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Chin. Phys. B  
  Chin. Phys. B--2015, Vol.24, No.11
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Direction-of-arrival estimation for co-located multiple-input multiple-output radar using structural sparsity Bayesian learning

Wen Fang-Qing, Zhang Gong, Ben De
Chin. Phys. B 2015, 24 (11): 110201 ;  doi: 10.1088/1674-1056/24/11/110201
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This paper addresses the direction of arrival (DOA) estimation problem for the co-located multiple-input multiple-output (MIMO) radar with random arrays. The spatially distributed sparsity of the targets in the background makes compressive sensing (CS) desirable for DOA estimation. A spatial CS framework is presented, which links the DOA estimation problem to support recovery from a known over-complete dictionary. A modified statistical model is developed to accurately represent the intra-block correlation of the received signal. A structural sparsity Bayesian learning algorithm is proposed for the sparse recovery problem. The proposed algorithm, which exploits intra-signal correlation, is capable being applied to limited data support and low signal-to-noise ratio (SNR) scene. Furthermore, the proposed algorithm has less computation load compared to the classical Bayesian algorithm. Simulation results show that the proposed algorithm has a more accurate DOA estimation than the traditional multiple signal classification (MUSIC) algorithm and other CS recovery algorithms.

Identifying the interactions in a colored dynamical network

Wu Zhao-Yan, Gong Xiao-Li
Chin. Phys. B 2015, 24 (11): 110202 ;  doi: 10.1088/1674-1056/24/11/110202
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The interactions of a colored dynamical network play a great role in its dynamical behaviour and are denoted by outer and inner coupling matrices. In this paper, the outer and inner coupling matrices are assumed to be unknown and need to be identified. A corresponding network estimator is designed for identifying the unknown interactions by adopting proper adaptive laws. Based on the Lyapunov function method and Barbalat’s lemma, the obtained result is analytically proved. A colored network coupled with chaotic Lorenz, Chen, and Lü systems is considered as a numerical example to illustrate the effectiveness of the proposed method.

Frequency-tunable transmon in a three-dimensional copper cavity

Pan Jia-Zheng, Cao Zhi-Min, Fan Yun-Yi, Zhou Yu, Lan Dong, Liu Yu-Hao, Chen Zhi-Ping, Li Yong-Chao, Cao Chun-Hai, Xu Wei-Wei, Kang Lin, Chen Jian, Yu Hai-Feng, Yu Yang, Sun Guo-Zhu, Wu Pei-Heng
Chin. Phys. B 2015, 24 (11): 110301 ;  doi: 10.1088/1674-1056/24/11/110301
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We have realized a frequency-tunable transmon in a three-dimensional cooper cavity using a direct current superconducting quantum interference device. Both the transition frequency of the transmon and the frequency of the dressed cavity can be varied with the applied external flux bias, which are well consistent with the theoretical model. The range of the variable transition frequency is from 5.188 GHz to 7.756 GHz. The energy relaxation time of the transmon is hundreds of nanoseconds.

Monogamous nature of symmetric N-qubit states of the W class: Concurrence and negativity tangle

P. J. Geetha, K. O. Yashodamma, Sudha
Chin. Phys. B 2015, 24 (11): 110302 ;  doi: 10.1088/1674-1056/24/11/110302
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Using Majorana representation of symmetric N-qubit pure states, we have examined the monogamous nature of the family of states with two-distinct spinors, the W class of states. We have evaluated the N-concurrence tangle and showed that all the states in this family have vanishing concurrence tangle. The negativity tangle for the W class of states is shown to be non-zero, illustrating the fact that the concurrence tangle is always lesser than or equal to the negativity tangle in a pure N-qubit state.

Effect of hydrostatic pressure and polaronic mass of the binding energy in a spherical quantum dot

A. Rejo Jeice, Sr. Gerardin Jayam, K. S. Joseph Wilson
Chin. Phys. B 2015, 24 (11): 110303 ;  doi: 10.1088/1674-1056/24/11/110303
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Simultaneous effect of hydrostatic pressure and polaronic mass on the binding energies of the ground and excited states of an on-center hydrogenic impurity confined in a GaAs/GaAlAs spherical quantum dot are theoretically investigated by the variational method within the effective mass approximation. The binding energy is calculated as a function of dot radius and pressure. Our findings proved that the hydrostatic pressure led to the decrease of confined energy and the increase of donor binding energy. Conduction band non-parabolicity and the polaron masses are effective in the donor binding energy which is significant for narrow dots not in the confined energy. The maximum donor binding energy achieved by the polaronic mass in the ground and excited states are 2%-19% for the narrow dots. The confined and donor binding energies approach zero as the dot size approaches infinity.

Dynamical decoupling pulses on the quantum correlations for the system of superconducting quantum circuit

Wang Dong-Mei, Qian Yi, Xu Jing-Bo, Yu You-Hong
Chin. Phys. B 2015, 24 (11): 110304 ;  doi: 10.1088/1674-1056/24/11/110304
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We investigate the influence of the dynamical decoupling pulses on the quantum correlations in a superconducting system consisting of two noninteracting qubits interacting with their own data buses. It is found that the geometric discord and entanglement between the two superconducting qubits can be increased by applying a train of π-phase pulses. We then proceed to explore how the decoupling pulses affect the quantum transfer of information between the two superconducting qubits by making use of the change of trace distance.

Entanglement dynamics of a three-qubit system with different interatomic distances

Feng Ling-Juan, Zhang Ying-Jie, Zhang Lu, Xia Yun-Jie
Chin. Phys. B 2015, 24 (11): 110305 ;  doi: 10.1088/1674-1056/24/11/110305
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We investigate the tripartite entanglement dynamics of three two-level atoms in a multi-mode vacuum field. By considering the influences of the interatomic distance and the initial condition on the lower bound of concurrence and the tripartite negativity, we show that an optimal interatomic distance can be found to minimize the collective damping. Interestingly, at the same optimal distance, the tripartite entanglement would be maximized in the open dynamics process. In the case of shorter interatomic distance, the tripartite entanglement can display the oscillatory behavior in the initial short-time limit and be trapped in a stationary value in the long-time limit. In addition, the tripartite entanglement for the general situation with different interatomic distances is also discussed.

Thermal entanglement of the Ising–Heisenberg diamond chain with Dzyaloshinskii–Moriya interaction

Qiao Jie, Zhou Bin
Chin. Phys. B 2015, 24 (11): 110306 ;  doi: 10.1088/1674-1056/24/11/110306
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We investigate the thermal entanglement in a spin-1/2 Ising-Heisenberg diamond chain, in which the vertical Heisenberg spin dimers alternate with single Ising spins. Due to the fact that the Dzyaloshinskii-Moriya (DM) interaction contributes to unusual and interesting magnetic properties in actual materials, and moreover it plays a significant role in the degree of the entanglement of the Heisenberg quantum spin systems, we focus on the effects of different DM interactions, including Dz and Dx, on the thermal entanglement of the Heisenberg spin dimer. The concurrence, as a measure of spin dimer entanglement, is calculated for different values of exchange interactions, DM interaction, external magnetic field, and temperature. It is found that the critical temperature and the critical magnetic field corresponding to the vanishing of entanglement increase with DM interaction, and the entanglement revival region gets larger by increasing DM interaction, thus DM interaction favors the formation of the thermal entanglement. It is observed that different DM interaction parameters (Dz and Dx) have remarkably different influences on the entanglement. Different from the case Dz, there is the non-monotonic variation of the concurrence with temperature in the case Dx, and additionally the DM interaction Dx can induce the entanglement near zero temperature in the case that the antiferromagnetic Ising-type interaction constant is larger than the antiferromagnetic Heisenberg interaction constant. It is also shown that for the same value of DM interaction the critical magnetic field of the case Dx is larger than that of the case Dz.

Passive decoy-state quantum key distribution using weak coherent pulses with modulator attenuation

Li Yuan, Bao Wan-Su, Li Hong-Wei, Zhou Chun, Wang Yang
Chin. Phys. B 2015, 24 (11): 110307 ;  doi: 10.1088/1674-1056/24/11/110307
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Passive decoy-state quantum key distribution is more desirable than the active one in some scenarios. It is also affected by the imperfections of the devices. In this paper, the influence of modulator attenuation on the passive decoy-state method is considered. We introduce and analyze the unbalanced Mach-Zehnder interferometer, briefly, and combining with the virtual source and imaginary unitary transformation, we characterize the passive decoy-state method using a weak coherent photon source with modulator attenuation. According to the attenuation parameter δ, the pass efficiencies are given. Then, the key generation rate can be acquired. From numerical simulations, it can be seen that modulator attenuation has a non-negligible influence on the performance of passive-state QKD protocol. Based on the research, the analysis method of virtual source and imaginary unitary transformation are preferred in analyzing passive decoy state protocol, and the passive decoy-state method is better than the active one and is close to the active vacuum + weak decoy state under the condition of having the same modulator attenuation.

Efficient schemes of joint remote preparation with a passive receiver via EPR pairs

Ma Song-Ya, Gao Cong, Luo Ming-Xing
Chin. Phys. B 2015, 24 (11): 110308 ;  doi: 10.1088/1674-1056/24/11/110308
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Novel schemes are put forward to execute the joint remote preparation of an arbitrary two-qubit state with a passive receiver via EPR pairs as the entangled channel. Compared with the previous protocols, the required multi-particle measurement is simplified and the classical communication cost is reduced. When the number of senders increases, the advantage is more evident. It means that the proposed schemes are more efficient in practice.

Optimized quantum random-walk search algorithm for multi-solution search

Zhang Yu-Chao, Bao Wan-Su, Wang Xiang, Fu Xiang-Qun
Chin. Phys. B 2015, 24 (11): 110309 ;  doi: 10.1088/1674-1056/24/11/110309
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This study investigates the multi-solution search of the optimized quantum random-walk search algorithm on the hypercube. Through generalizing the abstract search algorithm which is a general tool for analyzing the search on the graph to the multi-solution case, it can be applied to analyze the multi-solution case of quantum random-walk search on the graph directly. Thus, the computational complexity of the optimized quantum random-walk search algorithm for the multi-solution search is obtained. Through numerical simulations and analysis, we obtain a critical value of the proportion of solutions q. For a given q, we derive the relationship between the success rate of the algorithm and the number of iterations when q is no longer than the critical value.

Stability of a delayed predator–prey model in a random environment

Jin Yan-Fei, Xie Wen-Xian
Chin. Phys. B 2015, 24 (11): 110501 ;  doi: 10.1088/1674-1056/24/11/110501
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The stability of the first-order and second-order solution moments for a Harrison-type predator-prey model with parametric Gaussian white noise is analyzed in this paper. The moment equations of the system solution are obtained under Itô interpretations. The delay-independent stable condition of the first-order moment is identical to that of the deterministic delayed system, and the delay-independent stable condition of the second-order moment depends on the noise intensities. The corresponding critical time delays are determined once the stabilities of moments lose. Further, when the time delays are greater than the critical time delays, the system solution becomes unstable with the increase of noise intensities. Finally, some numerical simulations are given to verify the theoretical results.

Achieving high bit rate logical stochastic resonance in a bistable system by adjusting parameters

Yang Ding-Xin, Gu Feng-Shou, Feng Guo-Jin, Yang Yong-Min, Andrew Ball
Chin. Phys. B 2015, 24 (11): 110502 ;  doi: 10.1088/1674-1056/24/11/110502
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The phenomenon of logical stochastic resonance (LSR) in a nonlinear bistable system is demonstrated by numerical simulations and experiments. However, the bit rates of the logical signals are relatively low and not suitable for practical applications. First, we examine the responses of the bistable system with fixed parameters to different bit rate logic input signals, showing that an arbitrary high bit rate LSR in a bistable system cannot be achieved. Then, a normalized transform of the LSR bistable system is introduced through a kind of variable substitution. Based on the transform, it is found that LSR for arbitrary high bit rate logic signals in a bistable system can be achieved by adjusting the parameters of the system, setting bias value and amplifying the amplitudes of logic input signals and noise properly. Finally, the desired OR and AND logic outputs to high bit rate logic inputs in a bistable system are obtained by numerical simulations. The study might provide higher feasibility of LSR in practical engineering applications.

Chaotic synchronization in Bose–Einstein condensate of moving optical lattices via linear coupling

Zhang Zhi-Ying, Feng Xiu-Qin, Yao Zhi-Hai, Jia Hong-Yang
Chin. Phys. B 2015, 24 (11): 110503 ;  doi: 10.1088/1674-1056/24/11/110503
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A systematic study of the chaotic synchronization of Bose-Einstein condensed body is performed using linear coupling method based on Lyapunov stability theory, Sylvester’s criterion, and Gerschgorin disc theorem. The chaotic synchronization of Bose-Einstein condensed body in moving optical lattices is realized by linear coupling. The relationship between the synchronization time and coupling coefficient is obtained. Both the single-variable coupling and double-variable coupling are effective. The results of numerical calculation prove that the chaotic synchronization of double-variable coupling is faster than that of single-variable coupling and small coupling coefficient can achieve the chaotic synchronization. Weak noise has little influence on synchronization effect, so the linear coupling technology is suitable for the chaotic synchronization of Bose-Einstein condensate.

