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Chin. Phys. B  
  Chin. Phys. B--2018, Vol.27, No.1
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GENERAL

Improved reproducing kernel particle method for piezoelectric materials

Ji-Chao Ma, Gao-Feng Wei, Dan-Dan Liu
Chin. Phys. B 2018, 27 (1): 010201;  doi: 10.1088/1674-1056/27/1/010201
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In this paper, the normal derivative of the radial basis function (RBF) is introduced into the reproducing kernel particle method (RKPM), and the improved reproducing kernel particle method (IRKPM) is proposed. The method can decrease the errors on the boundary and improve the accuracy and stability of the algorithm. The proposed method is applied to the numerical simulation of piezoelectric materials and the corresponding governing equations are derived. The numerical results show that the IRKPM is more stable and accurate than the RKPM.

Finite-time robust control of uncertain fractional-order Hopfield neural networks via sliding mode control

Yangui Xi, Yongguang Yu, Shuo Zhang, Xudong Hai
Chin. Phys. B 2018, 27 (1): 010202;  doi: 10.1088/1674-1056/27/1/010202
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The finite-time control of uncertain fractional-order Hopfield neural networks is investigated in this paper. A switched terminal sliding surface is proposed for a class of uncertain fractional-order Hopfield neural networks. Then a robust control law is designed to ensure the occurrence of the sliding motion for stabilization of the fractional-order Hopfield neural networks. Besides, for the unknown parameters of the fractional-order Hopfield neural networks, some estimations are made. Based on the fractional-order Lyapunov theory, the finite-time stability of the sliding surface to origin is proved well. Finally, a typical example of three-dimensional uncertain fractional-order Hopfield neural networks is employed to demonstrate the validity of the proposed method.

A novel stable value iteration-based approximate dynamic programming algorithm for discrete-time nonlinear systems

Yan-Hua Qu, An-Na Wang, Sheng Lin
Chin. Phys. B 2018, 27 (1): 010203;  doi: 10.1088/1674-1056/27/1/010203
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The convergence and stability of a value-iteration-based adaptive dynamic programming (ADP) algorithm are considered for discrete-time nonlinear systems accompanied by a discounted quadric performance index. More importantly than sufficing to achieve a good approximate structure, the iterative feedback control law must guarantee the closed-loop stability. Specifically, it is firstly proved that the iterative value function sequence will precisely converge to the optimum. Secondly, the necessary and sufficient condition of the optimal value function serving as a Lyapunov function is investigated. We prove that for the case of infinite horizon, there exists a finite horizon length of which the iterative feedback control law will provide stability, and this increases the practicability of the proposed value iteration algorithm. Neural networks (NNs) are employed to approximate the value functions and the optimal feedback control laws, and the approach allows the implementation of the algorithm without knowing the internal dynamics of the system. Finally, a simulation example is employed to demonstrate the effectiveness of the developed optimal control method.

Quantum Monte Carlo study of hard-core bosons in Creutz ladder with zero flux

Yang Lin, Weichang Hao, Huaiming Guo
Chin. Phys. B 2018, 27 (1): 010204;  doi: 10.1088/1674-1056/27/1/010204
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The quantum phase of hard-core bosons in Creutz ladder with zero flux is studied. For a specific regime of the parameters (tx=tp, ty<0), the exact ground-state is found analytically, which is a dimerized insulator with one electron bound in each rung of the ladder. For the case tx,ty,tp>0, the system is exactly studied using quantum Monte Carlo (QMC) method without a sign problem. It is found that the system is a Mott insulator for small tp and a quantum phase transition to a superfluid phase is driven by increasing tp. The critical tpc is determined precisely by a scaling analysis. Since it is possible that the Creutz ladder is realized experimentally, the theoretical results are interesting to the cold-atom experiments.

Solution of the spin-one DKP oscillator under an external magnetic field in noncommutative space with minimal length

Bing-Qian Wang, Zheng-Wen Long, Chao-Yun Long, Shu-Rui Wu
Chin. Phys. B 2018, 27 (1): 010301;  doi: 10.1088/1674-1056/27/1/010301
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The spin-one Duffin-Kemmer-Petiau (DKP) oscillator under a magnetic field in the presence of the minimal length in the noncommutative coordinate space is studied by using the momentum space representation. The explicit form of energy eigenvalues is found, and the eigenfunctions are obtained in terms of the Jacobi polynomials. It shows that for the same azimuthal quantum number, the energy E increases monotonically with respect to the noncommutative parameter and the minimal length parameter. Additionally, we also report some special cases aiming to discuss the effect of the noncommutative coordinate space and the minimal length in the energy spectrum.

Non-Markovian speedup dynamics control of the damped Jaynes-Cummings model with detuning

Kai Xu, Wei Han, Ying-Jie Zhang, Heng Fan
Chin. Phys. B 2018, 27 (1): 010302;  doi: 10.1088/1674-1056/27/1/010302
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For a two-level atom in a lossy cavity, a scheme to manipulate the non-Markovian speedup dynamics has been proposed in the controllable environment (the lossy cavity field). We mainly focus on the effects of the qubit-cavity detuning and the qubit-cavity coupling strength κ on the non-Markovian speedup evolution of an open system. By controlling the environment, i.e., tuning and κ, two dynamical crossovers from Markovian to non-Markovian and from no-speedup to speedup are achieved. Furthermore, it is clearly found that increasing the coupling strength κ or detuning in some cases can make the environmental non-Markovianity stronger and hence can lead to faster evolution of the open system.

Quantum speed limit time of a two-level atom under different quantum feedback control

Min Yu, Mao-Fa Fang, Hong-Mei Zou
Chin. Phys. B 2018, 27 (1): 010303;  doi: 10.1088/1674-1056/27/1/010303
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We investigate the quantum speed limit time (QSLT) of a two-level atom under quantum-jump-based feedback control or homodyne-based feedback control. Our results show that the two different feedback control schemes have different influences on the evolutionary speed. By adjusting the feedback parameters, the quantum-jump-based feedback control can induce speedup of the atomic evolution from an excited state, but the homodyne-based feedback control cannot change the evolutionary speed. Additionally, the QSLT for the whole dynamical process is explored. Under the quantum-jump-based feedback control, the QSLT displays oscillatory behaviors, which implies multiple speed-up and speed-down processes during the evolution. While, the homodyne-based feedback control can accelerate the speed-up process and improve the uniform speed in the uniform evolution process.

Quantum coherence and non-Markovianity of an atom in a dissipative cavity under weak measurement

Yu Liu, Hong-Mei Zou, Mao-Fa Fang
Chin. Phys. B 2018, 27 (1): 010304;  doi: 10.1088/1674-1056/27/1/010304
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Quantum coherence and non-Markovianity of an atom in a dissipative cavity under weak measurement are investigated in this work. We find that:the quantum coherence obviously depends on the initial atomic state, the strength of the weak measurement and its reversal, the atom-cavity coupling constant and the non-Markovian effect. It is obvious that the weak measurement effect protects the coherence better. The quantum coherence is preserved more efficiently for larger atom-cavity coupling. The stronger the non-Markovian effect is, the more slowly the coherence reduces. The quantum coherence can be effectively protected by means of controlling these physical parameters.

Improved quantum randomness amplification with finite number of untrusted devices based on a novel extractor

Ming-Feng Xu, Wei Pan, Lian-Shan Yan, Bin Luo, Xi-Hua Zou, Peng-Hua Mu, Li-Yue Zhang
Chin. Phys. B 2018, 27 (1): 010305;  doi: 10.1088/1674-1056/27/1/010305
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Quantum randomness amplification protocols have increasingly attracted attention for their fantastic ability to amplify weak randomness to almost ideal randomness by utilizing quantum systems. Recently, a realistic noise-tolerant randomness amplification protocol using a finite number of untrusted devices was proposed. The protocol has the composable security against non-signalling eavesdroppers and could produce a single bit of randomness from weak randomness sources, which is certified by the violation of certain Bell inequalities. However, the protocol has a non-ignorable limitation on the min-entropy of independent sources. In this paper, we further develop the randomness amplification method and present a novel quantum randomness amplification protocol based on an explicit non-malleable two independent-source randomness extractor, which could remarkably reduce the above-mentioned specific limitation. Moreover, the composable security of our improved protocol is also proposed. Our results could significantly expand the application range for practical quantum randomness amplification, and provide a new insight on the practical design method for randomness extraction.

Novel quantum watermarking algorithm based on improved least significant qubit modification for quantum audio

Zhi-Guo Qu, Huang-Xing He, Tao Li
Chin. Phys. B 2018, 27 (1): 010306;  doi: 10.1088/1674-1056/27/1/010306
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As one of essential multimedia in quantum networks, the copyright protection of quantum audio has gradually become an important issue in the domain of quantum information hiding in the decades. In this paper, an improved quantum watermarking algorithm based on quantum audio by using least significant qubit (LSQb) modification is proposed. Compared with the previous achievements, it can effectively improve the robustness and security of watermark for copyright protection of quantum audio. In the new algorithm, the least significant bites and the peripheral least significant bits of the amplitudes are modified in terms of their logical consistency and correlation to enhance watermark robustness of resisting various illegal attacks. Furthermore, the new algorithm can avoid the weak robustness defect of many previous algorithms that directly embedded the watermark into the least significant bits. In order to implement the new algorithm, some specific quantum circuits are designed to obtain better applicability and scalability for embedding and extracting watermark. Finally, the simulation results including the values of audio waveforms and signal to noise ratios (SNR) prove that the new algorithm has good transparency, robustness, and security.

Soliton excitations in a polariton condensate with defects

Abderahim Mahmoud Belounis, Salem Kessal
Chin. Phys. B 2018, 27 (1): 010307;  doi: 10.1088/1674-1056/27/1/010307
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To study soliton excitations in a polariton condensate with defects, we use the Gross-Pitaevskii equation and its hydrodynamic form. An extra term is added to take into account the non-equilibrium nature of the polariton condensate and the presence of defects. The reductive perturbation method transforms these hydrodynamic equations into a modified Korteweg-de Vries equation in the long wavelength limit. We linearize this equation and study the soliton linear excitations. We give an analytic expression of traveling excitations using the variation of constants method. In the more general form, we show numerically that the excitations are oscillations, i.e., the amplitude and the width of the dark soliton oscillate simultaneously but in an opposite way.

Transitionless driving on local adiabatic quantum search algorithm

Feng-guang Li, Wan-su Bao, Shuo Zhang, Xiang Wang, He-liang Huang, Tan Li, Bo-wen Ma
Chin. Phys. B 2018, 27 (1): 010308;  doi: 10.1088/1674-1056/27/1/010308
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We apply the transitionless driving on the local adiabatic quantum search algorithm to speed up the adiabatic process. By studying quantum dynamics of the adiabatic search algorithm with the equivalent two-level system, we derive the transitionless driving Hamiltonian for the local adiabatic quantum search algorithm. We found that when adding a transitionless quantum driving term HD ≤ ft(t) on the local adiabatic quantum search algorithm, the success rate is 1 exactly with arbitrary evolution time by solving the time-dependent Schrödinger equation in eigen-picture. Moreover, we show the reason for the drastic decrease of the evolution time is that the driving Hamiltonian increases the lowest eigenvalues to a maximum of O ≤ ft(√N).

Current transport and mass separation for an asymmetric fluctuation system with correlated noises

Jie Wang, Li-Juan Ning
Chin. Phys. B 2018, 27 (1): 010501;  doi: 10.1088/1674-1056/27/1/010501
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We discuss the transport of an underdamped particle driven by an external fluctuation force in a spatially periodic asymmetric potential with correlated noises. The corresponding mathematical model is established. The movement of the steady current of an underdamped particle is presented by the method of the numerical simulation. It is indicated that the value of the current may be negative, zero, or positive. The external fluctuation force and correlated noises can effect the current direction. Under the appropriate parameters, the correlated noises intensity may even raise a reversal of the current. Besides, we have noticed a phenomenon that particles with different weight have different directions during movement by the impact of the correlated noises and external fluctuation force. Therefore, the Brownian particles can be effectively separated according to their masses.

Detection of meso-micro scale surface features based on microcanonical multifractal formalism

Yuanyuan Yang, Wei Chen, Tao Xie, William Perrie
Chin. Phys. B 2018, 27 (1): 010502;  doi: 10.1088/1674-1056/27/1/010502
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Synthetic aperture radar (SAR) is an effective tool to analyze the features of the ocean. In this paper, the microcanonical multifractal formalism is used to analyze SAR images to obtain meso-micro scale surface features. We use the Sobel operator to calculate the gradient of each point in the image, then operate continuous variable scale wavelet transform on this gradient matrix. The relationship between the wavelet coefficient and scale is built by linear regression. This relationship decides the singular exponents of every point in the image which contain local and global features. The manifolds in the ocean can be revealed by analyzing these exponents. The most singular manifold, which is related to the streamlines in the SAR images, can be obtained with a suitable threshold of the singular exponents. Experiments verify that application of the microcanonical multifractal formalism to SAR image analysis is effective for detecting the meso-micro scale surface information.

Topological horseshoe analysis and field-programmable gate array implementation of a fractional-order four-wing chaotic attractor

En-Zeng Dong, Zhen Wang, Xiao Yu, Zeng-Qiang Chen, Zeng-Hui Wang
Chin. Phys. B 2018, 27 (1): 010503;  doi: 10.1088/1674-1056/27/1/010503
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We present a fractional-order three-dimensional chaotic system, which can generate four-wing chaotic attractor. Dynamics of the fractional-order system is investigated by numerical simulations. To rigorously verify the chaos properties of this system, the existence of horseshoe in the four-wing attractor is presented. