of the trapped beam is found in a high-density region. For the Gaussian types of density distribution, the beam field submits nonlinearly oscillating regime. The results provide an efficient way to manipulate the dynamics of laser beam propagating in plasma.

Effects of N2/O2 flow rate on the surface properties and biocompatibility of nano-structured TiOxNy thin films prepared by high vacuum magnetron sputtering

Sehrish Saleem, R. Ahmad, Uzma Ikhlaq, R. Ayub, Jin Wei Hong, Xu Rui Zhen, Li Peng Hui, Khizra Abbas, Paul K. Chu
Chin. Phys. B, 2015, 24 (7): 075202 doi: 10.1088/1674-1056/24/7/075202
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NiTi shape memory alloys (SMA) have many biomedical applications due to their excellent mechanical and biocompatible properties. However, nickel in the alloy may cause allergic and toxic reactions, which limit some applications. In this work, titanium oxynitride films were deposited on NiTi samples by high vacuum magnetron sputtering for various nitrogen and oxygen gas flow rates. The x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) results reveal the presence of different phases in the titanium oxynitride thin films. Energy dispersive spectroscopy (EDS) elemental mapping of samples after immersion in simulated body fluids (SBF) shows that Ni is depleted from the surface and cell cultures corroborate the enhanced biocompatibility in vitro.

Continuous operation of 2.45-GHz microwave proton source for 306 hours with more than 50 mA DC beam

Peng Shi-Xiang, Zhang Ai-Lin, Ren Hai-Tao, Zhang Tao, Xu Yuan, Zhang Jing-Feng, Gong Jian-Hua, Guo Zhi-Yu, Chen Jia-Er
Chin. Phys. B, 2015, 24 (7): 075203 doi: 10.1088/1674-1056/24/7/075203
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This paper describes a long-term operation of the 2.45-GHz microwave proton source at Peking University. The DC proton beam of 50–55 mA with energy of 35 keV has been run for 306 hours continuously. Total beam availability, defined as 35-keV beam-on time divided by elapsed time, is higher than 99%. Water cooling machine failures cause all the downtime, and no plasma generator failure or high voltage breakdown is observed. The longest uninterrupted run time is 122 hours.

Modeling of the nanoparticle coagulation in pulsed radio-frequency capacitively coupled C2H2 discharges

Liu Xiang-Mei, Li Qi-Nan, Li Rui
Chin. Phys. B, 2015, 24 (7): 075204 doi: 10.1088/1674-1056/24/7/075204
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The role of pulse parameters on nanoparticle property is investigated self-consistently based on a couple of fluid model and aerosol dynamics model in a capacitively coupled parallel-plate acetylene (C2H2) discharge. In this model, the mass continuity equation, momentum balance equation, and energy balance equation for neutral gas are taken into account. Thus, the thermophoretic force arises when a gas temperature gradient exists. The typical results of this model are positive and negative ion densities, electron impact collisions rates, nanoparticle density, and charge distributions. The simulation is performed for duty ratio 0.4/0.7/1.0, as well as pulse modulation frequency from 40 kHz to 2.7 MHz for pure C2H2 discharges at a pressure of 500 mTorr. We find that the pulse parameters, especially the duty ratio, have a great affect on the dissociative attachment coefficient and the negative density. More importantly, by decreasing the duty ratio, nanoparticles start to diffuse to the wall. Under the action of gas flow, nanoparticle density peak is created in front of the pulse electrode, where the gas temperature is smaller.

Using a Mach–Zehnder interferometer to deduce nitrogen density mapping

F. Boudaoud, M. Lemerini
Chin. Phys. B, 2015, 24 (7): 075205 doi: 10.1088/1674-1056/24/7/075205
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This work presents an optical method using the Mach–Zehnder interferometer. We especially diagnose a pure nitrogen gas subjected to a point to plane corona discharge, and visualize the density spatial map. The interelectrode distance equals 6 mm and the variation of the optical path has been measured at different pressures: 220 Torr, 400 Torr, and 760 Torr. The interferograms are recorded with a CCD camera, and the numerical analysis of these interferograms is assured by the inverse Abel transformation. The nitrogen density is extracted through the Gladstone–Dale relation. The obtained results are in close agreement with values available in the literature.

Helix unwinding in ferroelectric liquid crystals induced by tilted electric field

Nail G. Migranov, Aleksey A. Kudreyko
Chin. Phys. B, 2015, 24 (7): 076101 doi: 10.1088/1674-1056/24/7/076101
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Helix unwinding in ferroelectric liquid crystals induced by an electric field is theoretically studied on the basis of the continuum theory. By applying a weak electric field tilted to the smectic layers, the contribution of the dielectric interaction energy density to the total free energy density is increased. Approximation methods are used to calculate the free energy for different tilt angles between the electric field and the smectic layers. The obtained results suggest selecting the optimal number of pitches in the film that matches to the minimum of the free energy.

Visible to deep ultraviolet range optical absorption of electron irradiated borosilicate glass

Wang Tie-Shan, Duan Bing-Huang, Tian Feng, Peng Hai-Bo, Chen Liang, Zhang Li-Min, Yuan Wei
Chin. Phys. B, 2015, 24 (7): 076102 doi: 10.1088/1674-1056/24/7/076102
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To study the room-temperature stable defects induced by electron irradiation, commercial borosilicate glasses were irradiated by 1.2 MeV electrons and then ultraviolet (UV) optical absorption (OA) spectra were measured. Two characteristic bands were revealed before irradiation, and they were attributed to silicon dangling bond (E'-center) and Fe3+ species, respectively. The existence of Fe3+ was confirmed by electron paramagnetic resonance (EPR) measurements. After irradiation, the absorption spectra revealed irradiation-induced changes, while the content of E'-center did not change in the deep ultraviolet (DUV) region. The slightly reduced OA spectra at 4.9 eV was supposed to transform Fe3+ species to Fe2+ species and this transformation leads to the appearance of 4.3 eV OA band. By calculating intensity variation, the transformation of Fe was estimated to be about 5% and the optical absorption cross section of Fe2+ species is calculated to be 2.2 times larger than that of Fe3+ species. Peroxy linkage (POL, ≡Si–O–O–Si≡), which results in a 3.7 eV OA band, is speculated not to be from Si–O bond break but from Si–O–B bond, Si–O–Al bond, or Si–O–Na bond break. The co-presence defect with POL is probably responsible for 2.9-eV OA band.

Microstructure evolution of Cu atomic islands on liquid surfaces in the ambient atmosphere

Zhang Xiao-Fei, Chen Hang, Yu Sen-Jiang
Chin. Phys. B, 2015, 24 (7): 076103 doi: 10.1088/1674-1056/24/7/076103
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We report the microstructure evolution of copper (Cu) nm-sized atomic islands on silicone oil surfaces in the ambient atmosphere. The origin of these nearly free sustaining atomic islands is explained by a three-stage growth model. The first stage is the nucleation and growth of atomic granules. Subsequently, the compact atomic islands grow by the aggregation of the atomic granules. Finally, they adhere to each other and form branched atomic islands. During the characteristic evolution, the atomic granules reconstruct and the average height of the atomic islands increases from 7.0± 1.0 nm to 13.0± 1.0 nm. The detailed evolution mechanism of the Cu atomic islands is presented.

Electric field effect in ultrathin zigzag graphene nanoribbons

Zhang Wen-Xing, Liu Yun-Xiao, Tian Hua, Xu Jun-Wei, Feng Lin
Chin. Phys. B, 2015, 24 (7): 076104 doi: 10.1088/1674-1056/24/7/076104
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The electric field effect in ultrathin zigzag graphene nanoribbons containing only three or four zigzag carbon chains is studied by first-principles calculations, and the change of conducting mechanism is observed with increasing in-plane electric field perpendicular to the ribbon. Wider zigzag graphene nanoribbons have been predicted to be spin-splitted for both valence band maximum (VBM) and conduction band minimum (CBM) with an applied electric field and become half-metal due to the vanishing band gap of one spin with increasing applied field. The change of VBM for the ultrathin zigzag graphene nanoribbons is similar to that for the wider ones when an electric field is applied. However, in the ultrathin zigzag graphene nanoribbons, there are two kinds of CBMs, one is spin-degenerate and the other is spin-splitted, and both are tunable by the electric field. Moreover, the two CBMs are spatially separated in momentum space. The conducting mechanism changes from spin-degenerate CBM to spin-splitted CBM with increasing applied electric field. Our results are confirmed by density functional calculations with both LDA and GGA functionals, in which the LDA always underestimates the band gap while the GGA normally produces a bigger band gap than the LDA.

Subthreshold behavior of AlInSb/InSb high electron mobility transistors

S. Theodore Chandra, N. B. Balamurugan, G. Lakshmi Priya, S. Manikandan
Chin. Phys. B, 2015, 24 (7): 076105 doi: 10.1088/1674-1056/24/7/076105
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We propose a scaling theory for single gate AlInSb/InSb high electron mobility transistors (HEMTs) by solving the two-dimensional (2D) Poisson equation. In our model, the effective conductive path effect (ECPE) is taken into account to overcome the problems arising from the device scaling. The potential in the effective conducting path is developed and a simple scaling equation is derived. This equation is solved to obtain the minimum channel potential Ødeff,min and the new scaling factor α to model the subthreshold behavior of the HEMTs. The developed model minimizes the leakage current and improves the subthreshold swing degradation of the HEMTs. The results of the analytical model are verified by numerical simulation with a Sentaurus TCAD device simulator.

Theoretical investigation of sulfur defects on structural, electronic, and elastic properties of ZnSe semiconductor

Muhammad Zafar, Shabbir Ahmed, M. Shakil, M. A. Choudhary, K. Mahmood
Chin. Phys. B, 2015, 24 (7): 076106 doi: 10.1088/1674-1056/24/7/076106
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The structural, electronic, and elastic properties of ZnSe1-xSx for the zinc blende structures have been studied by using the density functional theory. The calculations were performed using the plane wave pseudopotential method, as implemented in Quantum ESPRESSO. The exchange-correlation potential is treated with the local density approximation pz-LDA for these properties. Moreover, LDA+U approximation is employed to treat the “d” orbital electrons properly. A comparative study of the band gap calculated within both LDA and LDA+U schemes is presented. The analysis of results show considerable improvement in the calculation of band gap. The inclusion of compositional disorder increases the values of all elastic constants. In this study, it is found that elastic constants C11, C12, and C44 are mainly influenced by the compositional disorder. The obtained results are in good agreement with literature.

Load-redistribution strategy based on time-varying load against cascading failure of complex network

Liu Jun, Xiong Qing-Yu, Shi Xin, Wang Kai, Shi Wei-Ren
Chin. Phys. B, 2015, 24 (7): 076401 doi: 10.1088/1674-1056/24/7/076401
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Cascading failure can cause great damage to complex networks, so it is of great significance to improve the network robustness against cascading failure. Many previous existing works on load-redistribution strategies require global information, which is not suitable for large scale networks, and some strategies based on local information assume that the load of a node is always its initial load before the network is attacked, and the load of the failure node is redistributed to its neighbors according to their initial load or initial residual capacity. This paper proposes a new load-redistribution strategy based on local information considering an ever-changing load. It redistributes the loads of the failure node to its nearest neighbors according to their current residual capacity, which makes full use of the residual capacity of the network. Experiments are conducted on two typical networks and two real networks, and the experimental results show that the new load-redistribution strategy can reduce the size of cascading failure efficiently.

Effects of energy dissipation on anisotropic materials

Zhang Ling-Yun
Chin. Phys. B, 2015, 24 (7): 076501 doi: 10.1088/1674-1056/24/7/076501
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A model and its simulations are presented to describe the effects of energy dissipation on anisotropic systems. When the current electromigration is constant, energy dissipation depends on lattice constants, resistivity, and the angles along the longitudinal and transversal directions. It is shown that an orientation variation of the grain can significantly influence the energy dissipation for some anisotropic materials. Based on calculations for the grain model, the mechanism of grain growth and microstructure evolution under electromigration is explained. Theoretical implications about material selection and reliability are derived.

Nondestructive measurement of thermal contact resistance for the power vertical double-diffused metal-oxide-semiconductor

Li Rui, Guo Chun-Sheng, Feng Shi-Wei, Shi Lei, Zhu Hui, Wang Lin
Chin. Phys. B, 2015, 24 (7): 076601 doi: 10.1088/1674-1056/24/7/076601
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To obtain thermal contact resistance (TCR) between the vertical double-diffused metal-oxide-semiconductor (VDMOS) and the heat sink, we derived the relationship between the total thermal resistance and the contact force imposed on the VDMOS. The total thermal resistance from the chip to the heat sink is measured under different contact forces, and the TCR can be extracted nondestructively from the derived relationship. Finally, the experimental results are compared with the simulation results.

Phase diagram and collective modes in Rashba spin–orbit coupled BEC: Effect of in-plane magnetic field Hot!

Dong Dong, Zou Xu-Bo, Guo Guang-Can
Chin. Phys. B, 2015, 24 (7): 076701 doi: 10.1088/1674-1056/24/7/076701
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We studied the system of pure Rashba spin–orbit coupled Bose gas with an in-plane magnetic field. Based on the mean field theory, we obtained the zero temperature phase diagram of the system which exhibits three phases, plane wave (PW) phase, striped wave (SW) phase, and zero momentum (ZM) phase. It was shown that with a growing in-plane field, both SW and ZM phases will eventually turn into the PW phase. Furthermore, we adopted the Bogoliubov theory to study the excitation spectrum as well as the sound speed.

Unusual structural properties of polymers confined in a nanocylinder

Jiang Zhi-Bin, Peng Meng-Jie, Li Lin-Ling, Zhou Dong-Shan, Wang Rong, Xue Gi
Chin. Phys. B, 2015, 24 (7): 076801 doi: 10.1088/1674-1056/24/7/076801
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Structural properties of polymers confined in nanocylinders are investigated by Monte Carlo simulation, which is successfully used to consider the conformational property of constrained polymers. The conformational properties of the polymers close to the walls exhibit different features. The density profiles of polymers are enhanced near the wall of the nanocylinder, which shows that the packing densities differ near the wall and far from the wall. The highest densities near the wall of the nanocylinder decrease with increasing radius of the nanocylinder. Furthermore, the density excess is not only near the wall of the nanocylinder, but also shifts to the center of the nanocylinder at lower temperatures. The radius of gyration and the bond length of polymers in the nanocylinder show that the polymer chains tend to extend along the axis of the nanocylinder in highly confined nanocylinder and contract at lower temperature. Our results are very helpful in understanding the packing induced physical behaviors of polymers in nanocylinders, such as glass transition, crystallization, etc.

New method for fast morphological characterization of organic polycrystalline films by polarized optical microscopy

He Xiao-Chuan, Yang Jian-Bing, Yan Dong-Hang, Weng Yu-Xiang
Chin. Phys. B, 2015, 24 (7): 076803 doi: 10.1088/1674-1056/24/7/076803
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A new method to visualize the large-scale crystal grain morphology of organic polycrystalline films is proposed. First, optical anisotropic transmittance images of polycrystalline zinc phthalocyanine (ZnPc) films vacuum deposited by weak epitaxial growth (WEG) method were acquired with polarized optical microscopy (POM). Then morphology properties including crystal grain size, distribution, relative orientation, and crystallinity were derived from these images by fitting with a transition dipole model. At last, atomic force microscopy (AFM) imaging was carried out to confirm the fitting and serve as absolute references. This method can be readily generalized to other organic polycrystalline films, thus providing an efficient way to access the large-scale morphologic properties of organic polycrystalline films, which may prove to be useful in industry as a film quality monitoring method.


Structure-dependent metal—insulator transition in one-dimensional Hubbard superlattice

Zhang Liang-Liang, Huang Jin, Duan Cheng-Bo, Wang Wei-Zhong
Chin. Phys. B, 2015, 24 (7): 077101 doi: 10.1088/1674-1056/24/7/077101
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We investigate the charge and spin gaps, and the spin structure in half-filled one-dimensional Hubbard superlattices with one repulsive site and L0 free sites per unit cell. For odd L0, it is correlated metal at the particle–hole symmetric point, and then turns into band insulator beyond this point. For even L0, the system has a Mott insulator phase around the particle–hole symmetric point and undergoes a metal–insulator transition with on-site repulsion U increasing. For large U, there exists a multiperiodic spin structure, which results from the ferromagnetic (antiferromagnetic) correlation between the nearest neighboring repulsive sites for odd (even) L0.

Stacking fault energy, yield stress anomaly, and twinnability of Ni3Al: A first principles study

Liu Li-Li, Wu Xiao-Zhi, Wang Rui, Li Wei-Guo, Liu Qing
Chin. Phys. B, 2015, 24 (7): 077102 doi: 10.1088/1674-1056/24/7/077102
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Using first principles calculations combined with the quasiharmonic approach, we study the effects of temperature on the elastic constants, generalized stacking fault energies, and generalized planar fault energies of Ni3Al. The antiphase boundary energies, complex stacking fault energies, superlattice intrinsic stacking fault energies, and twinning energies decrease slightly with temperature. Temperature dependent anomalous yield stress of Ni3Al is predicted by the energy-based criterion based on elastic anisotropy and antiphase boundary energies. It is found that p increases with temperature and this can give a more accurate description of the anomalous yield stress in Ni3Al. Furthermore, the predicted twinnablity of Ni3Al is also decreasing with temperature.

Enhanced coercivity and remanence of PrCo5 nanoflakes prepared by surfactant-assisted ball milling with heat-treated starting powder

Zuo Wen-Liang, Zhao Xin, Xiong Jie-Fu, Shang Rong-Xiang, Zhang Ming, Hu Feng-Xia, Sun Ji-Rong, Shen Bao-Gen
Chin. Phys. B, 2015, 24 (7): 077103 doi: 10.1088/1674-1056/24/7/077103
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PrCo5 nanoflakes with strong texture and high coercivity of 8.15 kOe were prepared by surfactant-assisted ball milling with heat-treated starting powder. The thickness and length of the as-milled nanoflakes are mainly in the ranges of 50–100 nm and 0.5–3 μm, respectively. The x-ray diffraction patterns demonstrate that the heat treatment can increase the single phase and crystallinity of the PrCo5 compound, and combined with the demagnetization curves, indicate that the single phase and crystallinity are important for preparing high-coercivity and strong-textured rare earth permanent magnetic nanoflakes. In addition, the coercivity mechanism of the as-milled PrCo5 nanoflakes is studied by the angle dependence of coercivity for an aligned sample and the field dependence of coercivity, isothermal (IRM) and dc demagnetizing (DCD) remanence curves for an unaligned sample. The results indicate that the coercivity is dominated by co-existing mechanisms of pinning and nucleation. Furthermore, exchange coupling and dipolar coupling also co-exist in the sample.

Electronic and optical properties of lithium niobate under high pressure: A first-principles study

Sang Dan-Dan, Wang Qing-Lin, Han Chong, Chen Kai, Pan Yue-Wu
Chin. Phys. B, 2015, 24 (7): 077104 doi: 10.1088/1674-1056/24/7/077104
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We theoretically study the structural, electronic, and optical properties of lithium niobate under pressure using the plane-wave pseudopotential density functional theory by CASTEP code. It was found that there is a phase transition from the R3c structure to the Pnma structure at a pressure of 18.7 GPa. The Pnma structure was dynamically stable according to the calculation of phonon dispersion. From the charge density distributions, there exist covalent interactions along the Nb–O bond. The hybridization between O 2p and Nb 4d orbital in the Pnma phase increases with increasing pressure, while it is not changed in the R3c phase. With increasing pressure, the average Nb–O bond length decreases and the Nb–O bond population increases, indicating the increased covalent character between Nb and O atoms under high pressure at Pnma phase, which leads to the increased hybridization between O 2p and Nb 4d orbitals. Furthermore, the optical dielectric function, refractive index, extinction coefficient, electron energy, loss and reflectivity are calculated.

High performance trench MOS barrier Schottky diode with high-k gate oxide

Zhai Dong-Yuan, Zhu Jun, Zhao Yi, Cai Yin-Fei, Shi Yi, Zheng You-Liao
Chin. Phys. B, 2015, 24 (7): 077201 doi: 10.1088/1674-1056/24/7/077201
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A novel trench MOS barrier Schottky diode (TMBS) device with a high-k material introduced into the gate insulator is reported, which is named high-k TMBS. By simulation with Medici, it is found that the high-k TMBS can have 19.8% lower leakage current while maintaining the same breakdown voltage and forward turn-on voltage compared with the conventional regular trench TMBS.

Au and Ti induced charge redistributions on monolayer WS2

Zhu Hui-Li, Yang Wei-Huang, Wu Ya-Ping, Lin Wei, Kang Jun-Yong, Zhou Chang-Jie
Chin. Phys. B, 2015, 24 (7): 077301 doi: 10.1088/1674-1056/24/7/077301
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By using the first-principles calculations, structural and electronic properties of Au and Ti adsorbed WS2 monolayers are studied systematically. For Au-adsorbed WS2, metallic interface states are induced in the middle of the band gap across the Fermi level. These interface states origin mainly from the Au-6s states. As to the Ti adsorbed WS2, some delocalized interface states appear and follow the bottom of conduction band. The Fermi level arises into the conduction band and leads to the n-type conducting behavior. The n-type interface states are found mainly come from the Ti-3d and W-5d states due to the strong Ti–S hybridization. The related partial charge densities between Ti and S atoms are much higher and increased by an order of magnitude as compared with that of Au-adsorbed WS2. Therefore, the electron transport across the Ti-adsorbed WS2 system is mainly by the resonant transport, which would further enhances the electronic transparency when monolayer WS2 contacts with metal Ti. These investigations are of significant importance in understanding the electronic properties of metal atom adsorption on monolayer WS2 and offer valuable references for the design and fabrication of 2D nanodevices.

Coherent and tunable radiation with power enhancement from surface plasmon polaritons

Gong Sen, Zhong Ren-Bin, Hu Min, Chen Xiao-Xing, Zhang Ping, Zhao Tao, Liu Sheng-Gang
Chin. Phys. B, 2015, 24 (7): 077302 doi: 10.1088/1674-1056/24/7/077302
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Surface plasmon polaritons excited by an electron beam can be transformed into coherent and tunable light radiation waves with power enhancement in the simple structure of a metal film with a dielectric medium loading. In this paper, the process of the radiation transformation of this radiation, and the dependencies of the radiation characteristics on the parameters of the structure and the electron beam are studied in detail. The radiation power enhancement is greatly influenced by the beam energy and the film thickness in the infrared to ultraviolet frequency region. Up to 122 times radiation power enhancement and 6.5% radiation frequency tuning band can be obtained by optimizing the beam energy and the parameters of the film.

A low-threshold nanolaser based on hybrid plasmonic waveguides at the deep subwavelength scale

Li Zhi-Quan, Piao Rui-Qi, Zhao Jing-Jing, Meng Xiao-Yun, Tong Kai
Chin. Phys. B, 2015, 24 (7): 077303 doi: 10.1088/1674-1056/24/7/077303
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A novel nanolaser structure based on a hybrid plasmonic waveguide is proposed and investigated. The coupling between the metal nanowire and the high-index semiconductor nanowire with optical gain leads to a strong field enhancement in the air gap region and low propagation loss, which enables the realization of lasing at the deep subwavelength scale. By optimizing the geometric parameters of the structure, a minimal lasing threshold is achieved while maintaining the capacity of ultra-deep subwavelength mode confinement. Compared with the previous coupled nanowire pair based hybrid plasmonic structure, a lower threshold can be obtained with the same geometric parameters. The proposed nanolaser can be integrated into a miniature chip as a nanoscale light source and has the potential to be widely used in optical communication and optical sensing technology.

Energy distribution extraction of negative charges responsible for positive bias temperature instability

Ren Shang-Qing, Yang Hong, Wang Wen-Wu, Tang Bo, Tang Zhao-Yun, Wang Xiao-Lei, Xu Hao, Luo Wei-Chun, Zhao Chao, Yan Jiang, Chen Da-Peng, Ye Tian-Chun
Chin. Phys. B, 2015, 24 (7): 077304 doi: 10.1088/1674-1056/24/7/077304
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A new method is proposed to extract the energy distribution of negative charges, which results from electron trapping by traps in the gate stack of nMOSFET during positive bias temperature instability (PBTI) stress based on the recovery measurement. In our case, the extracted energy distribution of negative charges shows an obvious dependence on energy, and the energy level of the largest energy density of negative charges is 0.01 eV above the conduction band of silicon. The charge energy distribution below that energy level shows strong dependence on the stress voltage.

A novel physical parameter extraction approach for Schottky diodes

Wang Hao, Chen Xing, Xu Guang-Hui, Huang Ka-Ma
Chin. Phys. B, 2015, 24 (7): 077305 doi: 10.1088/1674-1056/24/7/077305
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Parameter extraction is an important step for circuit simulation methods that are based on physical models of semiconductor devices. A novel physical parameter extraction approach for Schottky diodes is proposed in this paper. By employing a set of analytical formulas, this approach extracts all of the necessary physical parameters of the diode chip in a unique way. It then extracts the package parasitic parameters with a curve-fitting method. To validate the proposed approach, a model HSMS-282c commercial Schottky diode is taken as an example. Its physical parameters are extracted and used to simulate the diode's electrical characteristics. The simulated results based on the extracted parameters are compared with the measurements and a good agreement is obtained, which verifies the feasibility and accuracy of the proposed approach.

Modulation of WNx/Ge Schottky barrier height by varying N composition of tungsten nitride

Wei Jiang-Bin, Chi Xiao-Wei, Lu Chao, Wang Chen, Lin Guang-Yang, Wu Huan-Da, Huang Wei, Li Cheng, Chen Song-Yan, Liu Chun-Li
Chin. Phys. B, 2015, 24 (7): 077306 doi: 10.1088/1674-1056/24/7/077306
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Modulation of the Schottky barrier heights was successfully demonstrated for WNx/p-Ge and WNx/n-Ge contacts by increasing the nitrogen component in the WNx films. The WN0.38/p-Ge contact exhibits rectifying characteristic and an apparent Schottky barrier of 0.49 eV while the WN0.38/n-Ge Schottky contact exhibits quasi-Ohmic current–voltage characteristics. Dipoles formed at the contact interface by the difference of the Pauling electronegativities of Ge and N are confirmed to alleviate the Fermi-level pinning effect.

Temperature-dependent bias-stress-induced electrical instability of amorphous indium-gallium-zinc-oxide thin-film transistors

Qian Hui-Min, Yu Guang, Lu Hai, Wu Chen-Fei, Tang Lan-Feng, Zhou Dong, Ren Fang-Fang, Zhang Rong, Zheng You-Liao, Huang Xiao-Ming
Chin. Phys. B, 2015, 24 (7): 077307 doi: 10.1088/1674-1056/24/7/077307
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The time and temperature dependence of threshold voltage shift under positive-bias stress (PBS) and the following recovery process are investigated in amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors. It is found that the time dependence of threshold voltage shift can be well described by a stretched exponential equation in which the time constant τ is found to be temperature dependent. Based on Arrhenius plots, an average effective energy barrier Eτstress= 0.72 eV for the PBS process and an average effective energy barrier Eτrecovery= 0.58 eV for the recovery process are extracted respectively. A charge trapping/detrapping model is used to explain the threshold voltage shift in both the PBS and the recovery process. The influence of gate bias stress on transistor performance is one of the most critical issues for practical device development.

Effect of interfacial coupling on rectification in organic spin rectifiers

Hu Gui-Chao, Zuo Meng-Ying, Li Ying, Zhang Zhao, Ren Jun-Feng, Wang Chuan-Kui
Chin. Phys. B, 2015, 24 (7): 077308 doi: 10.1088/1674-1056/24/7/077308
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The effect of interfacial coupling on rectification in an organic co-oligomer spin diode is investigated theoretically by considering spin-independent and spin-resolved couplings respectively. In the case of spin-independent coupling, an optimal interfacial coupling strength with a significant enhanced rectification ratio is found, whose value depends on the structural asymmetry of the molecule. In the case of spin-resolved coupling, we found that only the variation of the interfacial coupling with specific spin is effective to modulate the rectification, which is due to the spin-filtering property of the central asymmetric magnetic molecule. A transition of the spin-current rectification between parallel spin-current rectification and antiparallel spin-current rectification may be observed with the variation of the spin-resolved interfacial coupling. The interfacial effect on rectification is further analyzed from the spin-dependent transmission spectrum at different biases.

Optimization of intergrain connection in high-temperature superconductor Bi2Sr2CaCu2Ox

Li Cheng-Shan, Zhang Sheng-Nan, Hao Qing-Bin, Ma Xiao-Bo, Lu Tian-Ni, Zhang Ping-Xiang
Chin. Phys. B, 2015, 24 (7): 077401 doi: 10.1088/1674-1056/24/7/077401
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A modified spark plasma sintering (SPS) technique was developed for the fabrication of Bi2Sr2CaCu2Ox (Bi-2212) superconducting bulks with better intergrain connections. The influences of the modified SPS process on the microstructures, intergrain connections, and related superconducting properties were systematically analyzed. The modified SPS process can not only increase the final density of the bulk samples but also enhance the texture structures. Clean grain boundaries were obtained instead of the intergrain amorphous layers. Therefore the intergranular properties were obviously improved. Due to the better intergrain connections and the stronger flux pinning properties, the critical current densities of the Bi-2212 bulks obtained via the modified SPS process were greatly increased. The obtained improvements imply the possibility for the modified SPS technique to be used for enhancing the superconducting properties of the Bi-2212 tapes.

Tuning the magnetic anisotropy of CoFeB grown on flexible substrates

Zhang Hao, Li Yuan-Yuan, Yang Mei-Yin, Zhang Bao, Yang Guang, Wang Shou-Guo, Wang Kai-You
Chin. Phys. B, 2015, 24 (7): 077501 doi: 10.1088/1674-1056/24/7/077501
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The magnetic properties of CoFeB thin films grown on flexible polyimide substrates were investigated using a magneto-optical Kerr effect magnetometer. In-plane uniaxial magnetic anisotropy was observed in the virgin state. The strain induced by bending the flexible substrate was applied on the sample to change the magnetic properties of CoFeB. The strain induced uniaxial magnetic anisotropy changed linearly with the deformation by about 8.41×104 erg/cm3 at 1% of deformation. Our results prove the magnetic properties of CoFeB grown on flexible polyimide substrate can be tuned effectively by bending, which could be important for future flexible spintronics.

Anomalous microstructure and magnetocaloric properties in off-stoichiometric La–Fe–Si and its hydride

He Chun, Zhang Ming-Xiao, Shao Yan-Yan, Dong Jing-Du, Yan A-Ru, Liu Jian
Chin. Phys. B, 2015, 24 (7): 077503 doi: 10.1088/1674-1056/24/7/077503
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In the present work we reported the phase formation, microstructure, magnetocaloric effect and hydrogenation behavior of La-rich La1.7Fe11.6Si1.4 alloy. In this off-stoichiometric La(Fe,Si)13 alloy, the NaZn13-type La(Fe,Si)13 matrix phase shows faceted grains, with the Cr5B3-type La5Si3 used as the secondary phase distributed intergranularly. Such a peculiar morphology quickly forms upon one day annealing. In La1.7Fe11.6Si1.4 alloy, we have observed a significant field dependence of magnetostructural transition temperature (~ 6.3 K/T), resulting in a large and table-like entropy change (ΔS~ 18 J/kg·K in 2 T) over a broad temperature range (~ 10 K). Upon hydrogenation, the maximum value of ΔS keeps almost unchanged, while the Curie temperature increases up to 350 K. These results indicate that the investigated off-stoichiometric La(Fe,Si)13 alloy is a promising magnetic material for magnetic refrigerators.

FePt nano-stripes fabricated on anodic aluminum oxide templates

Deng Chen-Hua, Qiao Xin-Yu, Wang Fang, Fan Jiu-Ping, Zeng Hao, Xu Xiao-Hong
Chin. Phys. B, 2015, 24 (7): 077504 doi: 10.1088/1674-1056/24/7/077504
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A new FePt nanostructure with stripe-like patterns has been prepared by direct current (DC) magnetron sputtering on anodic aluminum oxide (AAO) templates. AAO templates anodized under low voltages (7 V) demonstrate self-organized, maze-like patterns, different from the conventional porous structures obtained at high voltages. FePt thin films deposited on such templates tend to replicate the morphology of the templates. Although there is no obvious spatial ordering, the dimensions of the FePt nano-stripes are highly uniform, due to the constrained growth along the transverse direction of the AAO pattern. The magnetic properties are strongly influenced by this unique morphology. While continuous films demonstrate strong exchange coupling, the dominant interaction in FePt nano-stripes with the same nominal thickness is magnetostatic. The morphology also dictates the magnetization reversal behaviors, with thin films dominated by domain nucleation; while nano-stripes incline to reverse their magnetization by spin rotation. Our work demonstrates that self-organized AAO templates can be used to control the morphology and magnetic behavior of FePt materials.

Equivalent circuit model including magnetic and thermo sources for the thermo–magneto–electric coupling effect in magnetoelectric laminates

Cui Xiao-Le, Zhou Hao-Miao
Chin. Phys. B, 2015, 24 (7): 077506 doi: 10.1088/1674-1056/24/7/077506
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The nonlinear thermo–magneto–mechanical magnetostrictive constitutive and the linear thermo–mechanical-electric piezoelectric constitutive are adopted in this paper. The bias magnetic field and ambient temperature are equivalent to a magnetic source and a thermo source, respectively. An equivalent circuit, which contains a magnetic source and a thermo source at the input, for the thermo–magneto–electric coupling effect in magnetoelectric (ME) laminates, is established. The theoretical models of the output voltage and static ME coefficient for ME laminates can be derived from this equivalent circuit model. The predicted static ME coefficient versus temperature curves are in excellent agreement with the experimental data available both qualitatively and quantitatively. It confirms the validity of the proposed model. Then the models are adopted to predict variations in the output voltages and ME coefficients in the laminates under different ambient temperatures, bias magnetic fields, and the volume ratios of magnetostrictive phases. This shows that the output voltage increases with both increasing temperature and increasing volume ratio of magnetostrictive phases; the ME coefficient decreases with increasing temperature; the ME coefficient shows an initial sharp increase and then decreases slowly with the increase in the bias magnetic field, and there is an optimum volume ratio of magnetostrictive phases that maximize the ME coefficient. This paper can not only provide a new idea for the study of the thermo–magneto–electric coupling characteristics of ME laminates, but also provide a theoretical basis for the design and application of ME laminates, operating under different sensors.

Piezoelectric and electro—optic properties of tetragonal (1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 single crystals by phenomenological theory

Qiu Jian-Hua, Wang Xiu-Qin, Yuan Ning-Yi, Ding Jian-Ning
Chin. Phys. B, 2015, 24 (7): 077701 doi: 10.1088/1674-1056/24/7/077701
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An eighth-order Landau–Devonshire theory is constructed to investigate the piezoelectric and electro–optic properties of tetragonal (1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 single crystals (x=0.38 and x=0.4). The dielectric stiffness coefficients of the Landau potential are obtained by fitting to the dielectric properties and the phase transition temperature between cubic phase and tetragonal phase. It is indicated that tetragonal (1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 single crystals have the first-order cubic-tetragonal phase transitions. The dielectric constants are in great agreement with the experimental results. The piezoelectric coefficients d33 and d31 at room temperature are about 145 pC/N and -62 pC/N respectively which are smaller than that of (1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 single crystals around the morphotropic phase boundary. Moreover, tetragonal (1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 single crystals have the linear electro–optic coefficients rc=33.7 pm/V and rc=28.8 pm/V for x=0.38 and x=0.4, respectively which are in accordance with the measurements.

Theoretical analysis of semi/non-polar InGaN/GaN light-emitting diodes grown on silicon substrates

Yu Lei, Zhang Yuan-Wen, Li Kai, Pi Hui, Diao Jia-Sheng, Wang Xing-Fu, Hu Wen-Xiao, Zhang Chong-Zhen, Song Wei-Dong, Shen Yue, Li Shu-Ti
Chin. Phys. B, 2015, 24 (7): 077801 doi: 10.1088/1674-1056/24/7/077801
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A theoretical study of polar and semi/non-polar InGaN/GaN light-emitting diodes (LEDs) with different internal surface polarization charges, which can be grown on Si substrates, is conducted by using APSYS software. In comparison with polar structure LEDs, the semi-polar structure exhibits a higher concentration of electrons and holes and radiative recombination rate, and its reduced built-in polarization field weakens the extent of band bending which causes the shift of peak emission wavelength. So the efficiency droop of semi-polar InGaN/GaN LEDs declines obviously and the optical power is significantly improved. In comparison with non-polar structure LEDs, although the concentration of holes and electrons as well as the radiative recombination rate of the semi-polar structure are better in the last two quantum wells (QWs) approaching the p-GaN side, the uniformity of distribution of carriers and radiative recombination rate for the non-polar structure is better. So the theoretical analysis indicates that the removal of the internal polarization field in the MQWs active regions for non-polar structure LEDs contributes to the uniform distribution of electrons and holes, and decreases the electron leakage. Thus it enhances the radiative recombination rate, and further improves the IQEs and optical powers, and shows the best photoelectric properties among these three structures.

Linear optical properties of defective KDP with oxygen vacancy: First-principles calculations

Chen Xin, Zhao Qian-Qian, Wang Xiao-Chun, Chen Jun, Ju Xin
Chin. Phys. B, 2015, 24 (7): 077802 doi: 10.1088/1674-1056/24/7/077802
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The linear optical properties of potassium dihydrogen phosphate (KDP) with oxygen vacancy are investigated with first-principles density functional theory calculations. We use Heyd–Scuseria–Ernzerhof (HSE06) functional to calculate the linear optical properties because of its accuracy in the band gap calculation. Compared with the perfect KDP, we found that due to the defect states located at the band gap, the defective KDP with oxygen vacancy has new optical adsorption within the energy region from 4.8 eV to 7.0 eV (the corresponding wavelength region is from 258 nm to 177 nm). As a result, the oxygen vacancy can decrease the damage threshold of KDP crystal. It may give a direction to the KDP production for laser system.

Real-time quantitative optical method to study temperature dependence of crack propagation process in colloidal photonic crystal film

Lin Dong-Feng, Xu Yu-Zhuan, Shi Jiang-Jian, Zhang Yu, Luo Yan-Hong, Li Dong-Mei, Meng Qing-Bo
Chin. Phys. B, 2015, 24 (7): 077803 doi: 10.1088/1674-1056/24/7/077803
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A real-time quantitative optical method to characterize crack propagation in colloidal photonic crystal film (CPCF) is developed based on particle deformation models and previous real-time crack observations. The crack propagation process and temperature dependence of the crack propagation rate in CPCF are investigated. By this method, the crack propagation rate is found to slow down gradually to zero when cracks become more numerous and dense. Meanwhile, with the temperature increasing, the crack propagation rate constant decreases. The negative temperature dependence of the crack propagation rate is due to the increase of van der Waals attraction, which finally results in the decrease of resultant force. The findings provide new insight into the crack propagation process in CPCF.

Doping inhomogeneity and staging of ultra-thin graphite intercalation compound flakes probed by visible and near-infrared Raman spectroscopy

Lu Yan, Zhang Xin, Wu Jiang-Bin, Li Xiao-Li, Li Qiao-Qiao, Tan Ping-Heng
Chin. Phys. B, 2015, 24 (7): 077804 doi: 10.1088/1674-1056/24/7/077804
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When ultra-thin graphite intercalation compounds (GICs) are deposited on the SiO2/Si substrate, it is found that their colors are dependent on the thickness of GIC flakes. The sample colors of ultrathin GIC flakes can no longer provide qualitative information on the stage index. Here, multi-wavelength Raman spectroscopy is thus applied to study the doping inhomogeneity and staging of ultra-thin GICs by FeCl3 intercalation. The G band intensity of stage-1 GIC flakes is strongly enhanced by 532-nm laser excitation, while that of stage-2 and stage-3 flakes exhibits strong intensity enhancement for 785-nm laser excitation. The near-infrared lasers are suggested to probe the doping inhomogeneity and staging of ultra-thin GIC flakes.


Influence of vacuum degree on growth of Bi2Te3 single crystal

Tang Yan-Kun, Zhao Wen-Juan, Zhu Hua-Qiang, Huang Yong-Chao, Cao Wei-Wei, Yang Qian, Yao Xiao-Yan, Zhai Ya, Dong Shuai
Chin. Phys. B, 2015, 24 (7): 078101 doi: 10.1088/1674-1056/24/7/078101
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Bi2Te3 single crystals were prepared by the solid-state reaction method. The effect of the vacuum on the growth of Bi2Te3 single crystals was studied with varying the oxygen content by controlling the air pressure in the silica tube. High quality Bi2Te3 single crystals have been obtained and there is no influence on the growth by an extremely small amount of oxygen in a high vacuum at 1.0 × 10-3 Pa. As the air pressure is increased at 1.0 × 10-2 Pa, oxygen only mainly impacts on the growth of the surface for the prepared samples. Micron-sized rod-like structure and flower-like clusters are observed on the surface. For the samples prepared at 1.0 × 10-1 Pa, x-ray diffraction data show that the yellow part on the surface is Bi2TeO5, while the Bi2Te3 single crystal is still the major phase as the inside part. More interestingly, various crystal morphologies are observed by scanning electron microscope for Bi2Te3 near the boundary between Bi2Te3 and Bi2TeO5. Possible growth mechanisms for Bi2Te3 with different morphologies are discussed in detail.

Normally-off metamorphic AlInAs/AlInAs HEMTs on Si substrates grown by MOCVD

Huang Jie, Li Ming, Lau Kei-May
Chin. Phys. B, 2015, 24 (7): 078102 doi: 10.1088/1674-1056/24/7/078102
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A combination of self-aligned fluoride-based plasma treatment and post-gate rapid thermal annealing was developed to fabricate a novel 120-nm T-shaped gate normally-off metamorphic Al0.49In0.51As/Ga0.47In0.53As HEMT device on a Si substrate grown by metal-organic chemical vapor deposition (MOCVD). A shift of the threshold voltage, from -0.42 V to 0.11 V was obtained and the shift can be effectively adjusted by the process parameter of CF4 plasma treatment. Furthermore, a side benefit of reducing the leakage current of the device up to two orders of magnitude was also observed. E-mode transistors with 120 nm gate length own fT up to 160 GHz and fmax of 140 GHz. These characteristics imply the potential of the fluoride-based plasma treatment technology for the fabrication of monolithic enhancement/depletion-mode mHEMTs, which also encourage the massive production with this low-cost technology.

Influence of Ag and Sn incorporation in In2S3 thin films

Lin Ling-Yan, Yu Jin-Ling, Cheng Shu-Ying, Lu Pei-Min
Chin. Phys. B, 2015, 24 (7): 078103 doi: 10.1088/1674-1056/24/7/078103
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Ag- and Sn-doped In2S3 thin films were deposited on glass substrates using the thermal evaporation technique. The doping was realized by thermal diffusion. The influences of Ag and Sn impurities on the electrical, structural, morphological, and optical properties of the In2S3 films were investigated. In all deposited samples, the x-ray diffraction spectra revealed the formation of cubic In2S3 phase. A significant increase in the crystallite size was observed after Ag doping, while the doping of Sn slightly decreased the crystallite size. The x-ray photoelectron spectroscopy verified the diffusion of Ag and Sn into the In2S3 films after annealing. The optical study illustrated that Ag doping resulted in a reduction of the optical band gap while Sn doping led to a widening of the gap. Optical properties were investigated to determine the optical constants. Besides, it was found that the resistivity decreases significantly either after Ag or Sn incorporation. The study demonstrates that the Sn-doped In2S3 thin films are more suitable for buffer layer application in solar cells than the Ag-doped In2S3 thin films.

Preparation and characterization of PTFE coating in new polymer quartz piezoelectric crystal sensor for testing liquor products

Gu Yu, Li Qiang
Chin. Phys. B, 2015, 24 (7): 078106 doi: 10.1088/1674-1056/24/7/078106
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A new method was developed based on the electron beam vacuum dispersion (EBVD) technology to prepare the PTFE polymer coating of the new polymer quartz piezoelectric crystal sensor for testing liquor products. The new method was applied in the new EBVD equipment which we designed. A real-time system monitoring the polymer coating's thickness was designed for the new EBVD equipment according to the quartz crystal microbalance (QCM) principle, playing an important role in preparing stable and uniform PTFE polymer coatings of the same thickness. 30 pieces of PTFE polymer coatings on the surface of the quartz crystal basis were prepared with the PTFE polymer ultrafine powder (purity ≥ 99.99%) as the starting material. We obtained 30 pieces of new PTFE polymer sensors. By using scanning electron microscopy (SEM), the structure of the PTFE polymer coating's column clusters was studied. One sample from the 30 pieces of new PTFE polymer sensors was analysed by SEM in four scales, i.e., 400×, 1000×, 10000×, and 25000×. It was shown that under the condition of high bias voltage and low bias current, uniformly PTFE polymer coating could be achieved, which indicates that the new EBVD equipment is suitable for mass production of stable and uniform polymer coating.

Tip-splitting instability in directional solidification based on bias field method

You Jia-Xue, Wang Zhi-Jun, Li Jun-Jie, Wang Jin-Cheng
Chin. Phys. B, 2015, 24 (7): 078107 doi: 10.1088/1674-1056/24/7/078107
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Tip splitting instability of cellular interface morphology in directional solidification is analyzed based on the bias field method proposed recently by Glicksman. The physical mechanism of tip instability is explained by analyzing the interface potential, the tangential energy flux, and the normal energy flux. A rigorous criterion for tip-splitting instability is established analytically, i.e., the ratio of the cellular tip radius to the cellular width α > √3/2/π≈0.3899, which is in good agreement with simulation results. This study also reveals that the cellular tip splitting instability is attributable to weak Gibbs–Thomson energy acting on the interface.

Effects of physical parameters on the cell-to-dendrite transition in directional solidification

Wei Lei, Lin Xin, Wang Meng, Huang Wei-Dong
Chin. Phys. B, 2015, 24 (7): 078108 doi: 10.1088/1674-1056/24/7/078108
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A quantitative cellular automaton model is used to study the cell-to-dendrite transition (CDT) in directional solidification. We give a detailed description of the CDT by carefully examining the influence of the physical parameters, including: the Gibbs–Thomson coefficient Γ, the solute diffusivity Dl, the solute partition coefficient k0, and the liquidus slope ml. It is found that most of the parameters agree with the Kurz and Fisher (KF) criterion, except for k0. The intrinsic relations among the critical velocity Vcd, the cellular primary spacing λc, max, and the critical spacing λcd are investigated.

Synthesis of graphene-supported monodisperse AuPd bimetallic nanoparticles for electrochemical oxidation of methanol

Xiao Hong-Jun, Shen Cheng-Min, Shi Xue-Zhao, Yang Su-Dong, Tian Yuan, Lin Shao-Xiong, Gao Hong-Jun
Chin. Phys. B, 2015, 24 (7): 078109 doi: 10.1088/1674-1056/24/7/078109
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Monodisperse AuPd bimetallic nanoparticles (NPs) with different compositions are synthesized by using oleylamine (OAm) as reducing reagent, stabilizer, and solvent. To obtain AuPd solid solution NPs, Pd–OAm and Au–OAm precursors are firstly prepared by mixing OAm with Palladium (II) acetylacetonate (Pd(acac)2) and HAuCl4, respectively. Then Pd–OAm and Au–OAm precursor solutions are injected into a hot oleylamine solution to form AuPd NPs. The size of these NPs ranges from 6.0 to 8.0 nm and the composition is controlled by varying the precursor ratio. The AuPd NPs are loaded onto reduced graphene oxide (RGO) sheets to make catalysts. Alloy NPs show high electrocatalytic activity and stability toward methanol oxidation in the alkaline media. Their catalytic activity for methanol oxidation is found to be dependent on the NP composition. As the Pd component increases, the peak current densities during the forward scan gradually increase and reach the maximum at AuPd2. The enhancement of alloy NPs for methanol oxidation can be attributed to a synergistic effect of Au and Pd on the surface of alloy NPs.

Instability of lithium bis(fluorosulfonyl)imide (LiFSI)–potassium bis(fluorosulfonyl)imide (KFSI) system with LiCoO2 at high voltage

Zhang Shu, Li Wen-Jun, Ling Shi-Gang, Li Hong, Zhou Zhi-Bin, Chen Li-Quan
Chin. Phys. B, 2015, 24 (7): 078201 doi: 10.1088/1674-1056/24/7/078201
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The cycling performance, impedance variation, and cathode surface evolution of the Li/LiCoO2 cell using LiFSI–KFSI molten salt electrolyte are reported. It is found that this battery shows poor cycling performance, with capacity retention of only about 67% after 20 cycles. It is essential to understand the origin of the instability. It is noticed that the polarization voltage and the impedance of the cell both increase slowly upon cycling. The structure and the properties of the pristine and the cycled LiCoO2 cathodes are investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). It is found that the LiCoO2 particles are corroded by this molten salt electrolyte, and the decomposition by-product covers the surface of the LiCoO2 cathode after 20 cycles. Therefore, the surface side reaction explains the instability of the molten salt electrolyte with LiCoO2.

Redox-mediated reversible modulation of the photoluminescence of single quantum dots

Li Ying, Liu Ren-Wei, Ma Li, Fan Su-Na, Li Hui, Hu Shu-Xin, Li Ming
Chin. Phys. B, 2015, 24 (7): 078202 doi: 10.1088/1674-1056/24/7/078202
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Precise control over the photoluminescence (PL) of single quantum dots (QDs) is important for their practical applications. We show that the PL of individual CdSe/ZnS core/shell QDs can be effectively enhanced and continuously modulated by electrochemically manipulating the electron transfer (ET) between the QDs and the attached redox-active ligands such as 2-mercaptoethanol (BME). We found that i) the ET from BME to the QDs' surface trap states suppresses the blinking of the QDs, ii) the ET from the QDs' conduction band to the oxidization product results in dimmed PL when BME is oxidized, and iii) further oxidization of BME results in a significant PL brightening. The single particle measurements help us unveil the important features hidden in ensemble measurements and understand the underlying mechanism of the PL modulation. The results also suggest a simple yet efficient method to produce bright and non-blinking QDs and offer opportunities for further development of high resolution fluorescent bioimaging and nanodevices.

Nano structure evolution in P3HT:PC61BM blend films due to the effects of thermal annealing or by adding solvent

Fan Xing, Zhao Su-Ling, Chen Yu, Zhang Jie, Yang Qian-Qian, Gong Wei, Yuan Meng-Yao, Xu Zheng, Xu Xu-Rong
Chin. Phys. B, 2015, 24 (7): 078401 doi: 10.1088/1674-1056/24/7/078401
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Crystallographic dynamics of blend films of regioregular poly(3-hexylthiophene) (P3HT) mixed with [6-6-]-phenyl-C61-butyric acid methyl ester (PC61BM) treated by thermal annealing or by adding solvent 1,8-diiodooctane (DIO) are characterized by 2D-grazing incidence x-ray diffraction (2D-GIXRD). The results show that the P3HT chains are primarily oriented with the thiophene ring edge-on to the substrate, with a small fraction of chains oriented plane-on. The interplanar spacing becomes narrow after being treated by DIO, and the coherence length of the P3HT crystallites increases after being treated by thermal annealing or DIO, which is accompanied by a change in the orientation angle of the P3HT lamellae. The increased ordering of P3HT packing induced by thermal annealing or adding DIO contributes to enhanced photovoltaic performance.

Low-Tc direct current superconducting quantum interference device magnetometer-based 36-channel magnetocardiography system in a magnetically shielded room

Qiu Yang, Li Hua, Zhang Shu-Lin, Wang Yong-Liang, Kong Xiang-Yan, Zhang Chao-Xiang, Zhang Yong-Sheng, Xu Xiao-Feng, Yang Kang, Xie Xiao-Ming
Chin. Phys. B, 2015, 24 (7): 078501 doi: 10.1088/1674-1056/24/7/078501
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We constructed a 36-channel magnetocardiography (MCG) system based on low-Tc direct current (DC) superconducting quantum interference device (SQUID) magnetometers operated inside a magnetically shielded room (MSR). Weakly damped SQUID magnetometers with large Steward–McCumber parameter βc (βc ≈ 5), which could directly connect to the operational amplifier without any additional feedback circuit, were used to simplify the readout electronics. With a flux-to-voltage transfer coefficient ∂V/∂Ø larger than 420 μV/Ø0, the SQUID magnetometers had a white noise level of about 5.5 fT·Hz-1/2 when operated in MSR. 36 sensing magnetometers and 15 reference magnetometers were employed to realize software gradiometer configurations. The coverage area of the 36 sensing magnetometers is 210× 210 mm2. MCG measurements with a high signal-to-noise ratio of 40 dB were done successfully using the developed system.

Characteristics of drain-modulated generation current in n-type metal-oxide-semiconductor field-effect transistor

Chen Hai-Feng, Guo Li-Xin, Zheng Pu-Yang, Dong Zhao, Zhang Qian
Chin. Phys. B, 2015, 24 (7): 078502 doi: 10.1088/1674-1056/24/7/078502
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Drain-modulated generation current IDMG induced by interface traps in an n-type metal-oxide-semiconductor field-effect transistor (nMOSFET) is investigated. The formation of IDMG ascribes to the change of the Si surface potential φs. This change makes the channel suffer transformation from the inversion state, depletion I state to depletion II state. The simulation result agrees with the experiment in the inversion and depletion I states. In the depletion II state, the theoretical curve goes into saturation, while the experimental curve drops quickly as VD increases. The reason for this unconformity is that the drain-to-gate voltage VDG lessens φs around the drain corner and controls the falling edge of the IDMG curve. The experiments of gate-modulated generation and recombination currents are also applied to verify the reasonability of the mechanism. Based on this mechanism, a theoretical model of the IDMG falling edge is set up in which IDMG has an exponential attenuation relation with VDG. Finally, the critical fitting coefficient t of the experimental curves is extracted. It is found that t=80 mV=3kT/q. This result fully shows the accuracy of the above mechanism.

Bandwidth improvement of high power uni-traveling-carrier photodiodes by reducing the series resistance and capacitance

Li Jin, Xiong Bing, Sun Chang-Zheng, Luo Yi, Wang Jian, Hao Zhi-Biao, Han Yan-Jun, Wang Lai, Li Hong-Tao
Chin. Phys. B, 2015, 24 (7): 078503 doi: 10.1088/1674-1056/24/7/078503
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A backside illuminated mesa-structure InGaAs/InP modified uni-traveling-carrier photodiode (MUTC-PD) with wide bandwidth and high saturation power is fabricated and investigated. The device structure is optimized to reduce the capacitance and resistance. For the 22-μm-diameter device, the maximum responsivity at 1.55 μm is 0.5 A/W, and the 3-dB cutoff frequency reaches up to 28 GHz. The output photocurrent at the 1-dB compression point is measured to be 54 mA at 25 GHz, with a corresponding output radio frequency (RF) power of up to 15.5 dBm. The saturation characteristics of the MUTC-PD are also verified by the electric field simulation, and electric field collapse is found to be the cause of the saturation phenomenon.

An improved recommendation algorithm via weakening indirect linkage effect

Chen Guang, Qiu Tian, Shen Xiao-Quan
Chin. Phys. B, 2015, 24 (7): 078901 doi: 10.1088/1674-1056/24/7/078901
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We propose an indirect-link-weakened mass diffusion method (IMD), by considering the indirect linkage and the source object heterogeneity effect in the mass diffusion (MD) recommendation method. Experimental results on the MovieLens, Netflix, and RYM datasets show that, the IMD method greatly improves both the recommendation accuracy and diversity, compared with a heterogeneity-weakened MD method (HMD), which only considers the source object heterogeneity. Moreover, the recommendation accuracy of the cold objects is also better elevated in the IMD than the HMD method. It suggests that eliminating the redundancy induced by the indirect linkages could have a prominent effect on the recommendation efficiency in the MD method.

Effect of body biasing on single-event induced charge collection in deep N-well technology

Ding Yi, Hu Jian-Guo, Qin Jun-Rui, Tan Hong-Zhou
Chin. Phys. B, 2015, 24 (7): 079401 doi: 10.1088/1674-1056/24/7/079401
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As the device size decreases, the soft error induced by space ions is becoming a great concern for the reliability of integrated circuits (ICs). At present, the body biasing technique is widely used in highly scaled technologies. In the paper, using the three-dimensional technology computer-aided design (TCAD) simulation, we analyze the effect of the body biasing on the single-event charge collection in deep N-well technology. Our simulation results show that the body biasing mainly affects the behavior of the source, and the effect of body biasing on the charge collection for the nMOSFET and pMOSFET is quite different. For the nMOSFET, the RBB will increase the charge collection, while the FBB will reduce the charge collection. For the pMOSFET, the effect of RBB on the SET pulse width is small, while the FBB has an adverse effect. Moreover, the differenceof the effect of body biasing on the charge collection is compared in deep N-well and twin well.

A novel multi-pin rectangular waveguide slow-wave structure based backward wave amplifier at 340 GHz

Zhang Kai-Chun, Qi Zhong-Kuo, Yang Zhao-Long
Chin. Phys. B, 2015, 24 (7): 079402 doi: 10.1088/1674-1056/24/7/079402
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A backward wave amplifier (BWA) in a terahertz regime with a novel slow-wave structure (SWS) composed of multi parallel grating pins inside a rectangular waveguide is analyzed. The multi-pin rectangular waveguide SWS possesses good performance and is compatible with micro-fabrication technologies. The dispersion and interaction impedance of the multi-pin SWS are presented. The stopbands of the modes cling together in a Brillouim zone. The SWS has a high interaction impedance that is suitable for the interaction of multi cylindrical beams. The design, which is based on three parallel pins supporting the wave–beam interaction with four cylindrical beams, is verified by three-dimensional particle-in-cell simulations. A BWA with the central frequency at 340 GHz is demonstrated, and the output power is more than 100 mW. A tuning frequency range of 15 GHz (333–348 GHz) is obtained with a gain of more than 20 dB.
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