Full-order sliding mode control of uncertain chaos in a permanent magnet synchronous motor based on a fuzzy extended state observer

Chen Qiang, Nan Yu-Rong, Zheng Heng-Huo, Ren Xue-Mei
Chin. Phys. B 2015, 24 (11): 110504 ;  doi: 10.1088/1674-1056/24/11/110504
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A full-order sliding mode control based on a fuzzy extended state observer is proposed to control the uncertain chaos in the permanent magnet synchronous motor. Through a simple coordinate transformation, the chaotic PMSM model is transformed into the Brunovsky canonical form, which is more suitable for the controller design. Based on the fuzzy control theory, a fuzzy extended state observer is developed to estimate the unknown states and uncertainties, and the restriction that all the system states should be completely measurable is avoided. Thereafter, a full-order sliding mode controller is designed to ensure the convergence of all system states without any chattering problem. Comparative simulations show the effectiveness and superior performance of the proposed control method.

Chaotic maps and biometrics-based anonymous three-party authenticated key exchange protocol without using passwords

Xie Qi, Hu Bin, Chen Ke-Fei, Liu Wen-Hao, Tan Xiao
Chin. Phys. B 2015, 24 (11): 110505 ;  doi: 10.1088/1674-1056/24/11/110505
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In three-party password authenticated key exchange (AKE) protocol, since two users use their passwords to establish a secure session key over an insecure communication channel with the help of the trusted server, such a protocol may suffer the password guessing attacks and the server has to maintain the password table. To eliminate the shortages of password-based AKE protocol, very recently, according to chaotic maps, Lee et al. [2015 Nonlinear Dyn. 79 2485] proposed a first three-party-authenticated key exchange scheme without using passwords, and claimed its security by providing a well-organized BAN logic test. Unfortunately, their protocol cannot resist impersonation attack, which is demonstrated in the present paper. To overcome their security weakness, by using chaotic maps, we propose a biometrics-based anonymous three-party AKE protocol with the same advantages. Further, we use the pi calculus-based formal verification tool ProVerif to show that our AKE protocol achieves authentication, security and anonymity, and an acceptable efficiency.

Performance characteristics of low-dissipative generalized Carnot cycles with external leakage losses

Huang Chuan-Kun, Guo Jun-Cheng, Chen Jin-Can
Chin. Phys. B 2015, 24 (11): 110506 ;  doi: 10.1088/1674-1056/24/11/110506
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Under the assumption of low-dissipation, a unified model of generalized Carnot cycles with external leakage losses is established. Analytical expressions for the power output and efficiency are derived. The general performance characteristics between the power output and the efficiency are revealed. The maximum power output and efficiency are calculated. The lower and upper bounds of the efficiency at the maximum power output are determined. The results obtained here are universal and can be directly used to reveal the performance characteristics of different Carnot cycles, such as Carnot heat engines, Carnot-like heat engines, flux flow engines, gravitational engines, chemical engines, two-level quantum engines, etc.

Composite behaviors of dual meminductor circuits

Zheng Ci-Yan, Yu Dong-Sheng, Liang Yan, Chen Meng-Ke
Chin. Phys. B 2015, 24 (11): 110701 ;  doi: 10.1088/1674-1056/24/11/110701
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This paper focuses on analyzing the composite dynamic behaviors of two meminductors in serial and parallel connections with different polarities. Based on the constitutive relations, two time-integral-of-flux (TIF) controlled meminductors are adopted to theoretically demonstrate the variation of memductance in terms of TIF, charge, flux, and current. By utilizing a floating memristor-less meminductor emulator, the theoretical analysis reported in this paper is confirmed via a PSPICE simulation study and hardware experiment. Good agreement among theoretical analysis, simulation, and hardware validation confirms that dual meminductor circuits in composite connections behave as a new meminductor with higher complexity.

TOPICAL REVIEW—Interface-induced high temperature superconductivity

In situ electrical transport measurement of superconductive ultrathin films

Liu Can-Hua, Jia Jin-Feng
Chin. Phys. B 2015, 24 (11): 110702 ;  doi: 10.1088/1674-1056/24/11/110702
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The discovery of an extraordinarily superconductive large energy gap in SrTiO3 supported single-layer FeSe films has recently initiated a great deal of research interests in surface-enhanced superconductivity and superconductive ultrathin films fabricated on crystal surfaces. On account of the instability of ultra-thin films in air, it is desirable to perform electrical transport measurement in ultra-high vaccum (UHV). Here we review the experimental techniques of in situ electrical transport measurement and their applications on superconductive ultrathin films.

First-principles study of FeSe epitaxial films on SrTiO3

Liu Kai, Gao Miao, Lu Zhong-Yi, Xiang Tao
Chin. Phys. B 2015, 24 (11): 117402 ;  doi: 10.1088/1674-1056/24/11/117402
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The discovery of high temperature superconductivity in FeSe films on SrTiO3 substrate has inspired great experimental and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of FeSe epitaxial films, emphasizing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermore, the comparison between FeSe monolayer and bilayer films on SrTiO3 is discussed.

Direct evidence of high temperature superconductivity in one-unit-cell FeSe films on SrTiO3 substrate by transport and magnetization measurements

Xing Ying, Wang Jian
Chin. Phys. B 2015, 24 (11): 117404 ;  doi: 10.1088/1674-1056/24/11/117404
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Zero resistance and Meissner effect are two crucial experimental evidences of superconductivity in determining a new kind of superconductor, which can be detected by transport and diamagnetic measurements. In this paper, we briefly review the main transport and magnetization results on the one unit cell (1-UC) FeSe films grown on SrTiO3 (STO) substrates from our team in recent years, which identify the high temperature superconductivity in 1-UC FeSe films.

What makes the Tc of FeSe/SrTiO3 so high?

Dung-Hai Lee
Chin. Phys. B 2015, 24 (11): 117405 ;  doi: 10.1088/1674-1056/24/11/117405
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This paper reviews some of the recent progresses in the study of high temperature superconductivity in the interface between a single unit cell FeSe and SrTiO3. It offers the author’s personal view of why Tc is high and how to further increase it.

In-situ spectroscopic studies and interfacial engineering on FeSe/oxide heterostructures: Insights on the interfacial superconductivity

Peng Rui, Xu Hai-Chao, Feng Dong-Lai
Chin. Phys. B 2015, 24 (11): 117902 ;  doi: 10.1088/1674-1056/24/11/117902
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The discovery of high temperature superconductivity in single-layer FeSe/SrTiO3 provides a new platform for exploring superconductivity and pursuing higher Tc (superconducting transition temperature) through fabricating artificial heterostructures. In this paper, we review the recent progress in studying and tuning the interfacial superconductivity in single-layer FeSe, through the combined in-situ spectroscopic studies and atomic-scale engineering. By fabricating artificial heterostructures, various interfacial factors were tuned, and the corresponding evolutions of electronic structure and superconducting gap behavior were investigated. These studies enrich the current understanding on the interfacial superconductivity, and provide clues for further enhancing Tc through interface engineering.

Stark-potential evaporative cooling of polar molecules in a novel optical-access opened electrostatic trap

Sun Hui, Wang Zhen-Xia, Wang Qin, Li Xing-Jia, Liu Jian-Ping, Yin Jian-Ping
Chin. Phys. B 2015, 24 (11): 113101 ;  doi: 10.1088/1674-1056/24/11/113101
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We propose a novel optical-access opened electrostatic trap to study the Stark-potential evaporative cooling of polar molecules by using two charged disk electrodes with a central hole of radius r0=1.5 mm, and derive a set of new analytical equations to calculate the spatial distributions of the electrostatic field in the above charged-disk layout. Afterwards, we calculate the electric-field distributions of our electrostatic trap and the Stark potential for cold ND3 molecules, and analyze the dependences of both the electric field and the Stark potential on the geometric parameters of our charged-disk scheme, and find an optimal condition to form a desirable trap with the same trap depth in the x, y, and z directions. Also, we propose a desirable scheme to realize an efficient loading of cold polar molecules in the weak-field-seeking states, and investigate the dependences of the loading efficiency on both the initial forward velocity of the incident molecular beam and the loading time by Monte Carlo simulations. Our study shows that the maximal loading efficiency of our trap scheme can reach about 95%, and the corresponding temperature of the trapped cold molecules is about 28.8 mK. Finally, we study the Stark-potential evaporative cooling for cold polar molecules in our trap by the Monte Carlo method, and find that our simulated evaporative cooling results are consistent with our developed analytical model based on trapping-potential evaporative cooling.

Field ionization process of Eu 4f76snp Rydberg states

Zhang Jing, Shen Li, Dai Chang-Jian
Chin. Phys. B 2015, 24 (11): 113201 ;  doi: 10.1088/1674-1056/24/11/113201
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The field ionization process of the Eu 4f76snp Rydberg states, converging to the first ionization limit, 4f76s 9S4, is systematically investigated. The spectra of the Eu 4f76snp Rydberg states are populated with three-step laser excitation, and detected by electric field ionization (EFI) method. Two different kinds of the EFI pulses are applied after laser excitation to observe the possible impacts on the EFI process. The exact EFI ionization thresholds for the 4f76snp Rydberg states can be determined by observing the corresponding EFI spectra. In particular, some structures above the EFI threshold are found in the EFI spectra, which may be interpreted as the effect from black body radiation (BBR). Finally, the scaling law of the EFI threshold for the Eu 4f76snp Rydberg states with the effective quantum number is built.

Ionizations and fragmentations of benzene, methylbenzene, and chlorobenzene in strong IR and UV laser fields

Zhang Jun-Feng, Lü Hang, Zuo Wan-Long, Xu Hai-Feng, Jin Ming-Xing, Ding Da-Jun
Chin. Phys. B 2015, 24 (11): 113301 ;  doi: 10.1088/1674-1056/24/11/113301
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Ionizations and fragmentations of benzene, methylbenzene, and chlorobenzene are studied in linearly polarized 50-fs, 800-nm and 400-nm strong laser fields using a time-of-flight mass spectrometer. It is shown that at low laser intensity, the parent ions are dominant for any one of the molecules in an 800-nm strong laser field, while extensive fragmentation is observed in a 400-nm laser field, which can be understood by the resonant photon absorption of molecular cations. The ratio of the yield of the parent ion to the yield of the total ion for each molecule is measured as a function of laser intensity in a range from 1.0×1013 W/cm2 to 4.0×1014 W/cm2, in either the 800-nm or 400-nm laser field. The results show that the fragmentation of the aromatic molecules increases significantly as the laser intensity is increased. Possible mechanisms for fragmentation in strong laser fields are discussed. Finally, the saturation intensity of ionization of the titled molecules is also determined.

Resonance enhanced electron impact excitation for P-like Cu XV

Li Shuang, Yan Jun, Li Chuan-Ying, Huang Min, Chen Chong-Yang
Chin. Phys. B 2015, 24 (11): 113401 ;  doi: 10.1088/1674-1056/24/11/113401
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Employing both the Dirac R-matrix and the relativistic distorted wave with independent process and isolated resonance approaches, we report resonance enhanced electron impact excitation data (specifically, effective collision strengths) among the lowest 41 levels from the n=3 configurations of Cu XV. The results show that the latter approach can obtain resonance contributions reasonably well for most excitations of Cu XV, though a comparison between the two approaches shows that the close-coupling effects are truly significant for rather weak excitations, especially for two-electron excitations from the 3s3p4 to 3s23p23d configuration. Resonance contributions are significant (more than two orders of magnitude) for many excitations and dramatically influence the line intensity ratios associated with density diagnostics.

Vector correlations study of the reaction N(2D)+ H2(X1Σg+)→NH(a1Δ)+ H(2S) with different collision energies and reagent vibration excitations

Li Yong-Qing, Zhang Yong-Jia, Zhao Jin-Feng, Zhao Mei-Yu, Ding Yong
Chin. Phys. B 2015, 24 (11): 113402 ;  doi: 10.1088/1674-1056/24/11/113402
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Vector correlations of the reaction N(2D)+ H2(X1Σg+)→NH(a1Δ)+ H(2S)are studied based on a recent DMBESEC PES for the first excited state of NH2 [J. Phys. Chem. A 114 9644 (2010)] by using a quasi-classical trajectory method. The effects of collision energy and the reagent initial vibrational excitation on cross section and product polarization are investigated for v = 0–5 and j = 0 states in a wide collision energy range (10–50 kcal/mol). The integral cross section could be increased by H2 vibration excitation remarkably based on the DMBE-SEC PES. The different phenomena of differential cross sections with different collision energies and reagent vibration excitations are explained. Particularly, the NH molecules are scattered mainly in the backward hemisphere at low vibration quantum number and evolve from backward to forward direction with increasing vibration quantum number, which could be explained by the fact that the vibrational excitation enlarges the H–H distance in the entrance channel, thus enhancing the probability of collision between N atom and H atom. A further study on product polarization demonstrates that the collision energy and vibrational excitation of the reagent remarkably influence the distributions of P(θr), P(φr), and P(θr,φr).

Kinetics of protein adsorption/desorption mediated by pH-responsive polymer layer

Su Xiao-Hang, Lei Qun-Li, Ren Chun-Lai
Chin. Phys. B 2015, 24 (11): 113601 ;  doi: 10.1088/1674-1056/24/11/113601
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We propose a new way of regulating protein adsorption by using a pH-responsive polymer. According to the theoretical results obtained from the molecular theory and kinetic approaches, both thermodynamics and kinetics of protein adsorption are verified to be well controlled by the solution pH. The kinetics and the amount of adsorbed proteins at equilibrium are greatly increased when the solution environment changes from acid to neutral. The reason is that the increased pH promotes the dissociation of the weak polyelectrolyte, resulting in more charged monomers and more stretched chains. Thus the steric repulsion within the polymer layer is weakened, which effectively lowers the barrier felt by the protein during the process of adsorption. Interestingly, we also find that the kinetics of protein desorption is almost unchanged with the variation of pH. It is because although the barrier formed by the polymer layer changes along with the change of pH, the potential at contact with the surface varies equally. Our results may provide useful insights into controllable protein adsorption/desorption in practical applications.

Photostop of iodine atoms from electrically oriented ICl molecules

Bao Da-Xiao, Deng Lian-Zhong, Xu Liang, Yin Jian-Ping
Chin. Phys. B 2015, 24 (11): 113702 ;  doi: 10.1088/1674-1056/24/11/113702
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The dynamics of photostopping iodine atoms from electrically oriented ICl molecules was numerically studied based on their orientational probability distribution functions. Velocity distributions of the iodine atoms and their production rates were investigated for orienting electrical fields of various intensities. For the ICl precursor beams with an initial rotational temperature of ~1 K, the production of the iodine atoms near zero speed will be improved by about ~5 times when an orienting electrical field of ~200 kV/cm is present. A production rate of ~0.5‰ is obtained for photostopped iodine atoms with speeds less than 10 m/s, which are suitable for magnetic trapping. The electrical orientation of ICl precursors and magnetic trapping of photostopped iodine atoms in situ can be conveniently realized with a pair of charged ring magnets. With the maximal value of the trapping field being ~0.28 T, the largest trapping speed is ~7.0 m/s for the iodine atom.

Determination of ion quantity by using low-temperature ion density theory and molecular dynamics simulation

Du Li-Jun, Song Hong-Fang, Li Hai-Xia, Chen Shao-Long, Chen Ting, Sun Huan-Yao, Huang Yao, Tong Xin, Guan Hua, Gao Ke-Lin
Chin. Phys. B 2015, 24 (11): 113703 ;  doi: 10.1088/1674-1056/24/11/113703
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In this paper, we report a method by which the ion quantity is estimated rapidly with an accuracy of 4%. This finding is based on the low-temperature ion density theory and combined with the ion crystal size obtained from experiment with the precision of a micrometer. The method is objective, straightforward, and independent of the molecular dynamics (MD) simulation. The result can be used as the reference for the MD simulation, and the method can improve the reliability and precision of MD simulation. This method is very helpful for intensively studying ion crystal, such as phase transition, spatial configuration, temporal evolution, dynamic character, cooling efficiency, and the temperature limit of the ions.

Rapid extraction of the phase shift of the cold-atom interferometer via phase demodulation

Cheng Bing, Wang Zhao-Ying, Xu Ao-Peng, Wang Qi-Yu, Lin Qiang
Chin. Phys. B 2015, 24 (11): 113704 ;  doi: 10.1088/1674-1056/24/11/113704
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Generally, the phase of the cold-atom interferometer is extracted from the atomic interference fringe, which can be obtained by scanning the chirp rate of the Raman lasers at a given interrogation time T. If mapping the phase shift for each T with a series of measurements, the extraction time is limited by the protocol of each T measurement, and therefore increases dramatically when doing fine mapping with a small step of T. Here we present a new method for rapid extraction of the phase shift via phase demodulation. By using this method, the systematic shifts can be mapped though the whole interference area. This method enables quick diagnostics of the potential cause of the phase shift in specific time. We demonstrate experimentally that this method is effective for the evaluation of the systematic errors of the cold atomic gravimeter. The systematic phase error induced by the quadratic Zeeman effect in the free-falling region is extracted by this method. The measured results correspond well with the theoretic prediction and also agree with the results obtained by the fringe fitting method for each T.

Temperature and number evolution of cold cesium atoms inside a wall-coated glass cell

Huang Jia-Qiang, Zhang Jian-Wei, Wang Shi-Guang, Wang Zheng-Bo, Wang Li-Jun
Chin. Phys. B 2015, 24 (11): 113701 ;  doi: 10.1088/1674-1056/24/11/113701
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We report an experimental study on the temperature and number evolution of cold cesium atoms diffusively cooled inside a wall-coated glass cell by measuring the absorption profile of the 62S1/2 (F=4)→2P3/2 (F’=5) transition line with a weak probe laser in the evolution process. We found that the temperature of the cold atoms first gradually decreases from 16 mK to 9 mK, and then rapidly increases. The number of cold atoms first declines slowly from 2.1×109 to 3.7×108 and then falls drastically. A theoretical model for the number evolution is built and includes the instantaneous temperature of the cold atoms and a fraction p, which represents the part of cold cesium atoms elastically reflected by the coated cell wall. The theory is overall in good agreement with the experimental result, and a nonzero value is obtained for the fraction p, which indicates that the cold cesium atoms are not all heated to the ambient temperature by a single collision with the coated cell wall. These results can provide helpful insight for precision measurements based on diffuse laser cooling.

Kramers-Kronig relation in a Doppler-broadened Λ -type three-level system

Wang Meng, Lu Xiao-Gang, Bai Jin-Hai, Pei Li-Ya, Miao Xing-Xu, Gao Yan-Lei, Wu Ling-An, Fu Pan-Ming, Yang Shi-Ping, Pang Zhao-Guang, Wang Ru-Quan, Zuo Zhan-Chun
Chin. Phys. B 2015, 24 (11): 114205 ;  doi: 10.1088/1674-1056/24/11/114205
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We measure the absorption and dispersion in a Doppler-broadened Λ -type three level system by resonant stimulated Raman spectroscopy with homodyne detection. Through studying the dressed state energies of the system, it is found that the absorption and dispersion satisfy the Kramers-Kronig relation. The absorption and dispersion spectra calculated by employing this relation agree well with our experimental observations.

Defocusing role in femtosecond filamentation: Higher-order Kerr effect or plasma effect?

Li Su-Yu, Guo Fu-Ming, Yang Yu-Jun, Jin Ming-Xing
Chin. Phys. B 2015, 24 (11): 114207 ;  doi: 10.1088/1674-1056/24/11/114207
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The femtosecond filamentation in the classical and high-order Kerr (HOK) models is numerically investigated by adopting multi-photon ionization (MPI) cross section with different values. It is found that in the case that the MPI cross section is relatively small, there exists a big difference between the electron density as well as clamped intensity calculated in the classical model and those calculated in the HOK one, while in the case that the MPI cross section is relatively large, the electron density and clamped intensity calculated in the two models are nearly in agreement with each other, and under this circumstance, even if the higher-order nonlinear terms do exist, the free-charge generation and the associated defocusing in a filament are enough to mask their effects. The different behaviors of the maximum intensity and on-axis electron density at the collapse position with the pulse duration provides an approach to determine which effect plays the dominant defocusing role. These results demonstrate that it is ionization that results in the difference between the two models.

Deflections of photoelectron classical trajectories in screened Coulomb potentials of H2+

Qin Bo-Ya, Wang Pei-Jie, He Feng
Chin. Phys. B 2015, 24 (11): 114208 ;  doi: 10.1088/1674-1056/24/11/114208
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The photoelectron momentum distribution of H2+ in circularly polarized laser fields is studied based on classical trajectory calculations. We screen Coulomb potentials at different radii, and trace trajectories of an ensemble of electrons in such screened Coulomb potentials and circularly polarized laser fields. Simulations show that electron trajectories are bent by Coulomb fields, resulting in the laser-intensity-dependent drift of photoelectron momentum distributions in the laser polarization plane. This study intuitively explains how Coulomb potentials modify photoelectron momenta.

Enhancement of photoacoustic tomography in the tissue with speed-of-sound variance using ultrasound computed tomography

Cheng Ren-Xiang, Tao Chao, Liu Xiao-Jun
Chin. Phys. B 2015, 24 (11): 114301 ;  doi: 10.1088/1674-1056/24/11/114301
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The speed-of-sound variance will decrease the imaging quality of photoacoustic tomography in acoustically inhomogeneous tissue. In this study, ultrasound computed tomography is combined with photoacoustic tomography to enhance the photoacoustic tomography in this situation. The speed-of-sound information is recovered by ultrasound computed tomography. Then, an improved delay-and-sum method is used to reconstruct the image from the photoacoustic signals. The simulation results validate that the proposed method can obtain a better photoacoustic tomography than the conventional method when the speed-of-sound variance is increased. In addition, the influences of the speed-of-sound variance and the fan-angle on the image quality are quantitatively explored to optimize the image scheme. The proposed method has a good performance even when the speed-of-sound variance reaches 14.2%. Furthermore, an optimized fan angle is revealed, which can keep the good image quality with a low cost of hardware. This study has a potential value in extending the biomedical application of photoacoustic tomography.

Dynamics of quantum Fisher information in a two-level system coupled to multiple bosonic reservoirs

Wang Guo-You, Guo You-Neng, Zeng Ke
Chin. Phys. B 2015, 24 (11): 114201 ;  doi: 10.1088/1674-1056/24/11/114201
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We consider the optimal parameter estimation for a two-level system coupled to multiple bosonic reservoirs. By using quantum Fisher information (QFI), we investigate the effect of the Markovian reservoirs’ number N on QFI in both weak and strong coupling regimes for a two-level system surrounded by N zero-temperature reservoirs of field modes initially in the vacua. The results show that the dynamics of QFI non-monotonically decays to zero with revival oscillations at some time in the weak coupling regime depending on the reservoirs’ parameters. Furthermore, we also present the relations between the QFI flow, the flows of energy and information, and the sign of the decay rate to gain insight into the physical processes characterizing the dynamics.

Sub-Poissonian photon emission in coupled double quantum dots–cavity system

Ye Han, Peng Yi-Wei, Yu Zhong-Yuan, Zhang Wen, Liu Yu-Min
Chin. Phys. B 2015, 24 (11): 114202 ;  doi: 10.1088/1674-1056/24/11/114202
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In this work, we theoretically analyze the few-photon emissions generated in a coupled double quantum dots (CDQDs)-single mode microcavity system, under continuous wave and pulse excitation. Compared with the uncoupled case, strong sub-Poissonian character is achieved in a CDQDs-cavity system at a certain laser frequency. Based on the proposed scheme, single photon generation can be obtained separately under QD-cavity resonant condition and off-resonant condition. For different cavity decay rates, we reveal that laser frequency detunings of minimum second-order autocorrelation function are discrete and can be divided into three regions. Moreover, the non-ideal situation where two QDs are not identical is discussed, indicating the robustness of the proposed scheme, which possesses sub-Poissonian character in a large QD difference variation range.

Fabrication of 16 W all-normal-dispersion mode-locked Yb-doped rod-type fiber laser with large-mode area

Lü Zhi-Guo, Teng Hao, Wang Li-Na, Wang Rui, Wang Jun-Li, Wei Zhi-Yi
Chin. Phys. B 2015, 24 (11): 114203 ;  doi: 10.1088/1674-1056/24/11/114203
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A mode-locked ytterbium-doped rod-type fiber laser with 85 μ core diameter is developed based on the nonlinear polarization evolution in an all-normal-dispersion ring cavity, in which a uniaxial birefringent plate is used as the spectral filter. Average power up to 16 W is obtained at the repetition rate of 58 MHz, and the pulse duration is compressed to 182 fs with a grating-pair compressor. The output laser pulses show very good beam quality and power stability.

Quantitative measurements of one-dimensional OH absolute concentration profiles in a methane/air flat flame by bi-directional laser-induced fluorescence

Yu Xin, Yang Zhen, Peng Jiang-Bo, Zhang Lei, Ma Yu-Fei, Yang Chao-Bo, Li Xiao-Hui, Sun Rui
Chin. Phys. B 2015, 24 (11): 114204 ;  doi: 10.1088/1674-1056/24/11/114204
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The one-dimensional (1D) spatial distributions of OH absolute concentration in methane/air laminar premixed flat flame under different equivalence ratios at atmospheric pressure are investigated by using bi-directional laser-induced fluorescence (LIF) detection scheme combined with the direct absorption spectroscopy. The effective peak absorption cross section and the average temperature at a height of 2 mm above the burner are obtained by exciting absorption on the Q1(8) rotational line in the m A2Σ+ (v’= 0) ≤←X2Π (v"= 0) at 309.240 nm. The measured values are 1.86×10-15 cm2 and 1719 K, respectively. Spatial filtering and frequency filtering methods of reducing noise are used to deal with the experimental data, and the smoothing effects are also compared using the two methods. The spatial distribution regularities of OH concentration are obtained with the equivalence ratios ranging from 0.8 to 1.3. The spatial resolution of the measured result is 84 μm. Finally, a comparison is made between the experimental result of this paper and other relevant study results.

Role of the aperture in Z-scan experiments: A parametric study

M. R. Rashidian Vaziri
Chin. Phys. B 2015, 24 (11): 114206 ;  doi: 10.1088/1674-1056/24/11/114206
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In close-aperture Z-scan experiments, a small aperture is conventionally located in the far-field thereby enabling the detection of slight changes in the laser beam profile due to the Kerr-lensing effect. In this work, by numerically solving the Fresnel-Kirchhoff diffraction integrals, the amount of transmitted power through apertures has been evaluated and a parametric study on the role of the various parameters that can influence this transmitted power has been done. In order to perform a comprehensive analysis, we have used a nonlinear phase shift optimized for nonlocal nonlinear media in our calculations. Our results show that apertures will result in the formation of symmetrical fluctuations on the wings of Z-scan transmittance curves. It is further shown that the appearance of these fluctuations can be ascribed to the natural diffraction of the Gaussian beam as it propagates up to the aperture plane. Our calculations reveal that the nonlocal parameter variations can shift the position of fluctuations along the optical axis, whereas their magnitude depends on the largeness of the induced nonlinear phase shift. It is concluded that since the mentioned fluctuations are produced by the natural diffraction of the Gaussian beam itself, one must take care not to mistakenly interpret them as noise and should not expect to eliminate them from experimental Z-scan transmittance curves by using apertures with different sizes.

Simultaneous reconstruction of temperature distribution and radiative properties in participating media using a hybrid LSQR–PSO algorithm

Niu Chun-Yang, Qi Hong, Huang Xing, Ruan Li-Ming, Wang Wei, Tan He-Ping
Chin. Phys. B 2015, 24 (11): 114401 ;  doi: 10.1088/1674-1056/24/11/114401
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A hybrid least-square QR decomposition (LSQR)-particle swarm optimization (LSQR-PSO) algorithm was developed to estimate the three-dimensional (3D) temperature distributions and absorption coefficients simultaneously. The outgoing radiative intensities at the boundary surface of the absorbing media were simulated by the line-of-sight (LOS) method, which served as the input for the inverse analysis. The retrieval results showed that the 3D temperature distributions of the participating media with known radiative properties could be retrieved accurately using the LSQR algorithm, even with noisy data. For the participating media with unknown radiative properties, the 3D temperature distributions and absorption coefficients could be retrieved accurately using the LSQR-PSO algorithm even with measurement errors. It was also found that the temperature field could be estimated more accurately than the absorption coefficients. In order to gain insight into the effects on the accuracy of temperature distribution reconstruction, the selection of the detection direction and the angle between two detection directions was also analyzed.

Discontinuity of mode transition and hysteresis in hydrogen inductively coupled plasma via a fluid model

Xu Hui-Jing, Zhao Shu-Xia, Gao Fei, Zhang Yu-Ru, Li Xue-Chun, Wang You-Nian
Chin. Phys. B 2015, 24 (11): 115201 ;  doi: 10.1088/1674-1056/24/11/115201
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A new type of two-dimensional self-consistent fluid model that couples an equivalent circuit module is used to investigate the mode transition characteristics and hysteresis in hydrogen inductively coupled plasmas at different pressures, by varying the series capacitance of the matching box. The variations of the electron density, temperature, and the circuit electrical properties are presented. As cycling the matching capacitance, at high pressure both the discontinuity and hysteresis appear for the plasma parameters and the transferred impedances of both the inductive and capacitive discharge components, while at low pressure only the discontinuity is seen. The simulations predict that the sheath plays a determinative role on the presence of discontinuity and hysteresis at high pressure, by influencing the inductive coupling efficiency of applied power. Moreover, the values of the plasma transferred impedances at different pressures are compared, and the larger plasma inductance at low pressure due to less collision frequency, as analyzed, is the reason why the hysteresis is not seen at low pressure, even with a wider sheath. Besides, the behaviors of the coil voltage and current parameters during the mode transitions are investigated. They both increase (decrease) at the E to H (H to E) mode transition, indicating an improved (worsened) inductive power coupling efficiency.

Au nanorods-incorporated plasmonic-enhanced inverted organic solar cells

Peng Ling, Mei Yang, Chen Shu-Fen, Zhang Yu-Pei, Hao Jing-Yu, Deng Ling-Ling, Huang Wei
Chin. Phys. B 2015, 24 (11): 115202 ;  doi: 10.1088/1674-1056/24/11/115202
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The effect of Au nanorods (NRs) on optical-to-electric conversion efficiency is investigated in inverted polymer solar cells, in which Au NRs are sandwiched between two layers of ZnO. Accompanied by the optimization of thickness of ZnO covered on Au NRs, a high-power conversion efficiency of 3.60% and an enhanced short-circuit current density (JSC) of 10.87 mA/cm2 are achieved in the poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC60BM)-based inverted cell and the power conversion efficiency (PCE) is enhanced by 19.6% compared with the control device. The detailed analyses of the light absorption characteristics, the simulated scattering induced by Au NRs, and the electromagnetic field around Au NRs show that the absorption improvement in the photoactive layer due to the light scattering from the longitudinal axis and the near-field increase around Au NRs induced by localized surface plasmon resonance plays a key role in enhancing the performances.

Production of intense attosecond vector beam pulse trains based on harmonics

Han Yu-Jing, Liao Guo-Qian, Chen Li-Ming, Li Yu-Tong, Wang Wei-Min, Zhang Jie
Chin. Phys. B 2015, 24 (11): 115203 ;  doi: 10.1088/1674-1056/24/11/115203
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We provide the first report on the harmonics generated by an intense femtosecond vector beam that is normally incident on a solid target. By using 2D particle-in-cell (PIC) codes, we observe the third and the fifth harmonic signals with the same vector structure as the driving beam, and obtain an attosecond vector beam pulse train. We also show that the conversion efficiencies of the third and the fifth harmonics reach their maxima for a plasma density of four times the critical density due to the plasma resonating with the driving force. This method provides a new means of generating intense extreme ultraviolet (XUV) vector beams via ultra-intense laser-driven harmonics.

Simulations of the L–H transition dynamics with different heat and particle sources

Li Hui-Dong, Wang Zhan-Hui, Jan Weiland, Feng Hao, Sun Wei-Guo
Chin. Phys. B 2015, 24 (11): 115204 ;  doi: 10.1088/1674-1056/24/11/115204
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It is crucial to increase the total stored energy by realizing the transition from a low confinement (L-mode) state to a high confinement (H-mode) state in magnetic confinement fusion. The L-H transition process is simulated by using the predictive transport code based on Weiland’s fluid model. Based on the equilibrium parameters obtained from equilibrium fitting (EFIT) in the experiment, the electron density ne, electron temperature Te, ion temperatures Ti, ion poloidal Vp, and toroidal momenta Vt are simulated self-consistently. The L-H transition dynamic behaviors with the formation of the transport barriers of ion and electron temperatures, the electron density, and the ion toroidal momenta are analyzed. During the L-H transition, the strong poloidal flow shear in the edge transport barrier region is observed. The crashes of the electron and ion temperature pedestals are also observed during the L-H transition. The effects of the heating and particle sources on the L-H transition process are studied systematically, and the critical power threshold of the L-H transition is also found.

Structural origin underlying the effect of cooling rate on solidification point

Li Chen-Hui, Han Xiu-Jun, Luan Ying-Wei, Li Jian-Guo
Chin. Phys. B 2015, 24 (11): 116101 ;  doi: 10.1088/1674-1056/24/11/116101
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Solidification behaviors of liquid aluminum at different cooling rates were examined via classical molecular dynamics simulation with an embedded atom method potential. The results demonstrate that solidification point decreases with increasing cooling rate. To explain this phenomenon, solid-like cluster in liquid was analyzed by the structural analysis method of bond order parameters. The results reveal that the size of the largest solid-like cluster in deeply undercooled liquid decreases with the increase of cooling rate, which can provide a structural interpretation to the above phenomenon.

Surface-type nonvolatile electric memory elements based on organic-on-organic CuPc-H2Pc heterojunction

Khasan S. Karimov, Zubair Ahmad, Farid Touati, M. Mahroof-Tahir, M. Muqeet Rehman, S. Zameer Abbas
Chin. Phys. B 2015, 24 (11): 116102 ;  doi: 10.1088/1674-1056/24/11/116102
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A novel surface-type nonvolatile electric memory elements based on organic semiconductors CuPc and H2Pc are fabricated by vacuum deposition of the CuPc and H2Pc films on preliminary deposited metallic (Ag and Cu) electrodes. The gap between Ag and Cu electrodes is 30-40 μm. For the current-voltage (I-V) characteristics the memory effect, switching effect, and negative differential resistance regions are observed. The switching mechanism is attributed to the electric-field-induced charge transfer. As a result the device switches from a low to a high-conductivity state and then back to a low conductivity state if the opposite polarity voltage is applied. The ratio of resistance at the high resistance state to that at the low resistance state is equal to 120-150. Under the switching condition, the electric current increases ~80-100 times. A comparison between the forward and reverse I-V characteristics shows the presence of rectifying behavior.

Electron irradiation-induced change of structure and damage mechanisms in multi-walled carbon nanotubes

Yang Jian-Qun, Li Xing-Ji, Liu Chao-Ming, Ma Guo-Liang, Gao Feng
Chin. Phys. B 2015, 24 (11): 116103 ;  doi: 10.1088/1674-1056/24/11/116103
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Owing to their unique structure and excellent electrical property, carbon nanotubes (CNTs) as an ideal candidate for making future electronic components have great application potentiality. In order to meet the requirements for space application in electronic components, it is necessary to study structural changes and damage mechanisms of multi-walled carbon nanotubes (MWCNTs), caused by the irradiations of 70 and 110 keV electrons. In the paper, the changes of structure and damage mechanisms in the irradiated MWCNTs, induced by the irradiations of 70 and 110 keV electrons, are investigated. The changes in surface morphology and structure of the irradiated MWCNT film are characterized using scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, x-ray diffraction analysis (XRD), and electron paramagnetic resonance (EPR) spectroscopy. It is found that the MWCNTs show different behaviors in structural changes after 70 and 110 keV electron irradiation due to different damage mechanisms. SEM results reveal that the irradiation of 70 keV electrons does not change surface morphology of the MWCNT film, while the irradiation of 110 keV electrons with a high fluence of 5×1015 cm-2 leads to evident morphological changes, such as the formation of a rough surface, the entanglement of nanotubes and the shrinkage of nanotubes. Based on Raman spectroscopy, XPS, and XRD analyses, it is confirmed that the irradiation of 70 keV electrons increases the interlayer spacing of the MWCNTs and disorders their structure through electronic excitations and ionization effects, while the irradiation of 110 keV electrons obviously reduces the interlayer spacing of the MWCNTs and improves their graphitic order through knock-on atom displacements. The improvement of the irradiated MWCNTs by 110 keV electrons is attributed to the restructuring of defect sites induced by knock-on atom displacements. EPR spectroscopic analyses reveal that the MWCNTs exposed to both 70 keV electrons and 110 keV electrons suffer ionization damage to some extent.

Relationship between bias voltage and microstructureas well as properties of CrAlYN films

Fu Ying-Ying, Li Hong-Xuan, Ji Li, Liu Xiao-Hong, Liu Liu, Zhou Hui-Di, Chen Jian-Min
Chin. Phys. B 2015, 24 (11): 116201 ;  doi: 10.1088/1674-1056/24/11/116201
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In this work, a series of CrAlYN films doped with 1 at.% yttrium were deposited by unbalanced reactive magnetron sputtering under different bias voltages. The effects of bias voltage on microstructure and properties of the CrAlYN films were subsequently investigated. It is found that all the as-deposited films have similar chemical composition and crystalline structure. However, the bias voltage has significant impact on the mechanical properties and oxidation resistance of the resulting films. Namely, the film deposited at 100 V has the highest hardness and best oxidation resistance, which are mainly attributed to its denser structure and higher Al content than others. In addition, the film obtained at 100 V exhibits superior oxidation resistance even at 1000 ℃, and good friction and wear properties at 600 and 800 ℃, and the latter two are mainly ascribed to the formation of compact transfer layer on the worn surfaces. However, this film experienced obvious wear loss at low testing temperatures (i.e., 200 and 400 ℃) due to the serious abrasive wear.

Mechanical, electronic, and thermodynamic properties of zirconium carbide from first-principles calculations

Yang Xiao-Yong, Lu Yong, Zheng Fa-Wei, Zhang Ping
Chin. Phys. B 2015, 24 (11): 116301 ;  doi: 10.1088/1674-1056/24/11/116301
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Mechanical, electronic, and thermodynamic properties of zirconium carbide have been systematically studied using the ab initio calculations. The calculated equilibrium lattice parameter, bulk modulus, and elastic constants are all well consistent with the experimental data. The electronic band structure indicates that the mixture of C 2p and Zr 4d and 4p orbitals around the Fermi level makes a large covalent contribution to the chemical bonds between the C and Zr atoms. The Bader charge analysis suggests that there are about 1.71 electrons transferred from each Zr atom to its nearest C atom. Therefore, the Zr-C bond displays a mixed ionic/covalent character. The calculated phonon dispersions of ZrC are stable, coinciding with the experimental measurement. A drastic expansion in the volume of ZrC is seen with increasing temperature, while the bulk modulus decreases linearly. Based on the calculated phonon dispersion curves and within the quasi-harmonic approximation, the temperature dependence of the heat capacities is obtained, which gives a good description compared with the available experimental data.

Nature of the band gap of halide perovskites ABX3 (A= CH3NH3, Cs; B= Sn, Pb; X= Cl, Br, I): First-principles calculations

Yuan Ye, Xu Run, Xu Hai-Tao, Hong Feng, Xu Fei, Wang Lin-Jun
Chin. Phys. B 2015, 24 (11): 116302 ;  doi: 10.1088/1674-1056/24/11/116302
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The electronic structures of cubic structure of ABX3 (A= CH3NH3,Cs; B= Sn, Pb; X= Cl, Br, I) are analyzed by density functional theory using the Perdew-Burke-Ernzerhof exchange-correlation functional and using the Heyd-Scuseria-Ernzerhof hybrid functional. The valence band maximum (VBM) is found to be made up by an antibonding hybridization of B s and X p states, whereas bands made up by the π antibonding of B p and X p states dominates the conduction band minimum (CBM). The changes of VBM, CBM, and band gap with ion B and X are then systematically summarized. The natural band offsets of ABX3 are partly given. We also found for all the ABX3 perovskite materials in this study, the bandgap increases with an increasing lattice parameter. This phenomenon has good consistency with the experimental results.

Carrier behavior of HgTe under high pressure revealed by Hall effect measurement

Hu Ting-Jing, Cui Xiao-Yan, Li Xue-Fei, Wang Jing-Shu, Lü Xiu-Mei, Wang Ling-Sheng, Yang Jing-Hai, Gao Chun-Xiao
Chin. Phys. B 2015, 24 (11): 116401 ;  doi: 10.1088/1674-1056/24/11/116401
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We investigate the carrier behavior of HgTe under high pressures up to 23 GPa using in situ Hall effect measurements. As the phase transitions from zinc blende to cinnabar, then to rock salt, and finally to Cmcm occur, all the parameters change discontinuously. The conductivity variation under compression is described by the carrier parameters. For the zinc blende phase, both the decrease of carrier concentration and the increase of mobility indicate the overlapped valence band and conduction band separates with pressure. Pressure causes an increase in the hole concentration of HgTe in the cinnabar phase, which leads to the carrier-type inversion and the lowest mobility at 5.6 GPa. In the phase transition process from zinc blende to rock salt, Te atoms are the major ones in atomic movements in the pressure regions of 1.0-1.5 GPa and 1.8-3.1 GPa, whereas Hg atoms are the major ones in the pressure regions of 1.5-1.8 GPa and 3.1-7.7 GPa. The polar optical scattering of the rock salt phase decreases with pressure.

Theoretical analysis of droplet transition from Cassie to Wenzel state

Liu Tian-Qing, Li Yan-Jie, Li Xiang-Qin, Sun Wei
Chin. Phys. B 2015, 24 (11): 116801 ;  doi: 10.1088/1674-1056/24/11/116801
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Whether droplets transit from the Cassie to the Wenzel state (C-W) on a textured surface is the touchstone that the superhydrophobicity of the surface is still maintained. However, the C-W transition mechanism, especially the spontaneous transition of small droplets, is still not very clear to date. The interface free energy gradient of a small droplet is firstly proposed and derived as the driving force for its C-W evolution in this study based on the energy and gradient analysis. Then the physical and mathematical model of the C-W transition is found after the C-W driving force or transition pressure, the resistance, and the parameters of the meniscus beneath the droplet are formulated. The results show that the micro/nano structural parameters significantly affect the C-W driving force and resistance. The smaller the pillar diameter and pitch, the minor the C-W transition pressure, and the larger the resistance. Consequently, the C-W transition is difficult to be completed for the droplets on nano-textured surfaces. Meanwhile if the posts are too short, the front of the curved liquid-air interface below the droplet will touch the structural substrate easily even though the three phase contact line (TPCL) has not depinned. When the posts are high enough, the TPCL beneath the drop must move firstly before the meniscus can reach the substrate. As a result, the droplet on a textured surface with short pillars is easy to complete its C-W evolution. On the other hand, the smaller the droplet, the easier the C-W shift, since the transition pressure becomes larger, which well explains why an evaporating drop will collapse spontaneously from composite to Wenzel state. Besides, both intrinsic and advancing contact angles affect the C-W transition as well. The greater the two angles, the harder the C-W transition. In the end, the C-W transition parameters and the critical conditions measured in literatures are calculated and compared, and the calculations accord well with the experimental results.

Residual occurrence and energy property of proteins in HNP model

Jiang Zhou-Ting, Dou Wen-Hui, Shen Yu, Sun Ting-Ting, Xu Peng
Chin. Phys. B 2015, 24 (11): 116802 ;  doi: 10.1088/1674-1056/24/11/116802
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Four categories of globular proteins, including all-α, all-β, α +β, and α/β types, are simplified as the off-lattice HNP model involving the secondary-structural information of each protein. The propensity of three types of residues, i.e., H, N, and P to form a secondary structure is investigated based on 146 protein samples. We find that P residues are easy to form α -helices, whereas H residues have a higher tendency to construct β-sheets. The statistical analysis also indicates that the occurrence of P residues is invariably higher than that of H residues, which is independent of protein category. Changes in bond-and non-bonded potential energies of all protein samples under a wide temperature range are presented by coarse-grained molecular dynamics (MD) simulation. The simulation results clearly show a linear relationship between the bond-stretching/bending potential energy and the reduced temperature. The bond-torsional and non-bonded potential energies show distinct transitions with temperature. The bond-torsional energy increases to the maximum and then decreases with the increase of temperature, which is opposite to the change in non-bonded potential energy. The transition temperature of non-bonded potential energy is independent of the protein category, while that of bond-torsional energy is closely related to the protein secondary structure, i.e., α -helix or β-sheet. The quantitatively bonded-and semi-quantitatively non-bonded potential energy of 24 α +β and 23 α/β protein samples are successfully predicted according to the statistical results obtained from MD simulations.

Low contact resistivity between Ni/Au and p-GaN through thin heavily Mg-doped p-GaN and p-InGaN compound contact layer

Li Xiao-Jing, Zhao De-Gang, Jiang De-Sheng, Chen Ping, Zhu Jian-Jun, Liu Zong-Shun, Le Ling-Cong, Yang Jing, He Xiao-Guang
Chin. Phys. B 2015, 24 (11): 116803 ;  doi: 10.1088/1674-1056/24/11/116803
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Thin heavily Mg-doped InGaN and GaN compound contact layer is used to form Ni/Au Ohmic contact to p-GaN. The growth conditions of the compound contact layer and its effect on the performance of Ni/Au Ohmic contact to p-GaN are investigated. It is confirmed that the specific contact resistivity can be lowered nearly two orders by optimizing the growth conditions of compound contact layer. When the flow rate ratio between Mg and Ga gas sources of p++-InGaN layer is 10.6% and the thickness of p++-InGaN layer is 3 nm, the lowest specific contact resistivity of 3.98×10-5Ω·cm2 is achieved. In addition, the experimental results indicate that the specific contact resistivity can be further lowered to 1.07×10-7Ω ·cm2 by optimizing the alloying annealing temperature to 520 ℃.

Phase diagram of the Fermi–Hubbard model with spin-dependent external potentials: A DMRG study

Wei Xing-Bo, Meng Ye-Ming, Wu Zhe-Ming, Gao Xian-Long
Chin. Phys. B 2015, 24 (11): 117101 ;  doi: 10.1088/1674-1056/24/11/117101
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We investigate a one-dimensional two-component system in an optical lattice of attractive interactions under a spin-dependent external potential. Based on the density-matrix renormalization group methods, we obtain its phase diagram as a function of the external potential imbalance and the strength of the attractive interaction through the analysis on the density profiles and the momentum pair correlation functions. We find that there are three different phases in the system, a coexisted fully polarized and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, a normal polarized phase, and a Bardeen-Cooper-Schrieffer (BCS) phase. Different from the systems of spin-independent external potential, where the FFLO phase is normally favored by the attractive interactions, in the present situation, the FFLO phases are easily destroyed by the attractive interactions, leading to the normal polarized or the BCS phase.

Effects of IIIB transition metals on optoelectronic and magnetic properties of HoMnO3: A first principles study

H. A. Rahnamaye Aliabad, Battal G. Yalcin
Chin. Phys. B 2015, 24 (11): 117102 ;  doi: 10.1088/1674-1056/24/11/117102
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The optoelectronic and magnetic properties of pure HoMnO3 and Ho0.67T0.33MnO3 (T = La, Y) alloys in hexagonal phase are theoretically investigated by using the first-principles calculations. The investigations are performed by means of the density functional theory through using the spin polarized generalized gradient approximation plus the Hubbard potential (SPGGA+U, Ueff= 3 eV). The studied material HoMnO3 exhibits two indirect band gaps: 1.58 eV for the spin-up state and 0.72 eV for the spin-down state along the S-G direction within the SPGGA+U approximation. It is found that the band gap of pure HoMnO3 for the spin-up state increases with increasing La and Y dopants. The results show that all of the studied materials have semi-metallic behaviors for the spin-up state and semiconducting character for the spin-down state. The substitutions of La and Y for Ho in HoMnO3 cause the static dielectric constant (ε0) to increase in the x direction but to decrease in the z direction. The calculated optical conductivity spectrum of HoMnO3 in a low energy range is in good agreement with the recent experimental data.

Improvement of switching characteristics by substrate bias in AlGaN/AlN/GaN heterostructure field effect transistors

Yang Ming, Lin Zhao-Jun, Zhao Jing-Tao, Wang Yu-Tang, Li Zhi-Yuan, Lü Yuan-Jie, Feng Zhi-Hong
Chin. Phys. B 2015, 24 (11): 117103 ;  doi: 10.1088/1674-1056/24/11/117103
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A simple and effective approach to improve the switching characteristics of AlGaN/AlN/GaN heterostructure field effect transistors (HFETs) by applying a voltage bias on the substrate is presented. With the increase of the substrate bias, the OFF-state drain current is much reduced and the ON-state current keeps constant. Both the ON/OFF current ratio and the subthreshold swing are demonstrated to be greatly improved. With the thinned substrate, the improvement of the switching characteristics with the substrate bias is found to be even greater. The above improvements of the switching characteristics are attributed to the interaction between the substrate bias induced electrical field and the bulk traps in the GaN buffer layer, which reduces the conductivity of the GaN buffer layer.

Tunneling magnetoresistance based on a Cr/graphene/Cr magnetotunnel junction

Luan Gui-Ping, Zhang Pei-Ran, Jiao Na, Sun Li-Zhong
Chin. Phys. B 2015, 24 (11): 117201 ;  doi: 10.1088/1674-1056/24/11/117201
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Using the density functional theory and the nonequilibrium Green’s function method, we studied the finite-bias quantum transport in a Cr/graphene/Cr magnetotunnel junction (MTJ) constructed by a single graphene layer sandwiched between two semi-infinite Cr(111) electrodes. We found that the tunneling magnetoresistance (TMR) ratio in this MTJ reached 108%, which is close to that of a perfect spin filter. Under an external positive bias, we found that the TMR ratio remained constant at 65%, in contrast to MgO-based MTJs, the TMR ratios of which decrease with increasing bias. These results indicate that the Cr/graphene/Cr MTJ is a promising candidate for spintronics applications.

Adiabatic quantum pump in a zigzag graphene nanoribbon junction

Zhang Lin
Chin. Phys. B 2015, 24 (11): 117202 ;  doi: 10.1088/1674-1056/24/11/117202
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The adiabatic electron transport is theoretically studied in a zigzag graphene nanoribbon (ZGNR) junction with two time-dependent pumping electric fields. By modeling a ZGNR p-n junction and applying the Keldysh Green’s function method, we find that a pumped charge current is flowing in the device at a zero external bias, which mainly comes from the photon-assisted tunneling process and the valley selection rule in an even-chain ZGNR junction. The pumped charge current and its ON and OFF states can be efficiently modulated by changing the system parameters such as the pumping frequency, the pumping phase difference, and the Fermi level. A ferromagnetic ZGNR device is also studied to generate a pure spin current and a fully polarized spin current due to the combined spin pump effect and the valley valve effect. Our finding might pave the way to manipulate the degree of freedom of electrons in a graphene-based electronic device.

Photoactive area modification in bulk heterojunctionorganic solar cells using optimization of electrochemicallysynthesized ZnO nanorods

Mehdi Ahmadi, Sajjad Rashidi Dafeh
Chin. Phys. B 2015, 24 (11): 117203 ;  doi: 10.1088/1674-1056/24/11/117203
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In this work, ZnO nanorod arrays grown by an electrochemical deposition method are investigated. The crucial parameters of length, diameter, and density of the nanorods are optimized over the synthesize process and nanorods growth time. Crystalline structure, morphologies, and optical properties of ZnO nanorod arrays are studied by different techniques such as x-ray diffraction, scanning electron microscope, atomic force microscope, and UV-visible transmission spectra. The ZnO nanorod arrays are employed in an inverted bulk heterojunction organic solar cell of Poly (3-hexylthiophene):[6-6] Phenyl-(6) butyric acid methyl ester to introduce more surface contact between the electron transporter layer and the active layer. Our results show that the deposition time is a very important factor to achieve the aligned and uniform ZnO nanorods with suitable surface density which is required for effective infiltration of active area into the ZnO nanorod spacing and make a maximum interfacial surface contact for electron collection, as overgrowing causes nanorods to be too dense and thick and results in high resistance and lower visible light transmittance. By optimizing the thickness of the active layer on top of ZnO nanorods, an improved efficiency of 3.17% with a high FF beyond 60% was achieved.

Transport properties of zigzag graphene nanoribbons adsorbed with single iron atom

Yang Yu-E, Xiao Yang, Yan Xiao-Hong, Dai Chang-Jie
Chin. Phys. B 2015, 24 (11): 117204 ;  doi: 10.1088/1674-1056/24/11/117204
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We have performed density-functional calculations of the transport properties of the zigzag graphene nanoribbon (ZGNR) adsorbed with a single iron atom. Two adsorption configurations are considered, i.e., iron adsorbed on the edge and on the interior of the nanoribbon. The results show that the transport features of the two configurations are similar. However, the transport properties are modified due to the scattering effects induced by coupling of the ZGNR band states to the localized 3d-orbital state of the iron atom. More importantly, one can find that several dips appear in the transmission curve, which is closely related to the above mentioned coupling. We expect that our results will have potential applications in graphene-based spintronic devices.

Surface states in crystals with low-index surfaces Hot!

Wang Hui-Ping, Tao Rui-Bao
Chin. Phys. B 2015, 24 (11): 117301 ;  doi: 10.1088/1674-1056/24/11/117301
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For most of the conventional crystals with low-index surfaces, the hopping between the nearest neighbor (1NN) crystal planes (CPs) is dominant and the ones from the nNN (2≤q < ∞) CPs are relatively weak, considered as small perturbations. The recent theoretical analysis[1] has demonstrated the absence of surface states at the level of the hopping approximation between the 1NN CPs when the original infinite crystal has the geometric reflection symmetry (GRS) for each CP. Meanwhile, based on the perturbation theory, it has also been shown that small perturbations from the hopping between the nNN (2≤n<∞) CPs and surface relaxation have no impact on the above conclusion. However, for the crystals with strong intrinsic spin-orbit coupling (SOC), the dominant terms of intrinsic SOC associate with two 1NN bond hoppings. Thus SOC will significantly contribute the hoppings from the 1NN and/or 2NN CPs except the ones within each CP. Here, we will study the effect of the hopping between the 2NN CPs on the surface states in model crystals with three different type structures (Type I: “…-P-P-P-P-…”, Type II: “…-P-Q-P-Q-” and Type III: “…-P=Q-P=Q-…” where P and Q indicate CPs and the signs “-” and “=” mark the distance between the 1NN CPs). In terms of analytical and numerical calculations, we study the behavior of surface states in three types after the symmetric/asymmetric hopping from the 2NN CPs is added. We analytically prove that the symmetric hopping from the 2NN CPs cannot induce surface states in Type I when each CP has only one electron mode. The numerical calculations also provide strong support for the conclusion, even up to 5NN. However, in general, the coupling from the 2NN CPs (symmetric and asymmetric) is favorable to generate surface states except Type I with single electron mode only.

Electron Raman scattering in semiconductor quantum well wire of cylindrical ring geometry

Re. Betancourt-Riera, Ri. Betancourt-Riera, J. M. Nieto Jalil, R. Riera
Chin. Phys. B 2015, 24 (11): 117302 ;  doi: 10.1088/1674-1056/24/11/117302
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We study the electron states and the differential cross section for an electron Raman scattering process in a semiconductor quantum well wire of cylindrical ring geometry. The electron Raman scattering developed here can be used to provide direct information about the electron band structures of these confinement systems. We assume that the system grows in a GaAs/Al0.35Ga0.65As matrix. The system is modeled by considering T = 0 K and also a single parabolic conduction band, which is split into a sub-band system due to the confinement. The emission spectra are discussed for different scattering configurations, and the selection rules for the processes are also studied. Singularities in the spectra are found and interpreted.

Dynamic responses of series parallel-plate mesoscopic capacitors to time-dependent external voltage

Wang Jin-Hua, Quan Jun
Chin. Phys. B 2015, 24 (11): 117303 ;  doi: 10.1088/1674-1056/24/11/117303
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We investigate the dynamic responses of the series parallel-plate mesoscopic capacitors to a time-dependent external voltage. The results indicate that the quantum coherence between two capacitors strongly depends on the frequency of the external voltage and the distance between the two capacitors (c-c distance). The behaviors of the series capacitance incompletely follow the Kirchhoff’s laws; only in the low frequency case or the limit of the c-c distance, the capacitance approaches to the classical series capacitance. In addition, the real part of the frequency-dependent capacitance shows a maximum and a minimum, which appear around the peak of the imaginary part. These phenomena may be associated with the plasmon excitation in the mesoscopic capacitors.

Low specific contact resistance on epitaxial p-type 4H-SiC with a step-bunching surface

Han Chao, Zhang Yu-Ming, Song Qing-Wen, Tang Xiao-Yan, Zhang Yi-Men, Guo Hui, Wang Yue-Hu
Chin. Phys. B 2015, 24 (11): 117304 ;  doi: 10.1088/1674-1056/24/11/117304
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This paper reports the performances of Ti/Al based ohmic contacts fabricated on highly doped p-type 4H-SiC epitaxial layer which has a severe step-bunching surface. Different contact schemes are investigated based on the Al:Ti composition with no more than 50 at.% Al. The specific contact resistance (SCR) is obtained to be as low as 2.6×10-6 Ω·cm2 for the bilayered Ti(100 nm)/Al(100 nm) contact treated with 3 min rapid thermal annealing (RTA) at 1000 ℃. The microstructure analyses examined by physical and chemical characterization techniques reveal an alloy-assisted ohmic contact formation mechanism, i.e., a high degree of alloying plays a decisive role in forming the interfacial ternary Ti3SiC2 dominating the ohmic behavior of the Ti/Al based contact. Furthermore, a globally covered Ti3SiC2 layer with (0001)-oriented texture can be formed, regardless of the surface step bunching as well as its structural evolution during the metallization annealing.

Trap states induced by reactive ion etching in AlGaN/GaN high-electron-mobility transistors

Luo Jun, Zhao Sheng-Lei, Mi Min-Han, Hou Bin, Yang Xiao-Lei, Zhang Jin-Cheng, Ma Xiao-Hua, Hao Yue
Chin. Phys. B 2015, 24 (11): 117305 ;  doi: 10.1088/1674-1056/24/11/117305
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Frequency-dependent conductance measurements were carried out to investigate the trap states induced by reactive ion etching in AlGaN/GaN high-electron-mobility transistors (HEMTs) quantitatively. For the non-recessed HEMT, the trap state density decreases from 2.48×1013 cm-2·eV-1 at an energy of 0.29 eV to 2.79×1012 cm-2·eV-1 at ET= 0.33 eV. In contrast, the trap state density of 2.38×1013-1.10×1014 cm-2·eV-1 is located at ET in a range of 0.30-0.33 eV for the recessed HEMT. Thus, lots of trap states with shallow energy levels are induced by the gate recess etching. The induced shallow trap states can be changed into deep trap states by 350 ℃ annealing process. As a result, there are two different types of trap sates, fast and slow, in the annealed HEMT. The parameters of the annealed HEMT are ET= 0.29-0.31 eV and DT = 8.16×1012-5.58×1013 cm-2·eV-1 for the fast trap states, and ET= 0.37-0.45 eV and DT = 1.84×1013-8.50×1013 cm-2·eV-1 for the slow trap states. The gate leakage currents are changed by the etching and following annealing process, and this change can be explained by the analysis of the trap states.

Influence of multi-deposition multi-annealing on time-dependent dielectric breakdown characteristics of PMOS with high-k/metal gate last process

Wang Yan-Rong, Yang Hong, Xu Hao, Wang Xiao-Lei, Luo Wei-Chun, Qi Lu-Wei, Zhang Shu-Xiang, Wang Wen-Wu, Yan Jiang, Zhu Hui-Long, Zhao Chao, Chen Da-Peng, Ye Tian-Chun
Chin. Phys. B 2015, 24 (11): 117306 ;  doi: 10.1088/1674-1056/24/11/117306
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A multi-deposition multi-annealing technique (MDMA) is introduced into the process of high-k/metal gate MOSFET for the gate last process to effectively reduce the gate leakage and improve the device’s performance. In this paper, we systematically investigate the electrical parameters and the time-dependent dielectric breakdown (TDDB) characteristics of positive channel metal oxide semiconductor (PMOS) under different MDMA process conditions, including the deposition/annealing (D&A) cycles, the D&A time, and the total annealing time. The results show that the increases of the number of D&A cycles (from 1 to 2) and D&A time (from 15 s to 30 s) can contribute to the results that the gate leakage current decreases by about one order of magnitude and that the time to fail (TTF) at 63.2% increases by about several times. However, too many D&A cycles (such as 4 cycles) make the equivalent oxide thickness (EOT) increase by about 1 Å and the TTF of PMOS worsen. Moreover, different D&A times and numbers of D&A cycles induce different breakdown mechanisms.

AlGaN/GaN high electron mobility transistorwith Al2O3+BCB passivation

Zhang Sheng, Wei Ke, Yu Le, Liu Guo-Guo, Huang Sen, Wang Xin-Hua, Pang Lei, Zheng Ying-Kui, Li Yan-Kui, Ma Xiao-Hua, Sun Bing, Liu Xin-Yu
Chin. Phys. B 2015, 24 (11): 117307 ;  doi: 10.1088/1674-1056/24/11/117307
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In this paper, Al2O3 ultrathin film used as the surface passivation layer for AlGaN/GaN high electron mobility transistor (HEMT) is deposited by thermal atomic layer deposition (ALD), thereby avoiding plasma-induced damage and erosion to the surface. A comparison is made between the surface passivation in this paper and the conventional plasma enhanced chemical vapor deposition (PECVD) SiN passivation. A remarkable reduction of the gate leakage current and a significant increase in small signal radio frequency (RF) performance are achieved after applying Al2O3+BCB passivation. For the Al2O3+BCB passivated device with a 0.7 μ gate, the value of fmax reaches up to 100 GHz, but it decreases to 40 GHz for SiN HEMT. The fmax/ft ratio (≥ 4) is also improved after Al2O3+BCB passivation. The capacitance-voltage (C-V) measurement demonstrates that Al2O3+BCB HEMT shows quite less density of trap states (on the order of magnitude of 1010 cm-2) than that obtained at commonly studied SiN HEMT.

Electronic properties of the SnSe-metal contacts: First-principles study

Dai Xian-Qi, Wang Xiao-Long, Li Wei, Wang Tian-Xing
Chin. Phys. B 2015, 24 (11): 117308 ;  doi: 10.1088/1674-1056/24/11/117308
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The geometries and electronic properties of SnSe/metal contact have been investigated using first-principles calculation. It is found that the geometries of monolayer SnSe were affected slightly when SnSe adsorbs on M (M=Ag, Au, Ta) substrate. Compared with the corresponding free-standing monolayer SnSe, the adsorbed SnSe undergoes a semiconductor-to-metal transition. The potential difference Δ V indicates that SnSe/Ta contact is the best candidate for the Schottky contact of the three SnSe/M contacts. Two types of current-in-plane (CIP) structure, where a freestanding monolayer SnSe is connected to SnSe/M, are identified as the n-type CIP structure in SnSe/Ag contact and p-type CIP structure in SnSe/Au and SnSe/Ta contact. The results can stimulate further investigation for the multifunctional SnSe/metal contact.

Structural, elastic, and electronic properties of recently discovered ternary silicide superconductor Li2IrSi3: An ab-initio study

M. A. Hadi, M. A. Alam, M. Roknuzzaman, M. T. Nasir, A. K. M. A. Islam, S. H. Naqib
Chin. Phys. B 2015, 24 (11): 117401 ;  doi: 10.1088/1674-1056/24/11/117401
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The structural, elastic, and electronic properties of the very recently discovered ternary silicide superconductor, Li2IrSi3, are calculated using an ab-initio technique. We adopt the plane-wave pseudopotential approach within the framework of the first-principles density functional theory (DFT) implemented by the CASTEP code. The calculated structural parameters show reasonable agreement with the experimental results. The elastic moduli of this interesting material are calculated for the first time. The electronic band structure and electronic energy density of states indicate the strong covalent Ir-Si and Si-Si bonding, which leads to the formation of the rigid structure of Li2IrSi3. Strong covalency gives rise to a high Debye temperature in this system. We discuss the theoretical results in detail in this paper.

Effective method to control the levitation force and levitation height in a superconducting maglev system

Yang Peng-Tao, Yang Wan-Min, Wang Miao, Li Jia-Wei, Guo Yu-Xia
Chin. Phys. B 2015, 24 (11): 117403 ;  doi: 10.1088/1674-1056/24/11/117403
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The influence of the width of the middle magnet in the permanent magnet guideways (PMGs) on the levitation force and the levitation height of single-domain yttrium barium copper oxide (YBCO) bulks has been investigated at 77 K under the zero field cooled (ZFC) state. It is found that the largest levitation force can be obtained in the system with the width of the middle magnet of the PMG equal to the size of the YBCO bulk when the gap between the YBCO bulk and PMG is small. Both larger levitation force and higher levitation height can be obtained in the system with the width of the middle magnet of the PMG larger than the size of the YBCO bulk. The stiffness of the levitation force between the PMG and the YBCO bulk is higher in the system with a smaller width of the middle magnet in the PMG. These results provide an effective way to control the levitation force and the levitation height for the superconducting maglev design and applications.

Electrochemical synthesis of alkali-intercalated iron selenide superconductors Hot!

Shen Shi-Jie, Ying Tian-Ping, Wang Gang, Jin Shi-Feng, Zhang Han, Lin Zhi-Ping, Chen Xiao-Long
Chin. Phys. B 2015, 24 (11): 117406 ;  doi: 10.1088/1674-1056/24/11/117406
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Electrochemical method has been used to insert K/Na into FeSe lattice to prepare alkali-intercalated iron selenides at room temperature. Magnetization measurement reveals that KxFe2Se2 and NaxFe2Se2 are superconductive at 31 K and 46 K, respectively. This is the first successful report of obtaining metal-intercalated FeSe-based high-temperature superconductors using electrochemical method. It provides an effective route to synthesize metal-intercalated layered compounds for new superconductor exploration.

Hysteresis loop behaviors of ferroelectric thin films: A Monte Carlo simulation study

C. M. Bedoya-Hincapié, H. H. Ortiz-Álvarez, E. Restrepo-Parra, J. J. Olaya-Flórez, J. E. Alfonso
Chin. Phys. B 2015, 24 (11): 117701 ;  doi: 10.1088/1674-1056/24/11/117701
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The ferroelectric response of bismuth titanate Bi4Ti3O12 (BIT) thin film is studied through a Monte Carlo simulation of hysteresis loops. The ferroelectric system is described by using a Diffour Hamiltonian with three terms: the electric field applied in the z direction, the nearest dipole-dipole interaction in the transversal (x-y) direction, and the nearest dipole-dipole interaction in the direction perpendicular to the thin film (the z axis). In the sample construction, we take into consideration the dipole orientations of the monoclinic and orthorhombic structures that can appear in BIT at low temperature in the ferroelectric state. The effects of temperature, stress, and the concentration of pinned dipole defects are assessed by using the hysteresis loops. The results indicate the changes in the hysteresis area with temperature and stress, and the asymmetric hysteresis loops exhibit evidence of the imprint failure mechanism with the emergence of pinned dipolar defects. The simulated shift in the hysteresis loops conforms to the experimental ferroelectric response.

Electric properties and phase transition behavior in lead lanthanum zirconate stannate titanate ceramics with low zirconate content

Zeng Tao, Lou Qi-Wei, Chen Xue-Feng, Zhang Hong-Ling, Dong Xian-Lin, Wang Gen-Shui
Chin. Phys. B 2015, 24 (11): 117702 ;  doi: 10.1088/1674-1056/24/11/117702
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The phase transitions, dielectric properties, and polarization versus electric field (P-E) hysteresis loops of Pb0.97La0.02(Zr0.42Sn0.58-xTix)O3 (0.13 ≤ x ≤ 0.18) (PLZST) bulk ceramics were systematically investigated. This study exhibited a sequence of phase transitions by analyzing the change of the P-E hysteresis loops with increasing temperature. The antiferroelectric (AFE) to ferroelectric (FE) phase boundary of PLZST with the Zr content of 0.42 was found to locate at the Ti content between 0.14 and 0.15. This work is aimed to improve the ternary phase diagram of lanthanum-doped PZST with the Zr content of 0.42 and will be a good reference for seeking high energy storage density in the PLZST system with low-Zr content.

Indium-tin oxide films obtained by DC magnetron sputtering for improved Si heterojunction solar cell applications

Gu Jin-Hua, Si Jia-Le, Wang Jiu-Xiu, Feng Ya-Yang, Gao Xiao-Yong, Lu Jing-Xiao
Chin. Phys. B 2015, 24 (11): 117703 ;  doi: 10.1088/1674-1056/24/11/117703
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The indium-tin oxide (ITO) film as the antireflection layer and front electrodes is of key importance to obtaining high efficiency Si heterojunction (HJ) solar cells. To obtain high transmittance and low resistivity ITO films by direct-current (DC) magnetron sputtering, we studied the impacts of the ITO film deposition conditions, such as the oxygen flow rate, pressure, and sputter power, on the electrical and optical properties of the ITO films. ITO films of resistivity of 4×10-4 Ω ·m and average transmittance of 89% in the wavelength range of 380-780 nm were obtained under the optimized conditions: oxygen flow rate of 0.1 sccm, pressure of 0.8 Pa, and sputtering power of 110 W. These ITO films were used to fabricate the single-side HJ solar cell without an intrinsic a-Si:H layer. However, the best HJ solar cell was fabricated with a lower sputtering power of 95 W, which had an efficiency of 11.47%, an open circuit voltage (Voc) of 0.626 V, a filling factor (FF) of 0.50, and a short circuit current density (Jsc) of 36.4 mA/cm2. The decrease in the performance of the solar cell fabricated with high sputtering power of 110 W is attributed to the ion bombardment to the emitter. The Voc was improved to 0.673 V when a 5 nm thick intrinsic a-Si:H layer was inserted between the (p) a-Si:H and (n) c-Si layer. The higher Voc of 0.673 V for the single-side HJ solar cell implies the excellent c-Si surface passivation by a-Si:H.

Spectral investigation of R, Ce:YAG (R: Pr3+, Eu3+, Gd3+) single crystals and their applications in white LEDs

Du Yong, Shao Chong-Yun, Dong Yong-Jun, Yang Qiu-Hong, Hua Wei
Chin. Phys. B 2015, 24 (11): 117801 ;  doi: 10.1088/1674-1056/24/11/117801
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Eu3+, Pr3+, or Gd3+ codoped Ce:YAG single crystals were grown by using the Czochralski method. The photoluminescence (PL) emission and excitation spectra and transmittance were measured and investigated. The additional red-emitting bands were observed in the PL emission spectra of Eu,Ce:YAG and Pr,Ce:YAG single crystals and the formation of noticeable peaks was studied with reference to the schematic energy level diagrams. A red-shifted phenomenon was observed in the PL emission spectrum of Gd,Ce:YAG. With codoped Eu3+, Pr3+, or Gd3+ ions, warmer white light was achieved for the white light emitting diodes and the color rendering index became higher.

Design of broad angular phase retarders for the complete polarization analysis of extreme ultraviolet radiation

Lin Cheng-You, Chen Shu-Jing, Chen Zhao-Yang, Ding Ying-Chun
Chin. Phys. B 2015, 24 (11): 117802 ;  doi: 10.1088/1674-1056/24/11/117802
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A method of designing broad angular phase retarders in the extreme ultraviolet (EUV) region is presented. The design is based on a standard Levenberg-Marquardt algorithm combined with a common merit function. Using this method, a series of broad angular EUV phase retarders were designed using aperiodic Mo/Si multilayers. At photon energy of 90 eV, broad angular phase retarders with 30°, 60°, and 90° phase retardations have been realized in the angular range of 39°-51°. By analyzing and comparing the performances of the designed broad angular phase retarders, we found that the Mo/Si multilayer with more layers could obtain higher phase retardation in broader angular range when used to design the broad angular phase retarder. Broad angular phase retarders possess lower sensitivity toward changing incident angle compared with the traditional phase retarders designed with transmission periodic multilayers, and can be used for the polarization control of broad angular EUV sources.

Characteristics of charge and discharge of PMMA samples due to electron irradiation

Feng Guo-Bao, Wang Fang, Hu Tian-Cun, Cao Meng
Chin. Phys. B 2015, 24 (11): 117901 ;  doi: 10.1088/1674-1056/24/11/117901
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In this study, using a comprehensive numerical simulation of charge and discharge processes, we investigate the formation and evolution of negative charge and discharge characteristics of a grounded PMMA film irradiated by a non-focused electron beam. Electron scattering and transport processes in the sample are simulated with the Monte Carlo and the finite-different time-domain (FDTD) methods, respectively. The properties of charge and discharge processes are presented by the evolution of internal currents, charge quantity, surface potential, and discharge time. Internal charge accumulation in the sample may reach saturation by primary electron (PE) irradiation providing the charge duration is enough. Internal free electrons will run off to the ground in the form of leakage current due to charge diffusion and drift during the discharge process after irradiation, while trapped electrons remain. The negative surface potential determined by the charging quantity decreases to its saturation in the charge process, and then increases in the discharge process. A larger thickness of the PMMA film will result in greater charge amount and surface potential in charge saturation and in final discharge state, while the electron mobility of the material has little effects on the final discharge state. Moreover, discharge time is less for smaller thickness or larger electron mobility. The presented results can be helpful for estimating and weakening the charging of insulating samples especially under the intermittent electron beam irradiation in related surface analysis or measurement.

Migration characterization of Ga and In adatoms on dielectric surface in selective MOVPE

Chen Wei-Jie, Han Xiao-Biao, Lin Jia-Li, Hu Guo-Heng, Liu Ming-Gang, Yang Yi-Bin, Chen Jie, Wu Zhi-Sheng, Liu Yang, Zhang Bai-Jun
Chin. Phys. B 2015, 24 (11): 118101 ;  doi: 10.1088/1674-1056/24/11/118101
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Migration characterizations of Ga and In adatoms on the dielectric surface in selective metal organic vapor phase epitaxy (MOVPE) were investigated. In the typical MOVPE environment, the selectivity of growth is preserved for GaN, and the growth rate of GaN micro-pyramids is sensitive to the period of the patterned SiO2 mask. A surface migration induced model was adopted to figure out the effective migration length of Ga adatoms on the dielectric surface. Different from the growth of GaN, the selective area growth of InGaN on the patterned template would induce the deposition of InGaN polycrystalline particles on the patterned SiO2 mask with a long period. It was demonstrated with a scanning electron microscope and energy dispersive spectroscopy that the In adatoms exhibit a shorter migration length on the dielectric surface.

Growth and fabrication of semi-polar InGaN/GaN multi-quantum well light-emitting diodes on microstructured Si (001) substrates

Chen Long, Payne Justin, Strate Jan, Li Cheng, Zhang Jian-Ming, Yu Wen-Jie, Di Zeng-Feng, Wang Xi
Chin. Phys. B 2015, 24 (11): 118102 ;  doi: 10.1088/1674-1056/24/11/118102
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Semi-polar (1-101) InGaN/GaN light-emitting diodes were prepared on standard electronic-grade Si (100) substrates. Micro-stripes of GaN and InGaN/GaN quantum wells on semi-polar facets were grown on intersecting 111 planes of microscale V-grooved Si in metal-organic vapor phase epitaxy, covering over 50% of the wafer surface area. In-situ optical reflectivity and curvature measurements demonstrate that the effect of the thermal expansion coefficient mismatch was greatly reduced. A cross-sectional analysis reveals low threading dislocation density on the top of most surfaces. On such prepared (1-101) GaN, an InGaN/GaN LED was fabricated. Electroluminescence over 5 mA to 60 mA is found with a much lower blue-shift than that on the c-plane device. Such structures therefore could allow higher efficiency light emitters with a weak quantum confined Stark effect throughout the visible spectrum.

Tunable electromagnetically induced transparency at terahertz frequencies in coupled graphene metamaterial

Ding Guo-Wen, Liu Shao-Bin, Zhang Hai-Feng, Kong Xiang-Kun, Li Hai-Ming, Li Bing-Xiang, Liu Si-Yuan, Li Hai
Chin. Phys. B 2015, 24 (11): 118103 ;  doi: 10.1088/1674-1056/24/11/118103
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A graphene-based metamaterial with tunable electromagnetically induced transparency (EIT)-like transmission is numerically studied in this paper. The proposed structure consists of a graphene layer composed of coupled cut-wire pairs printed on a substrate. The simulation confirms that an EIT-like transparency window can be observed due to indirect coupling in a terahertz frequency range. More importantly, the peak frequency of the transmission window can be dynamically controlled over a broad frequency range by varying the Fermi energy levels of the graphene layer through controlling the electrostatic gating. The proposed metamaterial structure offers an additional opportunity to design novel applications such as switches or modulators.

Direct synthesis of graphene nanosheets support Pd nanodendrites for electrocatalytic formic acid oxidation

Yang Su-Dong, Chen Lin
Chin. Phys. B 2015, 24 (11): 118104 ;  doi: 10.1088/1674-1056/24/11/118104
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We report a solvothermal method preparation of dendritic Pd nanoparticles (DPNs) and spherical Pd nanoparticles (SPNs) supported on reduced graphene oxide (RGO). Drastically different morphologies of Pd NPs with nanodendritic structures or spherical structures were observed on graphene by controlling the reduction degree of graphene oxide (GO) under mild conditions. In addition to being a commonplace substrate, GO plays a more important role that relies on its surface groups, which serves as a shape-directing agent to direct the dendritic growth. As a result, the obtained DPNs/RGO catalyst exhibits a significantly enhanced electro-catalytic behavior for the oxidation of formic acid compared to the SPNs/RGO catalyst.

Crossover from 2-dimensional to 3-dimensional aggregations of clusters on square lattice substrates

Cheng Yi, Zhu Yu-Hong, Pan Qi-Fa, Yang Bo, Tao Xiang-Ming, Ye Gao-Xiang
Chin. Phys. B 2015, 24 (11): 118105 ;  doi: 10.1088/1674-1056/24/11/118105
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A Monte Carlo study on the crossover from 2-dimensional to 3-dimensional aggregations of clusters is presented. Based on the traditional cluster-cluster aggregation (CCA) simulation, a modified growth model is proposed. The clusters (including single particles and their aggregates) diffuse with diffusion step length l (1 ≤ l ≤ 7) and aggregate on a square lattice substrate. If the number of particles contained in a cluster is larger than a critical size sc, the particles at the edge of the cluster have a possibility to jump onto the upper layer, which results in the crossover from 2-dimensional to 3-dimensional aggregations. Our simulation results are in good agreement with the experimental findings.

Dynamics of two polarized nanoparticles Hot!

Duan Xiao-Yong, Wang Zhi-Guo
Chin. Phys. B 2015, 24 (11): 118106 ;  doi: 10.1088/1674-1056/24/11/118106
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The intrinsic dynamics of two interacting electric polarized nanorods is theoretically investigated. The relative motion between them caused by electric dipole-dipole interaction is derived based on the generalized Lagrangian formulation. The results show that the relative translation and rotation are nonlinear and closely dependent on the initial configuration of the two nanorods. Furthermore, the general conditions of the initial configuration, which determine the two nanorods to repel or attract each other at the initial time, are obtained. The two-dimensional relative motion of the two nanorods shows that the antiparallel and head-to-tail ordering stable self-assembly are respectively formed in two planar initial configurations. For different three-dimensional initial configurations, the interesting dynamic relative attraction, repulsion, and oscillation with rotation are respectively realized. Finally, the theoretical schemes which realize the relaxing, direct head-to-tail ordering, and direct antiparallel ordering stable self-assembly are presented according to the different modes of the motion of the nanoparticles. Some of our results agree well with the results of experiments and simulations.

Spray forming and mechanical properties of a new type powder metallurgy superalloy

Jia Chong-Lin, Ge Chang-Chun, Xia Min, Gu Tian-Fu
Chin. Phys. B 2015, 24 (11): 118107 ;  doi: 10.1088/1674-1056/24/11/118107
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The deposited billet of a new type powder metallurgy (PM) superalloy FGH4095M for use in turbine disk manufacturing has been fabricated using spray forming technology. The metallurgical quality of the deposited billet was analyzed in terms of density, texture, and grain size. Comparative research was done on the microstructure and mechanical properties between the flat disk preform prepared with hot isostatic pressing (HIP) and the same alloy forgings prepared with HIP followed by isothermal forging (IF). The results show that the density of the spray-formed and nitrogen-atomized deposit billet is above 99% of the theoretical density, indicating a compact structure. The grains are uniform and fine. The billet has weak texture with a random distribution in the spray deposition direction and perpendicular to the direction of deposition. A part of atomizing nitrogen exists in the preform in the form of carbonitride. Nitrogen-induced microporosity causes the density reduction of the preform. Compared with the process of HIP+IF, the superalloy FGH4095M after HIP has better mechanical properties at both room temperature and high temperature. The sizes of the γ’ phase are finer in microstructure of the preform after HIP in comparison with the forgings after HIP+IF. This work shows that SF+HIP is a viable processing route for FGH4095M as a turbine-disk material.

Hugoniot curve calculation of nitromethane decomposition mixtures: A reactive force field molecular dynamics approach

Guo Feng, Zhang Hong, Hu Hai-Quan, Cheng Xin-Lu, Zhang Li-Yan
Chin. Phys. B 2015, 24 (11): 118201 ;  doi: 10.1088/1674-1056/24/11/118201
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We investigate the Hugoniot curve, shock-particle velocity relations, and Chapman-Jouguet conditions of the hot dense system through molecular dynamics (MD) simulations. The detailed pathways from crystal nitromethane to reacted state by shock compression are simulated. The phase transition of N2 and CO mixture is found at about 10 GPa, and the main reason is that the dissociation of the C-O bond and the formation of C-C bond start at 10.0-11.0 GPa. The unreacted state simulations of nitromethane are consistent with shock Hugoniot data. The complete pathway from unreacted to reacted state is discussed. Through chemical species analysis, we find that the C-N bond breaking is the main event of the shock-induced nitromethane decomposition.

Driven self-assembly of hard nanoplates on soft elastic shells

Zhang Yao-Yang, Hua Yun-Feng, Deng Zhen-Yu
Chin. Phys. B 2015, 24 (11): 118202 ;  doi: 10.1088/1674-1056/24/11/118202
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The driven self-assembly behaviors of hard nanoplates on soft elastic shells are investigated by using molecular dynamics (MD) simulation method, and the driven self-assembly structures of adsorbed hard nanoplates depend on the shape of hard nanoplates and the bending energy of soft elastic shells. Three main structures for adsorbed hard nanoplates, including the ordered aggregation structures of hard nanoplates for elastic shells with a moderate bending energy, the collapsed structures for elastic shells with a low bending energy, and the disordered aggregation structures for hard shells, are observed. The self-assembly process of adsorbed hard nanoplates is driven by the surface tension of the elastic shell, and the shape of driven self-assembly structures is determined on the basis of the minimization of the second moment of mass distribution. Meanwhile, the deformations of elastic shells can be controlled by the number of adsorbed rods as well as the length of adsorbed rods. This investigation can help us understand the complexity of the driven self-assembly of hard nanoplates on elastic shells.

Exponential flux-controlled memristor model and its floating emulator

Liu Wei, Wang Fa-Qiang, Ma Xi-Kui
Chin. Phys. B 2015, 24 (11): 118401 ;  doi: 10.1088/1674-1056/24/11/118401
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As commercial memristors are still unavailable in the market, mathematic models and emulators which can imitate the features of the memristor are meaningful for further research. In this paper, based on the analyses of characteristics of the q-φ curve, an exponential flux-controlled model, which has the quality that its memductance (memristance) will keep monotonically increasing or decreasing unless the voltage’s polarity reverses (if not approach the boundaries), is constructed. A new approach to designing the floating emulator of the memristor is also proposed. This floating structure can flexibly meet various demands for the current through the memristor (especially the demand for a larger current). The simulations and experiments are presented to confirm the effectiveness of this model and its floating emulator.

Improved dichotomous search frequency offset estimator for burst-mode continuous phase modulation

Zhai Wen-Chao, Li Zan, Si Jiang-Bo, Bai Jun
Chin. Phys. B 2015, 24 (11): 118402 ;  doi: 10.1088/1674-1056/24/11/118402
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A data-aided technique for carrier frequency offset estimation with continuous phase modulation (CPM) in burst-mode transmission is presented. The proposed technique first exploits a special pilot sequence, or training sequence, to form a sinusoidal waveform. Then, an improved dichotomous search frequency offset estimator is introduced to determine the frequency offset using the sinusoid. Theoretical analysis and simulation results indicate that our estimator is noteworthy in the following aspects. First, the estimator can operate independently of timing recovery. Second, it has relatively low outlier, i.e., the minimum signal-to-noise ratio (SNR) required to guarantee estimation accuracy. Finally, the most important property is that our estimator is complexity-reduced compared to the existing dichotomous search methods: it eliminates the need for fast Fourier transform (FFT) and modulation removal, and exhibits faster convergence rate without accuracy degradation.

Multi-mode coupling analysis of a sub-terahertz band planar corrugated Bragg reflector

Liu Guo, Luo Yong, Wang Jian-Xun, Shu Guo-Xiang
Chin. Phys. B 2015, 24 (11): 118403 ;  doi: 10.1088/1674-1056/24/11/118403
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Planar Bragg reflector operating in the sub-terahertz wavelength installed at the upstream end of a sheet beam backward wave oscillator (BWO) is very promising to minimize the whole circuit structure and make it more compact. In this paper, a sub-terahertz wavelength (0.18-0.22 THz) tunable planar Bragg reflector is numerically analyzed by using multi-mode coupling theory (MCT). The operating mode TE10 and dominant coupling mode TE01 are mainly considered in this theory. Reflection and transmission performance of the reflector are demonstrated in detail and the results, in excellent agreement with the theoretical analysis and simulation, are also presented in this paper. Self-and cross-coupling coefficients between these two modes are presented as well. The reflector behaviors with different Bragg dimensions are discussed and analyzed in the 0.16-0.22 THz range. The analysis in this paper can be of benefit to the design and fabrication of the whole BWO circuit.

Stability of magnetic tip/superconductor levitation systems

M. K. Alqadi
Chin. Phys. B 2015, 24 (11): 118404 ;  doi: 10.1088/1674-1056/24/11/118404
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The vertical stability of a magnetic tip over a superconducting material is investigated by using the critical state and the frozen image models. The analytical expressions of the stiffness and the vibration frequency about the equilibrium position are derived in term of the geometrical parameters of the magnet/superconductor system. It is found that the stability of the system depends on the shape of the superconductor as well as its thickness.

Excellent ethanol sensing properties based on Er2O3-Fe2O3 nanotubes

Liu Chang-Bai, He Ying, Wang Sheng-Lei
Chin. Phys. B 2015, 24 (11): 118501 ;  doi: 10.1088/1674-1056/24/11/118501
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In this work, pure α-Fe2O3 and Er2O3-Fe2O3 nanotubes were synthesized by a simple single-capillary electrospinning technology followed by calcination treatment. The morphologies and crystal structures of the as-prepared samples were characterized by scanning electron microscopy and x-ray diffraction, respectively. The gas-sensing properties of the as-prepared samples have been researched, and the result shows that the Er2O3-Fe2O3 nanotubes exhibit much better sensitivity to ethanol. The response value of Er2O3-Fe2O3 nanotubes to 10 ppm ethanol is 21 at the operating temperature 240°, which is 14 times larger than that of pure α-Fe2O3 nanotubes (response value is 1.5). The ethanol sensing properties of α-Fe2O3 nanotubes are remarkably enhanced by doping Er, and the lowest detection limit of Er2O3-Fe2O3 nanotubes is 300 ppb, to which the response value is about 2. The response and recovery times are about 4 s and 70 s to 10 ppm ethanol, respectively. In addition, the Er2O3-Fe2O3 nanotubes possess good selectivity and long-term stability.

Decondensation behavior of DNA chains induced by multivalent cations at high salt concentrations: Molecular dynamics simulations and experiments

Jiang Yang-Wei, Ran Shi-Yong, He Lin-Li, Wang Xiang-Hong, Zhang Lin-Xi
Chin. Phys. B 2015, 24 (11): 118701 ;  doi: 10.1088/1674-1056/24/11/118701
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Using molecular dynamics simulations and atomic force microscopy (AFM), we study the decondensation process of DNA chains induced by multivalent cations at high salt concentrations in the presence of short cationic chains in solutions. The typical simulation conformations of DNA chains with varying salt concentrations for multivalent cations imply that the concentration of salt cations and the valence of multivalent cations have a strong influence on the process of DNA decondensation. The DNA chains are condensed in the absence of salt or at low salt concentrations, and the compacted conformations of DNA chains become loose when a number of cations and anions are added into the solution. It is explicitly demonstrated that cations can overcompensate the bare charge of the DNA chains and weaken the attraction interactions between the DNA chains and short cationic chains at high salt concentrations. The condensation-decondensation transitions of DNA are also experimentally observed in mixing spermidine with λ-phage DNA at different concentrations of NaCl/MgCl2 solutions.

Effects of channel noise on synchronization transitions in delayed scale-free network of stochastic Hodgkin-Huxley neurons

Wang Bao-Ying, Gong Yu-Bing
Chin. Phys. B 2015, 24 (11): 118702 ;  doi: 10.1088/1674-1056/24/11/118702
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We numerically study the effect of the channel noise on the spiking synchronization of a scale-free Hodgkin-Huxley neuron network with time delays. It is found that the time delay can induce synchronization transitions at an intermediate and proper channel noise intensity, and the synchronization transitions become strongest when the channel noise intensity is optimal. The neurons can also exhibit synchronization transitions as the channel noise intensity is varied, and this phenomenon is enhanced at around the time delays that can induce the synchronization transitions. It is also found that the synchronization transitions induced by the channel noise are dependent on the coupling strength and the network average degree, and there is an optimal coupling strength or network average degree with which the synchronization transitions become strongest. These results show that by inducing synchronization transitions, the channel noise has a big regulation effect on the synchronization of the neuronal network. These findings could find potential implications for the information transmission in neural systems.

In situ calibrating optical tweezers with sinusoidal-wave drag force method

Li Di, Zhou Jin-Hua, Hu Xin-Yao, Zhong Min-Cheng, Gong Lei, Wang Zi-Qiang, Wang Hao-Wei, Li Yin-Mei
Chin. Phys. B 2015, 24 (11): 118703 ;  doi: 10.1088/1674-1056/24/11/118703
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We introduce a corrected sinusoidal-wave drag force method (SDFM) into optical tweezers to calibrate the trapping stiffness of the optical trap and conversion factor (CF) of photodetectors. First, the theoretical analysis and experimental result demonstrate that the correction of SDFM is necessary, especially the error of no correction is up to 11.25% for a bead of 5 μm in diameter. Second, the simulation results demonstrate that the SDFM has a better performance in the calibration of optical tweezers than the triangular-wave drag force method (TDFM) and power spectrum density method (PSDM) at the same signal-to-noise ratio or trapping stiffness. Third, in experiments, the experimental standard deviations of calibration of trapping stiffness and CF with the SDFM are about less than 50% of TDFM and PSDM especially at low laser power. Finally, the experiments of stretching DNA verify that the in situ calibration with the SDFM improves the measurement stability and accuracy.

Deductive way of reasoning about the internet AS level topology

Dávid Szabó, Attila Kőrösi, József Bíró, András Gulyás
Chin. Phys. B 2015, 24 (11): 118901 ;  doi: 10.1088/1674-1056/24/11/118901
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Our current understanding about the AS level topology of the Internet is based on measurements and inductive-type models which set up rules describing the behavior (node and edge dynamics) of the individual ASes and generalize the consequences of these individual actions for the complete AS ecosystem using induction. In this paper we suggest a third, deductive approach in which we have premises for the whole AS system and the consequences of these premises are determined through deductive reasoning. We show that such a deductive approach can give complementary insights into the topological properties of the AS graph. While inductive models can mostly reflect high level statistics (e.g., degree distribution, clustering, diameter), deductive reasoning can identify omnipresent subgraphs and peering likelihood. We also propose a model, called YEAS, incorporating our deductive analytical findings that produces topologies contain both traditional and novel metrics for the AS level Internet.

Universal relation for transport in non-sparse complex networks

Wang Yan, Yang Xiao-Rong
Chin. Phys. B 2015, 24 (11): 118902 ;  doi: 10.1088/1674-1056/24/11/118902
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Transport properties of a complex network can be reflected by the two-point resistance between any pair of two nodes. We systematically investigate a variety of typical complex networks encountered in nature and technology, in which we assume each link has unit resistance, and we find for non-sparse network connections a universal relation exists that the two-point resistance is equal to the sum of the inverse degree of two nodes up to a constant. We interpret our observations by the localization property of the network’s Laplacian eigenvectors. The findings in this work can possibly be applied to probe transport properties of general non-sparse complex networks.

Fin width and height dependence of bipolar amplification in bulk FinFETs submitted to heavy ion irradiation

Yu Jun-Ting, Chen Shu-Ming, Chen Jian-Jun, Huang Peng-Cheng
Chin. Phys. B 2015, 24 (11): 119401 ;  doi: 10.1088/1674-1056/24/11/119401
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FinFET technologies are becoming the mainstream process as technology scales down. Based on a 28-nm bulk p-FinFET device, we have investigated the fin width and height dependence of bipolar amplification for heavy-ion-irradiated FinFETs by 3D TCAD numerical simulation. Simulation results show that due to a well bipolar conduction mechanism rather than a channel (fin) conduction path, the transistors with narrower fins exhibit a diminished bipolar amplification effect, while the fin height presents a trivial effect on the bipolar amplification and charge collection. The results also indicate that the single event transient (SET) pulse width can be mitigated about 35% at least by optimizing the ratio of fin width and height, which can provide guidance for radiation-hardened applications in bulk FinFET technology.
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