Chin. Phys. B

Shannon information capacity of time reversal wideband multiple-input multiple-output system based on correlated statistical channels

Yu Yang(杨瑜), Bing-Zhong Wang(王秉中), Shuai Ding(丁帅)

Chin. Phys. B, 2016, 25 (5): 050101. DOI: 10.1088/1674-1056/25/5/050101

Abstract ( 745 )

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Utilizing channel reciprocity, time reversal (TR) technique increases the signal-to-noise ratio (SNR) at the receiver with very low transmitter complexity in complex multipath environment. Present research works about TR multiple-input multiple-output (MIMO) communication all focus on the system implementation and network building. The aim of this work is to analyze the influence of antenna coupling on the capacity of wideband TR MIMO system, which is a realistic question in designing a practical communication system. It turns out that antenna coupling stabilizes the capacity in a small variation range with statistical wideband channel response. Meanwhile, antenna coupling only causes a slight detriment to the channel capacity in a wideband TR MIMO system. Comparatively, uncorrelated stochastic channels without coupling exhibit a wider range of random capacity distribution which greatly depends on the statistical channel. The conclusions drawn from information difference entropy theory provide a guideline for designing better high-performance wideband TR MIMO communication systems.

Controlling chaos based on a novel intelligent integral terminal sliding mode control in a rod-type plasma torch

Safa Khari, Zahra Rahmani, Behrooz Rezaie

Chin. Phys. B, 2016, 25 (5): 050201. DOI: 10.1088/1674-1056/25/5/050201

Abstract ( 741 )

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An integral terminal sliding mode controller is proposed in order to control chaos in a rod-type plasma torch system. In this method, a new sliding surface is defined based on a combination of the conventional sliding surface in terminal sliding mode control and a nonlinear function of the integral of the system states. It is assumed that the dynamics of a chaotic system are unknown and also the system is exposed to disturbance and unstructured uncertainty. To achieve a chattering-free and high-speed response for such an unknown system, an adaptive neuro-fuzzy inference system is utilized in the next step to approximate the unknown part of the nonlinear dynamics. Then, the proposed integral terminal sliding mode controller stabilizes the approximated system based on Lyapunov's stability theory. In addition, a Bee algorithm is used to select the coefficients of integral terminal sliding mode controller to improve the performance of the proposed method. Simulation results demonstrate the improvement in the response speed, chattering rejection, transient response, and robustness against uncertainties.

Dynamics of cubic-quintic nonlinear Schrödinger equation with different parameters Suggested by editors

Wei Hua(花巍), Xue-Shen Liu(刘学深), Shi-Xing Liu(刘世兴)

Chin. Phys. B, 2016, 25 (5): 050202. DOI: 10.1088/1674-1056/25/5/050202

Abstract ( 754 )

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We study the dynamics of the cubic-quintic nonlinear Schrödinger equation by the symplectic method. The behaviors of the equation are discussed with harmonically modulated initial conditions, and the contributions from the quintic term are discussed. We observe the elliptic orbit, homoclinic orbit crossing, quasirecurrence, and stochastic motion with different nonlinear parameters in this system. Numerical simulations show that the changing processes of the motion of the system and the trajectories in the phase space are various for different cubic nonlinear parameters with the increase of the quintic nonlinear parameter.

Tunable two-axis spin model and spin squeezing in two cavities Suggested by editors

Lixian Yu(俞立先), Caifeng Li(李彩凤), Jingtao Fan(樊景涛), Gang Chen(陈刚), Tian-Cai Zhang(张天才), Suotang Jia(贾锁堂)

Chin. Phys. B, 2016, 25 (5): 050301. DOI: 10.1088/1674-1056/25/5/050301

Abstract ( 827 )

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Multi-mode cavities have now attracted much attention both experimentally and theoretically. In this paper, inspired by recent experiments of cavity-assisted Raman transitions, we realize a two-axis spin Hamiltonian H=q(J_x²+χJ_y²)+ω₀J_z in two cavities. This realized Hamiltonian has a distinct property that all parameters can be tuned independently. For proper parameters, the well-studied one- and two-axis twisting Hamiltonians are recovered, and the scaling of N^-1 of the maximal squeezing factor can occur naturally. On the other hand, in the two-axis twisting Hamiltonian, spin squeezing is usually reduced when increasing the atomic resonant frequency ω₀. Surprisingly, we find that by combining with the dimensionless parameter χ(>-1), this atomic resonant frequency ω₀ can enhance spin squeezing greatly. These results are beneficial for achieving the required spin squeezing in experiments.

Quantum information entropy for one-dimensional system undergoing quantum phase transition

Xu-Dong Song(宋旭东), Shi-Hai Dong(董世海), Yu Zhang(张宇)

Chin. Phys. B, 2016, 25 (5): 050302. DOI: 10.1088/1674-1056/25/5/050302

Abstract ( 845 )

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Calculations of the quantum information entropy have been extended to a non-analytically solvable situation. Specifically, we have investigated the information entropy for a one-dimensional system with a schematic “Landau” potential in a numerical way. Particularly, it is found that the phase transitional behavior of the system can be well expressed by the evolution of quantum information entropy. The calculated results also indicate that the position entropy S_x and the momentum entropy S_p at the critical point of phase transition may vary with the mass parameter M but their sum remains as a constant independent of M for a given excited state. In addition, the entropy uncertainty relation is proven to be robust during the whole process of the phase transition.

Path integral approach to electron scattering in classical electromagnetic potential

Chuang Xu(许闯), Feng Feng(冯锋), Ying-Jun Li(李英骏)

Chin. Phys. B, 2016, 25 (5): 050303. DOI: 10.1088/1674-1056/25/5/050303

Abstract ( 853 )

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As is known to all, the electron scattering in classical electromagnetic potential is one of the most widespread applications of quantum theory. Nevertheless, many discussions about electron scattering are based upon single-particle Schrodinger equation or Dirac equation in quantum mechanics rather than the method of quantum field theory. In this paper, by using the path integral approach of quantum field theory, we perturbatively evaluate the scattering amplitude up to the second order for the electron scattering by the classical electromagnetic potential. The results we derive are convenient to apply to all sorts of potential forms. Furthermore, by means of the obtained results, we give explicit calculations for the one-dimensional electric potential.

A hybrid-type quantum random number generator

Hai-Qiang Ma(马海强), Wu Zhu(朱武), Ke-Jin Wei(韦克金), Rui-Xue Li(李瑞雪), Hong-Wei Liu(刘宏伟)

Chin. Phys. B, 2016, 25 (5): 050304. DOI: 10.1088/1674-1056/25/5/050304

Abstract ( 637 )

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This paper proposes a well-performing hybrid-type truly quantum random number generator based on the time interval between two independent single-photon detection signals, which is practical and intuitive, and generates the initial random number sources from a combination of multiple existing random number sources. A time-to-amplitude converter and multichannel analyzer are used for qualitative analysis to demonstrate that each and every step is random. Furthermore, a carefully designed data acquisition system is used to obtain a high-quality random sequence. Our scheme is simple and proves that the random number bit rate can be dramatically increased to satisfy practical requirements.

Multi-hop teleportation based on W state and EPR pairs

Hai-Tao Zhan(占海涛), Xu-Tao Yu(余旭涛), Pei-Ying Xiong(熊佩颖), Zai-Chen Zhang(张在琛)

Chin. Phys. B, 2016, 25 (5): 050305. DOI: 10.1088/1674-1056/25/5/050305

Abstract ( 1084 )

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Multi-hop teleportation has significant value due to long-distance delivery of quantum information. Many studies about multi-hop teleportation are based on Bell pairs, partially entangled pairs or W state. The possibility of multi-hop teleportation constituted by partially entangled pairs relates to the number of nodes. The possibility of multi-hop teleportation constituted by double W states is (4/9)ⁿ after n-hop teleportation. In this paper, a multi-hop teleportation scheme based on W state and EPR pairs is presented and proved. The successful possibility of quantum information transmitted hop by hop through intermediate nodes is deduced. The possibility of successful transmission is (2/3)ⁿ after n-hop teleportation.

Bianchi type I in f(T) gravitational theories

M I Wanas, G G L Nashed, O A Ibrahim

Chin. Phys. B, 2016, 25 (5): 050401. DOI: 10.1088/1674-1056/25/5/050401

Abstract ( 610 )

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A tetrad field that is homogeneous and anisotropic which contains two unknown functions A(t) and B(t) of cosmic time is applied to the field equations of f(t), where T is the torsion scalar, T=T^μ_νρS_μ^νρ. We calculate the equation of continuity and rewrite it as a product of two brackets, the first is a function of f(t) and the second is a function of the two unknowns A(t) and B(t). We use two different relations between the two unknown functions A(t) and B(t) in the second bracket to solve it. Both of these relations give constant scalar torsion and solutions coincide with the de Sitter one. So, another assumption related to the contents of the matter fields is postulated. This assumption enables us to drive a solution with a non-constant value of the scalar torsion and a form of f(t) which represents ΛCDM.

Modeling the capability of penetrating a jammed crowd to eliminate freezing transition Suggested by editors

Mohammed Mahmod Shuaib

Chin. Phys. B, 2016, 25 (5): 050501. DOI: 10.1088/1674-1056/25/5/050501

Abstract ( 670 )

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Frozen state from jammed state is one of the most interesting aspects produced when simulating the multidirectional pedestrian flow of high density crowds. Cases of real life situations for such a phenomenon are not exhaustively treated. Our observations in the Hajj crowd show that freezing transition does not occur very often. On the contrary, penetrating a jammed crowd is a common aspect. We believe the kindness of pedestrians facing others whose walking is blocked is a main factor in eliminating the frozen state as well as in relieving the jammed state. We refine the social force model by incorporating a new social force to enable the simulated pedestrians to mimic the real behavior observed in the Hajj area. Simulations are performed to validate the work qualitatively.

New data assimilation system DNDAS for high-dimensional models

Qun-bo Huang(皇群博), Xiao-qun Cao(曹小群), Meng-bin Zhu(朱孟斌), Wei-min Zhang(张卫民), Bai-nian Liu(刘柏年)

Chin. Phys. B, 2016, 25 (5): 050502. DOI: 10.1088/1674-1056/25/5/050502

Abstract ( 748 )

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The tangent linear (TL) models and adjoint (AD) models have brought great difficulties for the development of variational data assimilation system. It might be impossible to develop them perfectly without great efforts, either by hand, or by automatic differentiation tools. In order to break these limitations, a new data assimilation system, dual-number data assimilation system (DNDAS), is designed based on the dual-number automatic differentiation principles. We investigate the performance of DNDAS with two different optimization schemes and subsequently give a discussion on whether DNDAS is appropriate for high-dimensional forecast models. The new data assimilation system can avoid the complicated reverse integration of the adjoint model, and it only needs the forward integration in the dual-number space to obtain the cost function and its gradient vector concurrently. To verify the correctness and effectiveness of DNDAS, we implemented DNDAS on a simple ordinary differential model and the Lorenz-63 model with different optimization methods. We then concentrate on the adaptability of DNDAS to the Lorenz-96 model with high-dimensional state variables. The results indicate that whether the system is simple or nonlinear, DNDAS can accurately reconstruct the initial condition for the forecast model and has a strong anti-noise characteristic. Given adequate computing resource, the quasi-Newton optimization method performs better than the conjugate gradient method in DNDAS.

A novel methodology for constructing a multi-wing chaotic and hyperchaotic system with a unified step function switching control

Chao-Xia Zhang(张朝霞), Si-Min Yu(禹思敏)

Chin. Phys. B, 2016, 25 (5): 050503. DOI: 10.1088/1674-1056/25/5/050503

Abstract ( 951 )

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This paper aims at developing a novel method of constructing a class of multi-wing chaotic and hyperchaotic system by introducing a unified step function. In order to overcome the essential difficulties in iteratively adjusting multiple parameters of conventional multi-parameter control, this paper introduces a unified step function controlled by a single parameter for constructing various multi-wing chaotic and hyperchaotic systems. In particular, to the best of the authors' knowledge, this is also the first time to find a non-equilibrium multi-wing hyperchaotic system by means of the unified step function control. According to the heteroclinic loop Shilnikov theorem, some properties for multi-wing attractors and its chaos mechanism are further discussed and analyzed. A circuit for multi-wing systems is designed and implemented for demonstration, which verifies the effectiveness of the proposed approach.

Two-point resistance of an m×n resistor network with an arbitrary boundary and its application in RLC network Suggested by editors

Zhi-Zhong Tan(谭志中)

Chin. Phys. B, 2016, 25 (5): 050504. DOI: 10.1088/1674-1056/25/5/050504

Abstract ( 863 )

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A rectangular m×n resistor network with an arbitrary boundary is investigated, and a general resistance formula between two nodes on an arbitrary axis is derived by the Recursion-Transform (RT) method, a problem that has never been resolved before, for the Green's function technique and the Laplacian matrix approach are inapplicable to it. To have the exact solution of resistance is important but it is difficult to obtain under the condition of arbitrary boundary. Our result is directly expressed in a single summation and mainly composed of characteristic roots, which contain both finite and infinite cases. Further, the current distribution is given explicitly as a byproduct of the method. Our framework can be effectively applied to RLC networks. As an application to the LC network, we find that our formulation leads to the occurrence of resonances at h₁ = 1-cosφ_i-sinφ_icotnφ_i. This somewhat curious result suggests the possibility of practical applications of our formulae to resonant circuits.

Dynamic properties of chasers in a moving queue based on a delayed chasing model

Ning Guo(郭宁), Jian-Xun Ding(丁建勋), Xiang Ling(凌翔), Qin Shi(石琴), Reinhart Kühne

Chin. Phys. B, 2016, 25 (5): 050505. DOI: 10.1088/1674-1056/25/5/050505

Abstract ( 592 )

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A delayed chasing model is proposed to simulate the chase behavior in the queue, where each member regards the closest one ahead as the target, and the leader is attracted to a target point with slight fluctuation. When the initial distances between neighbors possess an identical low value, the fluctuating target of the leader can cause an amplified disturbance in the queue. After a long period of time, the queue recovers the stable state from the disturbance, forming a straight-line-like pattern again, but distances between neighbors grow. Whether the queue can keep stable or not depends on initial distance, desired velocity, and relaxation time. Furthermore, we carry out convergence analysis to explain the divergence transformation behavior and confirm the convergence conditions, which is in approximate agreement with simulations.

Daily variation of radon gas and its short-lived progeny concentration near ground level and estimation of aerosol residence time

M Mohery, A M Abdallah, A Ali, S S Baz

Chin. Phys. B, 2016, 25 (5): 050701. DOI: 10.1088/1674-1056/25/5/050701

Abstract ( 636 )

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Atmospheric concentrations of radon (²²²Rn) gas and its short-lived progenies ²¹⁸Po, ²¹⁴Pb, and ²¹⁴Po were continuously monitored every four hours at the ground level in Jeddah city, Kingdom of Saudi Arabia. The measurements were performed three times every week, starting from November 2014 to October 2015. A method of electrostatic precipitation of positively charged ²¹⁸Po and ²¹⁴Po by a positive voltage was applied for determining ²²²Rn gas concentration. The short-lived ²²²Rn progeny concentration was determined by using a filter holder connected with the alpha-spectrometric technique. The meteorological parameters (relative air humidity, air temperature, and wind speed) were determined during the measurements of ²²²Rn and its progeny concentrations. ²²²Rn gas as well as its short-lived progeny concentration display a daily and seasonal variation with high values in the night and early morning hours as compared to low values at noon and in the afternoon. The observed monthly atmospheric concentrations showed a seasonal trend with the highest values in the autumn/winter season and the lowest values in the spring/summer season. Moreover, and in parallel with alpha-spectrometric measurements, a single filter-holder was used to collect air samples. The deposited activities of ²¹⁴Pb and the long-lived ²²²Rn daughter ²¹⁰Pb on the filter were measured with the gamma spectrometric technique. The measured activity concentrations of ²¹⁴Pb by both techniques were found to be relatively equal largely. The highest mean seasonally activity concentrations of ²¹⁰Pb were observed in the autumn/winter season while the lowest mean were observed in the spring/summer season. The mean residence time (MRT) of aerosol particles in the atmospheric air could be estimated from the activity ratios of ²¹⁰Pb/²¹⁴Pb.

Accurate double many-body expansion potential energy surface of HS₂(A²A') by scaling the external correlation Suggested by editors

Lu-Lu Zhang(张路路), Yu-Zhi Song(宋玉志), Shou-Bao Gao(高守宝), Yuan Zhang(张媛), Qing-Tian Meng(孟庆田)

Chin. Phys. B, 2016, 25 (5): 053101. DOI: 10.1088/1674-1056/25/5/053101

Abstract ( 708 )

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A globally accurate single-sheeted double many-body expansion potential energy surface is reported for the first excited state of HS₂ by fitting the accurate ab initio energies, which are calculated at the multireference configuration interaction level with the aug-cc-pV QZ basis set. By using the double many-body expansion-scaled external correlation method, such calculated ab initio energies are then slightly corrected by scaling their dynamical correlation. A grid of 2767 ab initio energies is used in the least-square fitting procedure with the total root-mean square deviation being 1.406 kcal· mol^-1. The topographical features of the HS₂(A²A') global potential energy surface are examined in detail. The attributes of the stationary points are presented and compared with the corresponding ab initio results as well as experimental and other theoretical data, showing good agreement. The resulting potential energy surface of HS₂(A²A') can be used as a building block for constructing the global potential energy surfaces of larger m S/H molecular systems and recommended for dynamic studies on the title molecular system.

Comparing two iteration algorithms of Broyden electron density mixing through an atomic electronic structure computation

Man-Hong Zhang(张满红)

Chin. Phys. B, 2016, 25 (5): 053102. DOI: 10.1088/1674-1056/25/5/053102

Abstract ( 806 )

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By performing the electronic structure computation of a Si atom, we compare two iteration algorithms of Broyden electron density mixing in the literature. One was proposed by Johnson and implemented in the well-known VASP code. The other was given by Eyert. We solve the Kohn-Sham equation by using a conventional outward/inward integration of the differential equation and then connect two parts of solutions at the classical turning points, which is different from the method of the matrix eigenvalue solution as used in the VASP code. Compared to Johnson's algorithm, the one proposed by Eyert needs fewer total iteration numbers.

Laser frequency locking based on Rydberg electromagnetically induced transparency

Yuechun Jiao(焦月春), Jingkui Li(李敬奎), Limei Wang(王丽梅), Hao Zhang(张好), Linjie Zhang(张临杰), Jianming Zhao(赵建明), Suotang Jia(贾锁堂)

Chin. Phys. B, 2016, 25 (5): 053201. DOI: 10.1088/1674-1056/25/5/053201

Abstract ( 919 )

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We present a laser frequency locking to Rydberg transition with electromagnetically induced transparency (EIT) spectra in a room-temperature cesium vapor cell. Cesium levels 6S_1/2, 6P_3/2, and the nD_5/2 state, compose a cascade three-level system, where a coupling laser drives Rydberg transition, and probe laser detects the EIT signal. The error signal, obtained by demodulating the EIT signal, is used to lock the coupling laser frequency to Rydberg transition. The laser frequency fluctuation, ~0.7 MHz, is obtained after locking on, with the minimum Allan variance to be 8.9× 10^-11. This kind of locking method can be used to stabilize the laser frequency to the excited transition.

Carrier envelope phase effect on the spatial distribution of high-order harmonic generation in asymmetric molecule Suggested by editors

Jun Zhang(张军), Hai-Feng Liu(刘海凤), Xue-Fei Pan(潘雪飞), Hui Du(杜慧), Jing Guo(郭静), Xue-Shen Liu(刘学深)

Chin. Phys. B, 2016, 25 (5): 053202. DOI: 10.1088/1674-1056/25/5/053202

Abstract ( 923 )

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The spatial distribution in high-order harmonic generation (HHG) from the asymmetric diatomic molecule HeH²⁺ is investigated by numerically solving the non-Born-Oppenheimer time-dependent Schrödinger equation (TDSE). The spatial distribution of the HHG spectra shows that there is little contribution in HHG around the geometric center of two nuclei (z = 1.17 a.u.) and the equilibrium internuclear position of the H nucleus (z = 3.11 a.u.). We demonstrate the carrier envelope phase (CEP) effect on the spatial distribution of HHG in a few-cycle laser pulse. The HHG process is investigated by the time evolution of the electronic density distribution. The time-frequency analysis of HHG from two nuclei in HeH²⁺ is presented to further explain the underlying physical mechanism.

Carrier-envelope phase measurement using plasmonic-field-enhanced high-order harmonic generation of H atom in few-cycle laser pulses

Wei Li(李伟), Guo-Li Wang(王国利), Xiao-Xin Zhou(周效信)

Chin. Phys. B, 2016, 25 (5): 053203. DOI: 10.1088/1674-1056/25/5/053203

Abstract ( 570 )

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We investigate the plasmonic-field-enhanced high-order harmonic generation (HHG) of H atom driven by few-cycle laser pulses, by solving the time-dependent Schrödinger equation (TDSE). Compared with the homogeneous field, HHG spectra generated by inhomogeneous field exhibit two-plateau structure. We analyze the origin of the two plateaus by using the semiclassical trajectory method. Our results from both classical and TDSE simulations show that the cutoffs of the two plateaus are dramatically affected by the carrier-envelope phase (CEP) of laser pulse in the inhomogeneous field, even for a little longer pulse. Thus, we can determine the CEP of driving laser based on the cutoff position of HHG generated in the inhomogeneous field.

Coulomb explosion of CS₂ molecule under an intense femtosecond laser field

Xiao Wang(王潇), Jian Zhang(张健), Shi-An Zhang(张诗按), Zhen-Rong Sun(孙真荣)

Chin. Phys. B, 2016, 25 (5): 053301. DOI: 10.1088/1674-1056/25/5/053301

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We experimentally demonstrate the Coulomb explosion process of CS₂ molecule under a near-infrared (800 nm) intense femtosecond laser field by a DC-sliced ion imaging technique. We obtain the DC-sliced images of these fragment ions S⁺, S²⁺, CS⁺, and CS²⁺ by breaking one C-S bond, and assign their Coulomb explosion channels by considering their kinetic energy release and angular distribution. We also numerically simulate the dissociation dynamics of parent ions CS₂^k+ (k=2-4) by a Coulomb potential approximation, and obtain the time evolution of Coulomb energy and kinetic energy release, which indicates that the dissociation time of parent ions CS₂^k+ decreases with the increase of the charge number k. These experimental and theoretical results can serve as a useful benchmark for those researchers who work in the related area.

Structural optimization and segregation behavior of quaternary alloy nanoparticles based on simulated annealing algorithm

Xin-Ze Lu(陆欣泽), Gui-Fang Shao(邵桂芳), Liang-You Xu(许两有), Tun-Dong Liu(刘暾东), Yu-Hua Wen(文玉华)

Chin. Phys. B, 2016, 25 (5): 053601. DOI: 10.1088/1674-1056/25/5/053601

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Alloy nanoparticles exhibit higher catalytic activity than monometallic nanoparticles, and their stable structures are of importance to their applications. We employ the simulated annealing algorithm to systematically explore the stable structure and segregation behavior of tetrahexahedral Pt-Pd-Cu-Au quaternary alloy nanoparticles. Three alloy nanoparticles consisting of 443 atoms, 1417 atoms, and 3285 atoms are considered and compared. The preferred positions of atoms in the nanoparticles are analyzed. The simulation results reveal that Cu and Au atoms tend to occupy the surface, Pt atoms preferentially occupy the middle layers, and Pd atoms tend to segregate to the inner layers. Furthermore, Au atoms present stronger surface segregation than Cu ones. This study provides a fundamental understanding on the structural features and segregation phenomena of multi-metallic nanoparticles.

Microwave-mediated magneto-optical trap for polar molecules ^Hot! Suggested by editors

Dizhou Xie(谢笛舟), Wenhao Bu(卜文浩), Bo Yan(颜波)

Chin. Phys. B, 2016, 25 (5): 053701. DOI: 10.1088/1674-1056/25/5/053701

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Realizing a molecular magneto-optical trap has been a dream for cold molecular physicists for a long time. However, due to the complex energy levels and the small effective Lande g-factor of the excited states, the traditional magneto-optical trap (MOT) scheme does not work very well for polar molecules. One way to overcome this problem is the switching MOT, which requires very fast switching of both the magnetic field and the laser polarizations. Switching laser polarizations is relatively easy, but fast switching of the magnetic field is experimentally challenging. Here we propose an alternative approach, the microwave-mediated MOT, which requires a slight change of the current experimental setup to solve the problem. We calculate the MOT force and compare it with the traditional MOT and the switching MOT scheme. The results show that we can operate a good MOT with this simple setup.

The 650-nm variable optical attenuator based on polymer/silica hybrid waveguide

Yue-Yang Yu(于跃洋), Xiao-Qiang Sun(孙小强), Lan-Ting Ji(姬兰婷), Guo-Bing He(何国冰), Xi-Bin Wang(王希斌), Yun-Ji Yi(衣云骥), Chang-Ming Chen(陈长鸣), Fei Wang(王菲), Da-Ming Zhang(张大明)

Chin. Phys. B, 2016, 25 (5): 054101. DOI: 10.1088/1674-1056/25/5/054101

Abstract ( 728 )

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Visible light variable optical attenuators (VOA) are essential devices in the application of channel power regulation and equalization in wavelength-division multiplexing cross-connect nodes in plastic optical fiber (POF) transmission systems. In this paper, a polymer/silica hybrid waveguide thermo-optic attenuator based on multimode interference (MMI) coupler is designed and fabricated to operate at 650 nm. The single-mode transmission condition, MMI coupler, and transition taper dimensions are optimized through the beam propagation method. Thermal analysis based on material properties provides the optimized heater placement angle. The fabricated VOA presents an attenuation of 26.5 dB with a 21-mW electrical input power at 650 nm. The rise time and fall time are 51.99 and 192 μs, respectively. The time-stability measurement results prove its working reliability.

Dynamic study of compressed electron layer driven by linearly polarized laser

Feng-chao Wang(王凤超)

Chin. Phys. B, 2016, 25 (5): 054102. DOI: 10.1088/1674-1056/25/5/054102

Abstract ( 693 )

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The dynamics of the compressed electron layer (CEL) are investigated when a linearly polarized (LP) laser pulse irradiates a plasma target. The turbulent motion of the CEL is investigated by a simple model, which is verified by particle-in-cell (PIC) simulations. It is found that the compressed layer disperses in a few cycles of the laser duration, because the CEL comes back with a large velocity in the opposite direction of the laser incident. A larger wavelength laser can be used to tailor the proton beam by reducing the turbulence of the CEL in the region of the LP laser acceleration.

Image encryption using random sequence generated from generalized information domain

Xia-Yan Zhang(张夏衍), Guo-Ji Zhang(张国基), Xuan Li(李璇), Ya-Zhou Ren(任亚洲), Jie-Hua Wu(伍杰华)

Chin. Phys. B, 2016, 25 (5): 054201. DOI: 10.1088/1674-1056/25/5/054201

Abstract ( 703 )

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A novel image encryption method based on the random sequence generated from the generalized information domain and permutation-diffusion architecture is proposed. The random sequence is generated by reconstruction from the generalized information file and discrete trajectory extraction from the data stream. The trajectory address sequence is used to generate a P-box to shuffle the plain image while random sequences are treated as keystreams. A new factor called drift factor is employed to accelerate and enhance the performance of the random sequence generator. An initial value is introduced to make the encryption method an approximately one-time pad. Experimental results show that the random sequences pass the NIST statistical test with a high ratio and extensive analysis demonstrates that the new encryption scheme has superior security.

Higher-order nonclassical effects generated by multiple-photon annihilation-then-creation and creation-then-annihilation coherent states

Hong-Chun Yuan(袁洪春), Xue-Xiang Xu(徐学翔), Jin Xiao(肖进), Chao Xiong(熊超), Xi-Fang Zhu(朱锡芳)

Chin. Phys. B, 2016, 25 (5): 054202. DOI: 10.1088/1674-1056/25/5/054202

Abstract ( 732 )

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We explore two observable nonclassical properties of quantum states generated by repeatedly operating annihilation-then-creation (AC) and creation-then-annihilation (CA) on the coherent state, respectively, such as higher-order sub-Poissonian statistics and higher-order squeezing-enhanced effect. The corresponding analytical expressions are derived in detail depending on m. By numerically comparing those quantum properties, it is found that these states above have very different nonclassical properties and nonclassicality is exhibited more strongly after AC operation than after CA operation.

Triple optomechanical induced transparency in a two-cavity system Suggested by editors

Shi-Chao Wu(吴士超), Li-Guo Qin(秦立国), Jun Jing(景俊), Guo-Hong Yang(杨国宏), Zhong-Yang Wang(王中阳)

Chin. Phys. B, 2016, 25 (5): 054203. DOI: 10.1088/1674-1056/25/5/054203

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We theoretically investigate the optomechanical induced transparency (OMIT) phenomenon in a two-cavity system which is composed of two optomechanical cavities. Both of the cavities consist of a fixed mirror and a high-Q mechanical resonator, and they couple to each other via a common waveguide. We show that in the presence of a strong pump field applied to one cavity and a weak probe field applied to the other, a triple-OMIT can be observed in the output field at the probe frequency. The two mechanical resonators in the two cavities are identical, but they lead to different quantum interference pathways. The transparency windows are induced by the coupling of the two cavities and the optical pressure radiated to the mechanical resonators, which can be controlled via the power of the pump field and the coupling strength of the two cavities.

Control of microwave signals using bichromatic electromechanically induced transparency in multimode circuit electromechanical systems

Cheng Jiang(江成), Yuanshun Cui(崔元顺), Xintian Bian(边心田), Xiaowei Li(李晓薇), Guibin Chen(陈贵宾)

Chin. Phys. B, 2016, 25 (5): 054204. DOI: 10.1088/1674-1056/25/5/054204

Abstract ( 590 )

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We theoretically investigate the tunable delay and advancement of microwave signals based on bichromatic electromechanically induced transparency in a three-mode circuit electromechanical system, where two nanomechanical resonators with closely spaced frequencies are independently coupled to a common microwave cavity. In the presence of a strong microwave pump field, we obtain two transparency windows accompanied by steep phase dispersion in the transmitted microwave probe field. The width of the transparency window and the group delay of the probe field can be controlled effectively by the power of the pump field. It is shown that the maximum group delay of 0.12 ms and the advancement of 0.27 ms can be obtained in the current experiments.

A 12.1-W SESAM mode-locked Yb:YAG thin disk laser

Yingnan Peng(彭英楠), Zhaohua Wang(王兆华), Dehua Li(李德华), Jiangfeng Zhu(朱江峰), Zhiyi Wei(魏志义)

Chin. Phys. B, 2016, 25 (5): 054205. DOI: 10.1088/1674-1056/25/5/054205

Abstract ( 783 )

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Pumped by a 940 nm fiber-coupled diode laser, a passively mode-locked Yb:YAG thin disk oscillator was demonstrated with a semiconductor saturable absorber mirror (SESAM). 12.1 W mode-locked pulses were obtained with pulse duration of 698 fs at the repetition rate of 57.43 MHz. Measurement showed that the beam quality was close to the diffraction limit.

Analysis of melt ejection during long pulsed laser drilling Suggested by editors

Ting-Zhong Zhang(张廷忠), Zhi-Chao Jia(贾志超), Hai-Chao Cui(崔海超), De-Hua Zhu(朱德华), Xiao-Wu Ni(倪晓武), Jian Lu(陆健)

Chin. Phys. B, 2016, 25 (5): 054206. DOI: 10.1088/1674-1056/25/5/054206

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In pulsed laser drilling, melt ejection greatly influences the keyhole shape and its quality as well, but its mechanism has not been well understood. In this paper, numerical simulation and experimental investigations based on 304 stainless steel and aluminum targets are performed to study the effects of material parameters on melt ejection. The numerical method is employed to predict the temperatures, velocity fields in the solid, liquid, and vapour front, and melt pool dynamics of targets as well. The experimental methods include the shadow-graphic technique, weight method, and optical microscope imaging, which are applied to real-time observations of melt ejection phenomena, measurements of collected melt and changes of target mass, observations of surface morphology and the cross-section of the keyhole, respectively. Numerical and experimental results show that the metallic material with high thermal diffusivity like aluminum is prone to have a thick liquid zone and a large quantity of melt ejection. Additionally, to the best of our knowledge, the liquid zone is used to illustrate the relations between melt ejection and material thermal diffusivity for the first time. The research result in this paper is useful for manufacturing optimization and quality control in laser-material interaction.

Lasing dynamics study by femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy

Wei Dang(党伟), Qing Liao(廖清), Peng-Cheng Mao(毛鹏程), Hong-Bing Fu(付红兵), Yu-Xiang Weng(翁羽翔)

Chin. Phys. B, 2016, 25 (5): 054207. DOI: 10.1088/1674-1056/25/5/054207

Abstract ( 740 )

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Femtosecond time-resolved fluorescence non-collinear optical parametric amplification spectroscopy (FNOPAS) is a versatile technique with advantages of high sensitivity, broad detection bandwidth, and intrinsic spectrum correction function. These advantages should benefit the study of coherent emission, such as measurement of lasing dynamics. In this letter, the FNOPAS was used to trace the lasing process in Rhodamine 6G (R6G) solution and organic semiconductor nano-wires. High-quality transient emission spectra and lasing dynamic traces were acquired, which demonstrates the applicability of FNOPAS in the study of lasing dynamics. Our work extends the application scope of the FNOPAS technique.

Optical bistability and multistability in a defect slab doped by GaAs/AlGaAs multiple quantum wells Suggested by editors

Seyyed Hossein Asadpour, G Solookinejad, M Panahi, E Ahmadi Sangachin

Chin. Phys. B, 2016, 25 (5): 054208. DOI: 10.1088/1674-1056/25/5/054208

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We proposed a new model for controlling the optical bistability (OB) and optical multistability (OM) in a defect slab doped with four-level GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5 nm GaAs wells and 15-nm Al_0.3 Ga_0.7As barriers. The effects of biexciton energy renormalization, exciton spin relaxation, and thickness of the slab on the OB and OM properties of the defect slab were theoretically investigated. We found that the transition from OB to OM or vice versa is possible by adjusting the controllable parameters in a lab. Moreover, the transmission, reflection, and absorption properties of the weak probe light through the slab were also discussed in detail.

A novel single-order diffraction grating: Random position rectangle grating Suggested by editors

Zuhua Yang(杨祖华), Qiangqiang Zhang(张强强), Jing Wang(王静), Quanping Fan(范全平), Yuwei Liu(刘钰薇), Lai Wei(魏来), Leifeng Cao(曹磊峰)

Chin. Phys. B, 2016, 25 (5): 054209. DOI: 10.1088/1674-1056/25/5/054209

Abstract ( 837 )

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Spectral diagnosis of radiation from laser plasma interaction and monochromation of radiation source are hot and important topics recently. Grating is one of the primary optical elements to do this job. Although easy to fabricate, traditional diffraction grating suffers from multi-order diffraction contamination. On the other hand, sinusoidal amplitude grating has the nonharmonic diffraction property, but it is too difficult to fabricate, especially for x-ray application. A novel nonharmonic diffraction grating named random position rectangle grating (RPRG) is proposed in this paper. Theoretical analysis and experiment results show that the RPRG is both higher order diffraction suppressing and not difficult to fabricate. Additionally, it is highly efficient; its first order absolute theoretical diffraction efficiency reaches 4.1%. Our result shows that RPRG is a novel tool for radiation diagnosis and monochromation.

Flow control of micro-ramps on supersonic forward-facing step flow

Qing-Hu Zhang(张庆虎), Tao Zhu(朱涛), Shihe Yi(易仕和), Anping Wu(吴岸平)

Chin. Phys. B, 2016, 25 (5): 054701. DOI: 10.1088/1674-1056/25/5/054701

Abstract ( 708 )

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The effects of the micro-ramps on supersonic turbulent flow over a forward-facing step (FFS) was experimentally investigated in a supersonic low-noise wind tunnel at Mach number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spatiotemporal resolution images and velocity fields of supersonic flow over the testing model were captured. The fine structures and their spatial evolutionary characteristics without and with the micro-ramps were revealed and compared. The large-scale structures generated by the micro-ramps can survive the downstream FFS flowfield. The micro-ramps control on the flow separation and the separation shock unsteadiness was investigated by PIV results. With the micro-ramps, the reduction in the range of the reversal flow zone in streamwise direction is 50% and the turbulence intensity is also reduced. Moreover, the reduction in the average separated region and in separation shock unsteadiness are 47% and 26%, respectively. The results indicate that the micro-ramps are effective in reducing the flow separation and the separation shock unsteadiness.

Structural transitions of SWNT filled with C₆₀ under high pressure

Yong-gang Zou(邹永刚), Li Xu(徐莉), Kun Tian(田锟), He Zhang(张贺), Xiao-hui Ma(马晓辉), Ming-guang Yao(姚明光)

Chin. Phys. B, 2016, 25 (5): 056101. DOI: 10.1088/1674-1056/25/5/056101

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Raman spectra of C₆₀ filled single-walled carbon nanotubes (C₆₀@SWNTs) with diameters of 1.3-1.5 nm have been studied under high pressure. A plateau in the pressure dependence of the G-band frequency at around 10 GPa was observed in both experiments with 514 nm and 830 nm excitation lasers, which is similar to the high pressure behaviors of pristine SWNTs. This structural transition has been assigned to the transformation into a peanut-like structure of the nanotubes. At pressure below 2 GPa, no obvious Raman signature related to the structural transition of nanotubes was observed, unlike what has been reported for C₇₀ filled nanotubes. We discussed this point in terms of the arrangement differences of C₆₀ and C₇₀ molecules inside the nanotubes. At higher pressure up to 15 GPa, a graphite-like pressure evolution was observed in our C₆₀@SWNTs.

Stability of concentration-related self-interstitial atoms in fusion material tungsten

Hong Zhang(张红), Shu-Long Wen(温述龙), Min Pan(潘敏), Zheng Huang(黄整), Yong Zhao(赵勇), Xiang Liu(刘翔), Ji-Ming Chen(谌继明)

Chin. Phys. B, 2016, 25 (5): 056102. DOI: 10.1088/1674-1056/25/5/056102

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Based on the density functional theory, we calculated the structures of the two main possible self-interstitial atoms (SIAs) as well as the migration energy of tungsten (W) atoms. It was found that the difference of the (110) and (111) formation energies is 0.05-0.3 eV. Further analysis indicated that the stability of SIAs is closely related to the concentration of the defect. When the concentration of the point defect is high, (110) SIAs are more likely to exist, (111) SIAs are the opposite. In addition, the vacancy migration probability and self-recovery zones for these SIAs were researched by making a detailed comparison. The calculation provided a new viewpoint about the stability of point defects for self-interstitial configurations and would benefit the understanding of the control mechanism of defect behavior for this novel fusion material.

Effect of pressure on electronic and thermoelectric properties of magnesium silicide: A density functional theory study

Kulwinder Kaur, Ranjan Kumar

Chin. Phys. B, 2016, 25 (5): 056401. DOI: 10.1088/1674-1056/25/5/056401

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We study the effect of pressure on electronic and thermoelectric properties of Mg₂}Si using the density functional theory and Boltzmann transport equations. The variation of lattice constant, band gap, bulk modulus with pressure is also analyzed. Further, the thermoelectric properties (Seebeck coefficient, electrical conductivity, electronic thermal conductivity) have been studied as a function of temperature and pressure up to 1200 K. The results show that Mg₂Si is an n-type semiconductor with a band gap of 0.21 eV. The negative value of the Seebeck coefficient at all pressures indicates that the conduction is due to electrons. With the increase in pressure, the Seebeck coefficient decreases and electrical conductivity increases. It is also seen that, there is practically no effect of pressure on the electronic contribution of thermal conductivity. The paper describes the calculation of the lattice thermal conductivity and figure of merit of Mg₂}Si at zero pressure. The maximum value of figure of merit is attained 1.83×10^-3 at 1000 K. The obtained results are in good agreement with the available experimental and theoretical results.

Thermal effect on endurance performance of 3-dimensional RRAM crossbar array Suggested by editors

Nianduan Lu(卢年端), Pengxiao Sun(孙鹏霄), Ling Li(李泠), Qi Liu(刘琦), Shibing Long(龙世兵), Hangbing Lv(吕杭炳), Ming Liu(刘明)

Chin. Phys. B, 2016, 25 (5): 056501. DOI: 10.1088/1674-1056/25/5/056501

Abstract ( 759 )

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Three-dimensional (3D) crossbar array architecture is one of the leading candidates for future ultra-high density nonvolatile memory applications. To realize the technological potential, understanding the reliability mechanisms of the 3D RRAM array has become a field of intense research. In this work, the endurance performance of the 3D 1D1R crossbar array under the thermal effect is investigated in terms of numerical simulation. It is revealed that the endurance performance of the 3D 1D1R array would be seriously deteriorated under thermal effects as the feature size scales down to a relatively small value. A possible method to alleviate the thermal effects is provided and verified by numerical simulation.

Thermodynamic behaviour of Rashba quantum dot in the presence of magnetic field

Sukirti Gumber, Manoj Kumar, Pradip Kumar Jha, Man Mohan

Chin. Phys. B, 2016, 25 (5): 056502. DOI: 10.1088/1674-1056/25/5/056502

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The thermodynamic properties of an InSb quantum dot have been investigated in the presence of Rashba spin-orbit interaction and a static magnetic field. The energy spectrum and wave-functions for the system are obtained by solving the Schrodinger wave-equation analytically. These energy levels are employed to calculate the specific heat, entropy, magnetization and susceptibility of the quantum dot system using canonical formalism. It is observed that the system is susceptible to maximum heat absorption at a particular value of magnetic field which depends on the Rashba coupling parameter as well as the temperature. The variation of specific heat shows a Schottky-like anomaly in the low temperature limit and rapidly converges to the value of 2k_B with the further increase in temperature. The entropy of the quantum dot is found to be inversely proportional to the magnetic field but has a direct variation with temperature. The substantial effect of Rashba spin-orbit interaction on the magnetic properties of quantum dot is observed at low values of magnetic field and temperature.

Topological phase boundary in a generalized Kitaev model

Da-Ping Liu(刘大平)

Chin. Phys. B, 2016, 25 (5): 057101. DOI: 10.1088/1674-1056/25/5/057101

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We study the effects of the next-nearest-neighbor hopping and nearest-neighbor interactions on topological phases in a one-dimensional generalized Kitaev model. In the noninteracting case, we define a topological number and calculate exactly the phase diagram of the system. With addition of the next-nearest-neighbor hopping, the change of phase boundary between the topological and trivial regions can be described by an effective shift of the chemical potential. In the interacting case, we obtain the entanglement spectrum, the degeneracies of which correspond to the topological edge modes, by using the infinite time-evolving block decimation method. The results show that the interactions change the phase boundary as adding an effective chemical potential which can be explained by the change of the average number of particles.

First-principles study of the elastic and thermodynamic properties of thorium hydrides at high pressure

Xiao-Lin Zhang(张晓林), Yuan-Yuan Wu(武媛媛), Xiao-Hong Shao(邵晓红), Yong Lu(鲁勇), Ping Zhang(张平)

Chin. Phys. B, 2016, 25 (5): 057102. DOI: 10.1088/1674-1056/25/5/057102

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The high pressure behaviors of Th₄H₁₅ and ThH₂ are investigated by using the first-principles calculations based on the density functional theory (DFT). From the energy-volume relations, the bct phase of ThH₂ is more stable than the fcc phase at ambient conditions. At high pressure, the bct ThH₂ and bcc Th₄H₁₅ phases are more brittle than they are at ambient pressure from the calculated elastic constants and the Poisson ratio. The thermodynamic stability of the bct phase ThH₂ is determined from the calculated phonon dispersion. In the pressure domain of interest, the phonon dispersions of bcc Th₄H₁₅ and bct ThH₂ are positive, indicating the dynamical stability of these two phases, while the fcc ThH₂ is unstable. The thermodynamic properties including the lattice vibration energy, entropy, and specific heat are predicted for these stable phases. The vibrational free energy decreases with the increase of the temperature, and the entropy and the heat capacity are proportional to the temperature and inversely proportional to the pressure. As the pressure increases, the resistance to the external pressure is strengthened for Th₄H₁₅ and ThH₂.

First-principles study of strain effect on the formation and electronic structures of oxygen vacancy in SrFeO₂

Wei Zhang(张玮), Jie Huang(黄洁)

Chin. Phys. B, 2016, 25 (5): 057103. DOI: 10.1088/1674-1056/25/5/057103

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Motivated by recent experimental observations of metallic conduction in the quasi-two-dimensional SrFeO₂, we study the epitaxial strain effect on the formation and electronic structures of oxygen vacancy (V_o) by first-principles calculations. The bulk SrFeO₂ is found to have the G-type antiferromagnetic ordering (G-AFM) at zero strain, which agrees with the experiment. Under compressive strain the bulk SrFeO₂ keeps the G-AFM and has the trend of Mott insulator-metal transition. Different from most of the previous similar work about the strain effect on V_o, both the tensile strain and the compressive strain enhance the V_o formation. It is found that the competitions between the band energies and the electrostatic interactions are the dominant mechanisms in determining the V_o formation. We confirm that the V_o in SrFeO₂ would induce the n-type conductivity where the donor levels are occupied by the delocalized d_x²-_y² electrons. It is suggested that the vanishing of n-type conductivity observed by the Hall measurement on the strained films are caused by the shift of donor levels into the conduction band. These results would provide insightful information for the realization of metallic conduction in SrFeO₂.

Pressure induced magnetic and semiconductor-metal phase transitions in Cr₂MoO₆

San-Dong Guo(郭三栋)

Chin. Phys. B, 2016, 25 (5): 057104. DOI: 10.1088/1674-1056/25/5/057104

Abstract ( 1016 )

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We investigate magnetic ordering and electronic structures of Cr₂MoO₆ under hydrostatic pressure. To overcome the band gap problem, the modified Becke and Johnson exchange potential is used to investigate the electronic structures of Cr₂MoO₆. The insulating nature at the experimental crystal structure is produced, with a band gap of 1.04 eV, and the magnetic moment of the Cr atom is 2.50 μ_B, compared to an experimental value of about 2.47 μ_B. The calculated results show that an antiferromagnetic inter-bilayer coupling-ferromagnetic intra-bilayer coupling to a ferromagnetic inter-bilayer coupling-antiferromagnetic intra-bilayer coupling phase transition is produced with the pressure increasing. The magnetic phase transition is simultaneously accompanied by a semiconductor-metal phase transition. The magnetic phase transition can be explained by the Mo-O hybridization strength, and ferromagnetic coupling between two Cr atoms can be understood by empty Mo-d bands perturbing the nearest O-p orbital.

Homopolar bonds in Se-rich Ge—As—Se chalcogenide glasses

Si-Wei Xu(许思维), Rong-Ping Wang(王荣平), Zhi-Yong Yang(杨志勇), Li Wang(王丽), Luther-Davies Barry

Chin. Phys. B, 2016, 25 (5): 057105. DOI: 10.1088/1674-1056/25/5/057105

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We have prepared three groups of Ge-As-Se glasses in which the Se content is 5.5 mol%, 10 mol%, and 20 mol% rich, respectively. We explored the possibility of suppressing the formation of the Ge-Ge and As-As homopolar bonds in the glasses. Thermal kinetics analysis indicated that the 5.5 mol% Se-rich Ge_11.5As₂₄Se_64.5 glass exhibits the minimum fragility and thus is most stable against structural relaxation. Analysis of the Raman spectra of the glasses indicated that the Ge-Ge and As-As homopolar bonds could be almost completely suppressed in 20 mol% Se-rich Ge₁₅As₁₄Se₇₁ glass.

Optical study of charge dynamics in CaCo₂As₂ Suggested by editors

Wei Zhang(张威), Bing Xu(许兵), Run Yang(杨润), Jin-Yun Liu(刘金云), Hao Yang(杨浩), Xiang-Gang Qiu(邱祥冈)

Chin. Phys. B, 2016, 25 (5): 057201. DOI: 10.1088/1674-1056/25/5/057201

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We present an infrared spectroscopy study of charge dynamics in CaCo₂As₂ single crystal. In this material, the optical conductivity can be described by two Drude components with different scattering rates (1/τ): a broad incoherent background and a narrow Drude component. By monitoring the temperature dependence, we find that only the narrow Drude component is temperature-dependent and determines the transport properties. Especially a Fermi liquid behavior of carriers is revealed by the T² behavior in the dc resistivity ρ_n and scattering rate 1/τ_n, indicating a coherent nature of quasiparticles in the narrow Drude subsystem.

First-principles modeling hydrogenation of bilayered boron nitride Suggested by editors

Jing Wang(王静), Peng Zhang(张鹏), Xiang-Mei Duan(段香梅)

Chin. Phys. B, 2016, 25 (5): 057301. DOI: 10.1088/1674-1056/25/5/057301

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We have investigated the structural and electronic characteristics of hydrogenated boron-nitride bilayer (H-BNBN-H) using first-principles calculations. The results show that hydrogenation can significantly reduce the energy gap of the BN-BN into the visible-light region. Interestingly, the electric field induced by the interface dipoles helps to promote the formation of well-separated electron-hole pairs, as demonstrated by the charge distribution of the VBM and CBM. Moreover, the applied bias voltage on the vertical direction of the bilayer could modulate the band gap, resulting in transition from semiconductor to metal. We conclude that H-BNBN-H could improve the solar energy conversion efficiency, which may provide a new way for tuning the electronic devices to meet different environments and demands.

Electronic and magnetic properties at rough alloyed interfaces of Fe/Co on Au substrates: An augmented space study Suggested by editors

Priyadarshini Parida, Biplab Ganguli

Chin. Phys. B, 2016, 25 (5): 057302. DOI: 10.1088/1674-1056/25/5/057302

Abstract ( 633 )

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We studied the interface electronic and magnetic properties of Fe/Co deposited on Au substrate and researched the effects of roughness at the interfaces within augmented space formalism (ASF). The full calculation is carried out by recursion and tight-binding linear muffin tin orbital (TB-LMTO) methods. The amount of roughness is different at different atomic layers. The formalism is also applied to sharp interface, when interdiffusion of atoms is negligible. Our results of one monolayer transition metal agree with other reported results. A realistic rough interface is also modeled with three and four monolayers of transition metals, deposited on Au substrates.

Tunable Fano resonances and plasmonic hybridization of gold triangle-rod dimer nanostructure

Meng Huang(黄萌), Dong Chen(陈栋), Li Zhang(张利), Jun Zhou(周骏)

Chin. Phys. B, 2016, 25 (5): 057303. DOI: 10.1088/1674-1056/25/5/057303

Abstract ( 704 )

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A gold dimer structure consisting of a notched triangle nanoslice and a rectangle nanorod is proposed to produce distinct Fano resonance. Owing to the coupling between the dipole plasmon mode of the nanorod and the dipole or quadrupole plasmon mode of the nanoslice, the extinction spectrum with a deep Fano dip is formed and can be well fitted by the Fano interference model for different geometry parameters. In addition, Fano resonance of the gold dimer nanostructure also intensely depends on the polarization direction of incident light. Moreover, Fano resonance of the triangle-rod trimer is also analyzed by adding another nanorod into the former dimer and exhibits the splitting of plasmonic resonant peak in high order coupling modes. The plasmonic hybridizations in these nanostructures have been analyzed for revealing the physical origin of the Fano resonance.

High-performance germanium n⁺/p junction by nickel-induced dopant activation of implanted phosphorus at low temperature

Wei Huang(黄巍), Chao Lu(陆超), Jue Yu(余珏), Jiang-Bin Wei(魏江镔), Chao-Wen Chen(陈超文), Jian-Yuan Wang(汪建元), Jian-Fang Xu(徐剑芳), Chen Wang(王尘), Cheng Li(李成), Song-Yan Chen(陈松岩), Chun-Li Liu(刘春莉), Hong-Kai Lai(赖虹凯)

Chin. Phys. B, 2016, 25 (5): 057304. DOI: 10.1088/1674-1056/25/5/057304

Abstract ( 744 )

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High-performance Ge n⁺/p junctions were fabricated at a low formation temperature from 325℃ to 400℃ with a metal(nickel)-induced dopant activation technique. The obtained NiGe electroded Ge n⁺/p junction has a rectification ratio of 5.6×10⁴ and a forward current of 387 A/cm² at -1 V bias. The Ni-based metal-induced dopant activation technique is expected to meet the requirement of the shallow junction of Ge MOSFET.

Magnetoresistance and exchange bias in high Mn content melt-spun Mn₄₆Ni₄₂Sn₁₁Sb₁ alloy ribbon

Qingxue Huang(黄庆学), Fenghua Chen(陈峰华), Mingang Zhang(张敏刚), Xiaohong Xu(许小红)

Chin. Phys. B, 2016, 25 (5): 057305. DOI: 10.1088/1674-1056/25/5/057305

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Highly textured Heusler alloy Mn₄₆Ni₄₂Sn₁₁Sb₁ ribbons were prepared by melt spinning. The annealed high Mn content Mn₄₆Ni₄₂Sn₁₁Sb₁ ribbon cross-section microstructure, crystal structure, martensitic transformation (MT), and magnetoresistance (MR) properties were investigated. The MR in the annealed ribbon was assessed by the magnetic field direction perpendicular to the ribbon surface with the magnetic field up to 30 kOe. The large negative value of 25% for MR was obtained at 244 K. The exchange bias (EB) effects of the as-spun and annealed ribbons were investigated. After annealing, the EB effects have been improved by about 25 Oe at the temperature of 50 K. The magnetizations have increased approximately by 10% more than the as-spun ribbon.

Contact resistance asymmetry of amorphous indium-gallium-zinc-oxide thin-film transistors by scanning Kelvin probe microscopy ^Hot! Suggested by editors

Chen-Fei Wu(武辰飞), Yun-Feng Chen(陈允峰), Hai Lu(陆海), Xiao-Ming Huang(黄晓明), Fang-Fang Ren(任芳芳), Dun-Jun Chen(陈敦军), Rong Zhang(张荣), You-Dou Zheng(郑有炓)

Chin. Phys. B, 2016, 25 (5): 057306. DOI: 10.1088/1674-1056/25/5/057306

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In this work, a method based on scanning Kelvin probe microscopy is proposed to separately extract source/drain (S/D) series resistance in operating amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors. The asymmetry behavior of S/D contact resistance is deduced and the underlying physics is discussed. The present results suggest that the asymmetry of S/D contact resistance is caused by the difference in bias conditions of the Schottky-like junction at the contact interface induced by the parasitic reaction between contact metal and a-IGZO. The overall contact resistance should be determined by both the bulk channel resistance of the contact region and the interface properties of the metal-semiconductor junction.

Electron transport in electrically biased inverse parabolic double-barrier structure

M Bati, S Sakiroglu, I Sokmen

Chin. Phys. B, 2016, 25 (5): 057307. DOI: 10.1088/1674-1056/25/5/057307

Abstract ( 574 )

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A theoretical study of resonant tunneling is carried out for an inverse parabolic double-barrier structure subjected to an external electric field. Tunneling transmission coefficient and density of states are analyzed by using the non-equilibrium Green's function approach based on the finite difference method. It is found that the resonant peak of the transmission coefficient, being unity for a symmetrical case, reduces under the applied electric field and depends strongly on the variation of the structure parameters.

Length dependence of rectification in organic co-oligomer spin rectifiers

Gui-Chao Hu(胡贵超), Zhao Zhang(张朝), Ying Li(李营), Jun-Feng Ren(任俊峰), Chuan-Kui Wang(王传奎)

Chin. Phys. B, 2016, 25 (5): 057308. DOI: 10.1088/1674-1056/25/5/057308

Abstract ( 660 )

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The rectification ratio of organic magnetic co-oligomer diodes is investigated theoretically by changing the molecular length. The results reveal two distinct length dependences of the rectification ratio: for a short molecular diode, the charge-current rectification changes little with the increase of molecular length, while the spin-current rectification is weakened sharply by the length; for a long molecular diode, both the charge-current and spin-current rectification ratios increase quickly with the length. The two kinds of dependence switch at a specific length accompanied with an inversion of the rectifying direction. The molecular ortibals and spin-resolved transmission analysis indicate that the dominant mechanism of rectification suffers a change at this specific length, that is, from asymmetric shift of molecular eigenlevels to asymmetric spatial localization of wave functions upon the reversal of bias. This work demonstrates a feasible way to control the rectification in organic co-oligomer spin diodes by adjusting the molecular length.

Developing Josephson junction array chips for microvolt applications

Wenhui Cao(曹文会), Jinjin Li(李劲劲), Yuan Zhong(钟源), Yuan Gao(高原), Honghui Li(李红晖), Zengmin Wang(王曾敏), Qing He(贺青)

Chin. Phys. B, 2016, 25 (5): 057401. DOI: 10.1088/1674-1056/25/5/057401

Abstract ( 727 )

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Josephson junction array chips for microvolt applications have been designed and fabricated. A voltage step as small as 1 μV has been observed for a single junction in the array when it is driven by 483.59 MHz microwave. By selecting different parts of the array, it can output a voltage from 1 μV to 256 μV. The flat region of the voltage steps is over 200 μA. This kind of array is useful for potential microvolt applications.

Effect of residual stress on nematic domains in BaFe_2-xNi_xAs₂ studied by angular magnetoresistance

Ding Hu(胡定), Wenliang Zhang(张汶良), Yuan Wei(魏源), Xiaotian Zhang(张笑天), Cong Ren(任聪), Lei Shan(单磊), Yi-feng Yang(杨义峰), Huiqian Luo(罗会仟), Shiliang Li(李世亮)

Chin. Phys. B, 2016, 25 (5): 057402. DOI: 10.1088/1674-1056/25/5/057402

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We have studied the angular magnetoresistance of iron pnictides BaFe_2-xNi_xAs₂, which shows clear 180 degree periodicity as fitted by a cosine function. In the x = 0.065 sample, the phase of the two-fold symmetry changes 90 degrees above the tetragonal-to-orthorhombic structural transition temperature T_s. Since the phase at low temperature is associated with the rotation of orthorhombic domains by magnetic field, we show that even vacuum grease can push the presence of orthorhombic domains at temperatures much higher than T_s. Our results suggest that residual stress may have significant effects in studying the nematic orders and its fluctuations in iron pnictides.

First-principles investigation of electronic structure, effective carrier masses, and optical properties of ferromagnetic semiconductor CdCr₂S₄ Suggested by editors

Xu-Hui Zhu(朱旭辉), Xiang-Rong Chen(陈向荣), Bang-Gui Liu(刘邦贵)

Chin. Phys. B, 2016, 25 (5): 057501. DOI: 10.1088/1674-1056/25/5/057501

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The electronic structures, the effective masses, and optical properties of spinel CdCr₂S₄ are studied by using the full-potential linearized augmented planewave method and a modified Becke-Johnson exchange functional within the density-functional theory. Most importantly, the effects of the spin-orbit coupling (SOC) on the electronic structures and carrier effective masses are investigated. The calculated band structure shows a direct band gap. The electronic effective mass and the hole effective mass are analytically determined by reproducing the calculated band structures near the BZ center. SOC substantially changes the valence band top and the hole effective masses. In addition, we calculated the corresponding optical properties of the spinel structure CdCr₂S₄. These should be useful to deeply understand spinel CdCr₂S₄ as a ferromagnetic semiconductor for possible semiconductor spintronic applications.

Abnormal variation of magnetic properties with Ce content in (PrNdCe)₂Fe₁₄B sintered magnets prepared by dual alloy method

Xue-Feng Zhang(张雪峰), Jian-Ting Lan(兰剑亭), Zhu-Bai Li(李柱柏), Yan-Li Liu(刘艳丽), Le-Le Zhang(张乐乐), Yong-Feng Li(李永峰), Qian Zhao(赵倩)

Chin. Phys. B, 2016, 25 (5): 057502. DOI: 10.1088/1674-1056/25/5/057502

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Resource-saving (PrNdCe)₂Fe₁₄B sintered magnets with nominal composition (PrNd)_15-xCe_xFe₇₇B₈ (x=0-10) were prepared using a dual alloy method by mixing (PrNd)₅Ce₁₀Fe₇₇B₈ with (PrNd)₁₅Fe₇₇B₈ powders. For Ce atomic percent of 1% and 2%, coercivity decreases dramatically. With further increase of Ce atomic percent, the coercivity increases, peaks at 6.38 kOe in (PrNd)₁₁Ce₄Fe₇₇B₈, and then declines gradually. The abnormal dependence of coercivity is likely related to the inhomogeneity of rare earth chemical composition in the intergranular phase, where PrNd concentration is strongly dependent on the additive amount of (PrNd)₅Ce₁₀Fe₇₇B₈ powders. In addition, for Ce atomic percent of 8%, 7%, and 6% the coercivity is higher than that of magnets prepared by the conventional method, which shows the advantage of the dual alloy method in preparing high abundant rare earth magnets.

Dielectric and piezoelectric properties of (110) oriented Pb(Zr_1-xTi_x)O₃ thin films

Jian-Hua Qiu(邱建华), Zhi-Hui Chen(陈智慧), Xiu-Qin Wang(王秀琴), Ning-Yi Yuan(袁宁一), Jian-Ning Ding(丁建宁)

Chin. Phys. B, 2016, 25 (5): 057701. DOI: 10.1088/1674-1056/25/5/057701

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A phenomenological Landau-Devonshire theory is developed to investigate the ferroelectric, dielectric, and piezoelectric properties of (110) oriented Pb(Zr_1-xTi_x)O₃ (x=0.4, 0.5, 0.6, and 0.7) thin films. At room temperature, the tetragonal a₁ phase, the orthorhombic a₂c phase, the triclinic γ₁ phase, and the triclinic γ₂ phase are stable. The appearance of the negative polarization component P₂ in the a₂c phase and the γ₁ phase is attributed to the nonlinear coupling terms in the thermodynamic potential. The γ phase of the Pb(Zr_1-xTi_x)O₃ thin films has better dielectric and piezoelectric properties than the a₂c phase and the a₁ phase. The largest dielectric and piezoelectric coefficients are obtained in the Pb(Zr_0.5Ti_0.5)O₃ thin film. The piezoelectric coefficient of 110-150 pm/V is obtained in the (110) oriented Pb(Zr_0.5Ti_0.5)O₃ thin film, and the Pb(Zr_0.3Ti_0.7)O₃ thin film has the remnant polarization and relative dielectric constant of 50 μC/cm² and 100, respectively, which are in agreement with the experimental measurements reported in the literature.

Distribution of electron traps in SiO₂/HfO₂ nMOSFET

Xiao-Hui Hou(侯晓慧), Xue-Feng Zheng(郑雪峰), Ao-Chen Wang(王奥琛), Ying-Zhe Wang(王颖哲), Hao-Yu Wen(文浩宇), Zhi-Jing Liu(刘志镜), Xiao-Wei Li(李小炜), Yin-He Wu(吴银河)

Chin. Phys. B, 2016, 25 (5): 057702. DOI: 10.1088/1674-1056/25/5/057702

Abstract ( 771 )

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In this paper, the principle of discharge-based pulsed I-V technique is introduced. By using it, the energy and spatial distributions of electron traps within the 4-nm HfO₂ layer have been extracted. Two peaks are observed, which are located at ΔE~-1.0 eV and -1.43 eV, respectively. It is found that the former one is close to the SiO₂/HfO₂ interface and the latter one is close to the gate electrode. It is also observed that the maximum discharge time has little effect on the energy distribution. Finally, the impact of electrical stress on the HfO₂ layer is also studied. During stress, no new electron traps and interface states are generated. Meanwhile, the electrical stress also has no impact on the energy and spatial distribution of as-grown traps. The results provide valuable information for theoretical modeling establishment, material assessment, and reliability improvement for advanced semiconductor devices.

Observation of positive and small electron affinity of Si-doped AlN films grown by metalorganic chemical vapor deposition on n-type 6H-SiC

Feng Liang(梁锋), Ping Chen(陈平), De-Gang Zhao(赵德刚), De-Sheng Jiang(江德生), Zhi-Juan Zhao(赵志娟), Zong-Shun Liu(刘宗顺), Jian-Jun Zhu(朱建军), Jing Yang(杨静), Wei Liu(刘炜), Xiao-Guang He(何晓光), Xiao-Jing Li(李晓静), Xiang Li(李翔), Shuang-Tao Liu(刘双韬), Hui Yang(杨辉), Li-Qun Zhang(张立群), Jian-Ping Liu(刘建平), Yuan-Tao Zhang(张源涛), Guo-Tong Du(杜国同)

Chin. Phys. B, 2016, 25 (5): 057703. DOI: 10.1088/1674-1056/25/5/057703

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We have investigated the electron affinity of Si-doped AlN films (N_Si= 1.0 ×10¹⁸-1.0×10¹⁹ cm^-3) with thicknesses of 50, 200, and 400 nm, synthesized by metalorganic chemical vapor deposition (MOCVD) under low pressure on the n-type (001)6H-SiC substrates. The positive and small electron affinity of AlN films was observed through the ultraviolet photoelectron spectroscopy (UPS) analysis, where an increase in electron affinity appears with the thickness of AlN films increasing, i.e., 0.36 eV for the 50-nm-thick one, 0.58 eV for the 200-nm-thick one, and 0.97 eV for the 400-nm-thick one. Accompanying the x-ray photoelectron spectroscopy (XPS) analysis on the surface contaminations, it suggests that the difference of electron affinity between our three samples may result from the discrepancy of surface impurity contaminations.

Field-induced phase transitions in chiral smectic liquid crystals studied by the constant current method

H Dhaouadi, R Zgueb, O Riahi, F Trabelsi, T Othman

Chin. Phys. B, 2016, 25 (5): 057704. DOI: 10.1088/1674-1056/25/5/057704

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In ferroelectric liquid crystals, phase transitions can be induced by an electric field. The current constant method allows these transition to be quickly localized and thus the (E,T) phase diagram of the studied product can be obtained. In this work, we make a slight modification to the measurement principles based on this method. This modification allows the characteristic parameters of ferroelectric liquid crystal to be quantitatively measured. The use of a current square signal highlights a phenomenon of ferroelectric hysteresis with remnant polarization at null field, which points out an effect of memory in this compound.

Effect of size and indium-composition on linear and nonlinear optical absorption of InGaN/GaN lens-shaped quantum dot

Ahmed S Jbara, Zulkafli Othaman, M A Saeed

Chin. Phys. B, 2016, 25 (5): 057801. DOI: 10.1088/1674-1056/25/5/057801

Abstract ( 693 )

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Based on the Schrödinger equation for envelope function in the effective mass approximation, linear and nonlinear optical absorption coefficients in a multi-subband lens quantum dot are investigated. The effects of quantum dot size on the interband and intraband transitions energy are also analyzed. The finite element method is used to calculate the eigenvalues and eigenfunctions. Strain and In-mole-fraction effects are also studied, and the results reveal that with the decrease of the In-mole fraction, the amplitudes of linear and nonlinear absorption coefficients increase. The present computed results show that the absorption coefficients of transitions between the first excited states are stronger than those of the ground states. In addition, it has been found that the quantum dot size affects the amplitudes and peak positions of linear and nonlinear absorption coefficients while the incident optical intensity strongly affects the nonlinear absorption coefficients.

Origin of strain-induced resonances in flexible terahertz metamaterials

Xiu-Yun Sun(孙秀云), Li-Ren Zheng(郑立人), Xiao-Ning Li(李枭宁), Hua Xu(徐华), Xian-Ting Liang(梁先庭), Xian-Peng Zhang(张贤鹏), Yue-Hui Lu(鲁越晖), Young-Pak Lee, Joo-Yull Rhee, Wei-Jie Song(宋伟杰)

Chin. Phys. B, 2016, 25 (5): 057802. DOI: 10.1088/1674-1056/25/5/057802

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Two types of flexible terahertz metamaterials were fabricated on polyethylene naphthalate (PEN) substrates. The unit cell of one type consists of two identical split-ring resonators (SRRs) that are arranged face-to-face (i.e., FlexMetaF); the unit cell of the other type has nothing different but is arranged back-to-back (i.e., FlexMetaB). FlexMetaF and FlexMetaB illustrate the similar transmission dips under zero strain because the excitation of fundamental inductive-capacitive (LC) resonance is mainly dependent on the geometric structure of individual SRR. However, if a gradually variant strain is applied to bend FlexMetaF and FlexMetaB, the new resonant peaks appear: in the case of FlexMetaF, the peaks are located at the lower frequencies; in the case of FlexMetaB, the peaks appear at the frequencies adjacent to the LC resonance. The origin and evolution of strain-induced resonances are studied. The origin is ascribed to the detuning effect and the different responses to strain from FlexMetaF and FlexMetaB are associated with the coupling effect. These findings may improve the understanding on flexible terahertz metamaterials and benefit their applications in flexible or curved devices.

Broadband tunability of surface plasmon resonance in graphene-coating silica nanoparticles ^Hot! Suggested by editors

Zhe Shi(史哲), Yang Yang(杨阳), Lin Gan(甘霖), Zhi-Yuan Li(李志远)

Chin. Phys. B, 2016, 25 (5): 057803. DOI: 10.1088/1674-1056/25/5/057803

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Graphene decorated nanomaterials and nanostructures can potentially be used in military and medical science applications. In this article, we study the optical properties of a graphene wrapping silica core-shell spherical nanoparticle under illumination of external light by using the Mie theory. We find that the nanoparticle can exhibit surface plasmon resonance (SPR) that can be broadly tuned from mid infrared to near infrared via simply changing the geometric parameters. A simplified equivalent dielectric permittivity model is developed to better understand the physics of SPR, and the calculation results agree well qualitatively with the rigorous Mie theory. Both calculations suggest that a small radius of graphene wrapping nanoparticle with high Fermi level could move the SPR wavelength of graphene into the near infrared regime.

Temperature dependent direct-bandgap light emission and optical gain of Ge

Zhi Liu(刘智), Chao He(何超), Dongliang Zhang(张东亮), Chuanbo Li(李传波), Chunlai Xue(薛春来), Yuhua Zuo(左玉华), Buwen Cheng(成步文)

Chin. Phys. B, 2016, 25 (5): 057804. DOI: 10.1088/1674-1056/25/5/057804

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Band structure, electron distribution, direct-bandgap light emission, and optical gain of tensile strained, n-doped Ge at different temperatures were calculated. We found that the heating effects not only increase the electron occupancy rate in the Γ valley of Ge by thermal excitation, but also reduce the energy difference between its Γ valley and L valley. However, the light emission enhancement of Ge induced by the heating effects is weakened with increasing tensile strain and n-doping concentration. This phenomenon could be explained by that Ge is more similar to a direct bandgap material under tensile strain and n-doping. The heating effects also increase the optical gain of tensile strained, n-doped Ge at low temperature, but decrease it at high temperature. At high temperature, the hole and electron distributions become more flat, which prevent obtaining higher optical gain. Meanwhile, the heating effects also increase the free-carrier absorption. Therefore, to obtain a higher net maximum gain, the tensile strained, n-doped Ge films on Si should balance the gain increased by the heating effects and the optical loss induced by the free-carrier absorption.

Effects of Mg doping in the quantum barriers on the efficiency droop of GaN based light emitting diodes

Yang Liu(刘扬), Yongchun Yang(杨永春)

Chin. Phys. B, 2016, 25 (5): 058101. DOI: 10.1088/1674-1056/25/5/058101

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The effects of Mg doping in the quantum barriers (QBs) on the efficiency droop of GaN based light emitting diodes (LEDs) were investigated through a duel wavelength method. Barrier Mg doping would lead to the enhanced hole transportation and reduced polarization field in the quantum wells (QWs), both may reduce the efficiency droop. However, heavy Mg doping in the QBs would strongly deteriorate the crystal quality of the QWs grown after the doped QB. When increasing the injection current, the carriers would escape from the QWs between n-GaN and the doped QB and recombine non-radiatively in the QWs grown after the doped QB, leading to a serious efficiency droop.

First-principles study of the structural, electronic, and magnetic properties of double perovskite Sr₂FeReO₆ containing various imperfections

Yan Zhang(张研), Li Duan(段理), Vincent Ji, Ke-Wei Xu(徐可为)

Chin. Phys. B, 2016, 25 (5): 058102. DOI: 10.1088/1674-1056/25/5/058102

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The structural, electronic, and magnetic properties of double perovskite Sr₂FeReO₆ containing eight different imperfections of Fe_Re or Re_Fe antisites, Fe1-Re1 or Fe1-Re4 interchanges, V_Fe, V_Re, V_O or V_Sr vacancies have been studied by using the first-principles projector augmented wave (PAW) within generalized gradient approximation as well as taking into account the on-site Coulomb repulsive interaction (GGA+U). No obvious structural changes are observed for the imperfect Sr₂FeReO₆ containing Fe_Re or Re_Fe antisites, Fe1-Re1 or Fe1-Re4 interchanges, or V_Sr vacancy defects. However, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) V_Fe or V_Re (V_O) vacancies. The half-metallic (HM) character is maintained for the imperfect Sr₂FeReO₆ containing Fe_Re or Re_Fe antisites, Fe1-Re4 interchange, V_Fe, V_O or V_Sr vacancies, while it vanishes when the Fe1-Re1 interchange or V_Re vacancy is presented. So the Fe1-Re1 interchange and the V_Re vacancy defects should be avoided to preserve the HM character of Sr₂FeReO₆ and thus usage in spintronic devices. In the Fe_Re or Re_Fe antisites, Fe1-Re1 or Fe1-Re4 interchanges cases, the spin moments of the Fe (Re) cations situated on Re (Fe) antisites are in an antiferromagnetic coupling with those of the Fe(Re) cations on the regular sites. In the V_Fe, V_Re, V_O, or V_Sr vacancies cases, a ferromagnetic coupling is obtained within each cation sublattice, while the two cation sublattices are coupled antiferromagnetically. The total magnetic moments μ_tot ( μ_B/f.u.) of the imperfect Sr₂FeReO₆ containing eight different defects decrease in the sequence of V_Sr vacancy (3.50), V_Re vacancy (3.43), Fe_Re antisite (2.74), V_O vacancy (2.64), V_Fe vacancy (2.51), Re_Fe antisite (2.29), Fe1-Re4 interchange (1.96), Fe1-Re1 interchange (1.87), and the mechanisms of the saturation magnetization reduction have been analyzed.

Enhanced circular dichroism based on the dual-chiral metamaterial in terahertz regime

Jian Shao(邵健), Jie Li(李杰), Ying-Hua Wang(王英华), Jia-Qi Li(李家奇), Zheng-Gao Dong(董正高), Lin Zhou(周林)

Chin. Phys. B, 2016, 25 (5): 058103. DOI: 10.1088/1674-1056/25/5/058103

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The obvious circular dichroism (CD) and optical activity can be obtained based on the chiral metamaterial due to the plasmon-enhanced effect, which is very attractive for future compact devices with enhanced capabilities of light manipulation. In this paper, we propose a dual-chiral metamaterial composed of bilayer asymmetric split ring resonators (ASRR) that are in mirror-symmetry shape. It is demonstrated that the CD can get enhancement in the terahertz regime. Moreover, the CD can be further improved by modulating the asymmetry of ASRR. The enhanced CD effect in the terahertz regime has great potential applications in sensing, biomedical imaging, and molecular recognition.

Characterization of atomic-layer MoS₂ synthesized using a hot filament chemical vapor deposition method

Ying-Zi Peng(彭英姿), Yang Song(宋扬), Xiao-Qiang Xie(解晓强), Yuan Li(李源), Zheng-Hong Qian(钱正洪), Ru Bai(白茹)

Chin. Phys. B, 2016, 25 (5): 058104. DOI: 10.1088/1674-1056/25/5/058104

Abstract ( 694 )

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Atomic-layer MoS₂ ultrathin films are synthesized using a hot filament chemical vapor deposition method. A combination of atomic force microscopy (AFM), x-ray diffraction (XRD), high-resolution transition electron microscopy (HRTEM), photoluminescence (PL), and x-ray photoelectron spectroscopy (XPS) characterization methods is applied to investigate the crystal structures, valence states, and compositions of the ultrathin film areas. The nucleation particles show irregular morphology, while for a larger size somewhere, the films are granular and the grains have a triangle shape. The films grow in a preferred orientation (002). The HRTEM images present the graphene-like structure of stacked layers with low density of stacking fault, and the interlayer distance of plane is measured to be about 0.63 nm. It shows a clear quasi-honeycomb-like structure and 6-fold coordination symmetry. Room-temperature PL spectra for the atomic layer MoS₂ under the condition of right and left circular light show that for both cases, the A1 and B1 direct excitonic transitions can be observed. In the meantime, valley polarization resolved PL spectra are obtained. XPS measurements provide high-purity samples aside from some contaminations from the air, and confirm the presence of pure MoS₂. The stoichiometric mole ratio of S/Mo is about 2.0-2.1, suggesting that sulfur is abundant rather than deficient in the atomic layer MoS₂ under our experimental conditions.

Structural and photoluminescence studies on europium-doped lithium tetraborate (Eu:Li₂B₄O₇) single crystal grown by microtube Czochralski (μT-Cz) technique

A Kumaresh, R Arun Kumar, N Ravikumar, U Madhusoodanan, B S Panigrahi, K Marimuthu, M Anuradha

Chin. Phys. B, 2016, 25 (5): 058105. DOI: 10.1088/1674-1056/25/5/058105

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Rare earth europium (Eu³⁺)-doped lithium tetraborate (Eu:Li₂B₄O₇) crystal is grown from its stoichiometric melt by microtube Czochralski pulling technique (μT-Cz) for the first time. The grown crystals are subjected to powder x-ray diffraction (PXRD) analysis which reveals the tetragonal crystal structure of the crystals. UV-vis-NIR spectral analysis is carried out to study the optical characteristics of the grown crystals. The crystal is transparent in the entire visible region, and the lower cutoff is observed to be at 304 nm. The existence of BO₃ and BO₄ bonding structure and the molecular associations are analyzed by Fourier transform infrared (FTIR) spectroscopy. The results of excitation and emission-photoluminescence spectra of europium ion incorporated in lithium tetraborate (LTB) single crystal reveal that the observations of peaks at 258, 297, and 318 nm in the excitation spectra and peaks at 579, 591, 597, 613, and 651 nm are observed in the emission spectra. The chromaticity coordinates are calculated from the emission spectra, and the emission intensity of the grown crystal is characterized through a CIE 1931 (Commission International d'Eclairage) color chromaticity diagram.

Influences of different structures on the characteristics of H₂O-based and O₃-based La_xAl_yO films deposited by atomic layer deposition

Chen-Xi Fei(费晨曦), Hong-Xia Liu(刘红侠), Xing Wang(汪星), Dong-Dong Zhao(赵冬冬), Shu-Long Wang(王树龙), Shu-Peng Chen(陈树鹏)

Chin. Phys. B, 2016, 25 (5): 058106. DOI: 10.1088/1674-1056/25/5/058106

Abstract ( 757 )

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H₂O-based and O₃-based La_xAl_yO nanolaminate films were deposited on Si substrates by atomic layer deposition (ALD). Structures and performances of the films were changed by different barrier layers. The effects of different structures on the electrical characteristics and physical properties of the La_xAl_yO films were studied. Chemical bonds in the La_xAl_yO films grown with different structures and different oxidants were also investigated with x-ray photoelectron spectroscopy (XPS). The preliminary testing results indicate that the La_xAl_yO films with different structures and different oxidants show different characteristics, including dielectric constant, equivalent oxide thickness (EOT), electrical properties, and stability.

A nano-scale mirror-like surface of Ti-6Al-4V attained by chemical mechanical polishing

Chenliang Liang(梁晨亮), Weili Liu(刘卫丽), Shasha Li(李沙沙), Hui Kong(孔慧), Zefang Zhang(张泽芳), Zhitang Song(宋志棠)

Chin. Phys. B, 2016, 25 (5): 058301. DOI: 10.1088/1674-1056/25/5/058301

Abstract ( 713 )

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Metal Ti and its alloys have been widely utilized in the fields of aviation, medical science, and micro-electro-mechanical systems, for its excellent specific strength, resistance to corrosion, and biological compatibility. As the application of Ti moves to the micro or nano scale, however, traditional methods of planarization have shown their short slabs. Thus, we introduce the method of chemical mechanical polishing (CMP) to provide a new way for the nano-scale planarization method of Ti alloys. We obtain a mirror-like surface, whose flatness is of nano-scale, via the CMP method. We test the basic mechanical behavior of Ti-6Al-4V (Ti64) in the CMP process, and optimize the composition of CMP slurry. Furthermore, the possible reactions that may take place in the CMP process have been studied by electrochemical methods combined with x-ray photoelectron spectroscopy (XPS). An equivalent circuit has been built to interpret the dynamic of oxidation. Finally, a model has been established to explain the synergy of chemical and mechanical effects in the CMP of Ti-6Al-4V.

A G-band terahertz monolithic integrated amplifier in 0.5-μm InP double heterojunction bipolar transistor technology

Ou-Peng Li(李欧鹏), Yong Zhang(张勇), Rui-Min Xu(徐锐敏), Wei Cheng(程伟), Yuan Wang(王元), Bing Niu(牛斌), Hai-Yan Lu(陆海燕)

Chin. Phys. B, 2016, 25 (5): 058401. DOI: 10.1088/1674-1056/25/5/058401

Abstract ( 847 )

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Design and characterization of a G-band (140-220 GHz) terahertz monolithic integrated circuit (TMIC) amplifier in eight-stage common-emitter topology are performed based on the 0.5-μm InGaAs/InP double heterojunction bipolar transistor (DHBT). An inverted microstrip line is implemented to avoid a parasitic mode between the ground plane and the InP substrate. The on-wafer measurement results show that peak gains are 20 dB at 140 GHz and more than 15-dB gain at 140-190 GHz respectively. The saturation output powers are -2.688 dBm at 210 GHz and -2.88 dBm at 220 GHz, respectively. It is the first report on an amplifier operating at the G-band based on 0.5-μm InP DHBT technology. Compared with the hybrid integrated circuit of vacuum electronic devices, the monolithic integrated circuit has the advantage of reliability and consistency. This TMIC demonstrates the feasibility of the 0.5-μm InGaAs/InP DHBT amplifier in G-band frequencies applications.

Levitation and lateral forces between a point magnetic dipole and a superconducting sphere

H M Al-Khateeb, M K Alqadi, F Y Alzoubi, B Albiss, M K Hasan (Qaseer), N Y Ayoub

Chin. Phys. B, 2016, 25 (5): 058402. DOI: 10.1088/1674-1056/25/5/058402

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The dipole-dipole interaction model is employed to investigate the angular dependence of the levitation and lateral forces acting on a small magnet in an anti-symmetric magnet/superconducting sphere system. Breaking the symmetry of the system enables us to study the lateral force which is important in the stability of the magnet above a superconducting sphere in the Meissner state. Under the assumption that the lateral displacement of the magnet is small compared to the physical dimensions of our proposed system, analytical expressions are obtained for the levitation and lateral forces as a function of the geometrical parameters of the superconductor as well as the height, the lateral displacement, and the orientation of the magnetic moment of the magnet. The dependence of the levitation force on the height of the levitating magnet is similar to that in the symmetric magnet/superconducting sphere system within the range of proposed lateral displacements. It is found that the levitation force is linearly dependent on the lateral displacement whereas the lateral force is independent of this displacement. A sinusoidal variation of both forces as a function of the polar and azimuthal angles specifying the orientation of the magnetic moment is observed. The relationship between the stability and the orientation of the magnetic moment is discussed for different orientations.

Increasing energy relaxation time of superconducting qubits with nonmagnetic infrared filter and shield Suggested by editors

Yuhao Liu(刘宇浩), Mengmeng Li(李蒙蒙), Dong Lan(兰栋), Guangming Xue(薛光明), Xinsheng Tan(谭新生), Haifeng Yu(于海峰), Yang Yu(于扬)

Chin. Phys. B, 2016, 25 (5): 058501. DOI: 10.1088/1674-1056/25/5/058501

Abstract ( 942 )

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One of the primary origins of the energy relaxation in superconducting qubits is the quasiparticle loss. The quasiparticles can be excited remarkably by infrared radiation. In order to minimize the density of quasiparticle and increase the qubit relaxation time, we design and fabricate the infrared filter and shield for superconducting qubits. In comparison with previous filters and shields, a nonmagnetic dielectric is used as the infrared absorbing material, greatly suppressing the background magnetic fluctuations. The filters can be made to impedance-match with other microwave devices. Using the as-fabricated infrared filter and shield, we increased the relaxation time of a transmon qubit from 519 ns to 1125 ns.

A novel circuit design for complementary resistive switch-based stateful logic operations

Xiao-Ping Wang(王小平), Lin Chen(陈林), Yi Shen(沈轶), Bo-Wen Xu(徐博文)

Chin. Phys. B, 2016, 25 (5): 058502. DOI: 10.1088/1674-1056/25/5/058502

Abstract ( 850 )

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Recently, it has been demonstrated that memristors can be utilized as logic operations and memory elements. In this paper, we present a novel circuit design for complementary resistive switch (CRS)-based stateful logic operations. The proposed circuit can automatically write the destructive CRS cells back to the original states. In addition, the circuit can be used in massive passive crossbar arrays which can reduce sneak path current greatly. Moreover, the steps for CRS logic operations using our proposed circuit are reduced compared with previous circuit designs. We validate the effectiveness of our scheme through Hspice simulations on the logic circuits.

High-speed waveguide-integrated Ge/Si avalanche photodetector

Hui Cong(丛慧), Chunlai Xue(薛春来), Zhi Liu(刘智), Chuanbo Li(李传波), Buwen Cheng(步成文), Qiming Wang(王启明)

Chin. Phys. B, 2016, 25 (5): 058503. DOI: 10.1088/1674-1056/25/5/058503

Abstract ( 857 )

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Waveguide-integrated Ge/Si heterostructure avalanche photodetectors (APDs) were designed and fabricated using a CMOS-compatible process on 8-inch SOI substrate. The structure of the APD was designed as separate-absorption-charge-multiplication (SACM) using germanium and silicon as absorption region and multiplication region, respectively. The breakdown voltage (V_b) of such a device is 19 V at reverse bias and dark current appears to be 0.71 μA at 90% of the V_b. The device with a 10-μ length and 7-μ width of Ge layer shows a maximum 3-dB bandwidth of 17.8 GHz at the wavelength of 1550 nm. For the device with a 30-μ-length Ge region, gain-bandwidth product achieves 325 GHz.

Two-color light-emitting diodes with polarization-sensitive high extraction efficiency based on graphene

H Sattarian, S Shojaei, E Darabi

Chin. Phys. B, 2016, 25 (5): 058504. DOI: 10.1088/1674-1056/25/5/058504

Abstract ( 656 )

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In the present study, graphene photonic crystals are employed to enhance the light extraction efficiency (LEE) of two-color, red and blue, light-emitting diode (LED). The transmission characteristics of one-dimensional (1D) Fibonacci graphene photonic crystal LED (FGPC-LED) are investigated by using the transfer matrix method and the scaling study is presented. We analyzed the influence of period, thickness, and permittivity in the structure to enhance the LEE. The transmission spectrum of 1D FGPC has been optimized in detail. In addition, the effects of the angle of incidence and the state of polarization are investigated. As the main result, we found the optimum values of relevant parameters to enhance the extraction of red and blue light from an LED as well as provide perfect omnidirectional and high peak transmission filters for the TE and TM modes.

Stability of weighted spectral distribution in a pseudo tree-like network model

Bo Jiao(焦波), Yuan-ping Nie(聂原平), Cheng-dong Huang(黄赪东), Jing Du(杜静), Rong-hua Guo(郭荣华), Fei Huang(黄飞), Jian-mai Shi(石建迈)

Chin. Phys. B, 2016, 25 (5): 058901. DOI: 10.1088/1674-1056/25/5/058901

Abstract ( 792 )

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The comparison of networks with different orders strongly depends on the stability analysis of graph features in evolving systems. In this paper, we rigorously investigate the stability of the weighted spectral distribution (i.e., a spectral graph feature) as the network order increases. First, we use deterministic scale-free networks generated by a pseudo tree-like model to derive the precise formula of the spectral feature, and then analyze the stability of the spectral feature based on the precise formula. Except for the scale-free feature, the pseudo tree-like model exhibits the hierarchical and small-world structures of complex networks. The stability analysis is useful for the classification of networks with different orders and the similarity analysis of networks that may belong to the same evolving system.

Distributed event-triggered consensus tracking of second-order multi-agent systems with a virtual leader

Jie Cao(曹劼), Zhi-Hai Wu(吴治海), Li Peng(彭力)

Chin. Phys. B, 2016, 25 (5): 058902. DOI: 10.1088/1674-1056/25/5/058902

Abstract ( 841 )

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This paper investigates the consensus tracking problems of second-order multi-agent systems with a virtual leader via event-triggered control. A novel distributed event-triggered transmission scheme is proposed, which is intermittently examined at constant sampling instants. Only partial neighbor information and local measurements are required for event detection. Then the corresponding event-triggered consensus tracking protocol is presented to guarantee second-order multi-agent systems to achieve consensus tracking. Numerical simulations are given to illustrate the effectiveness of the proposed strategy.

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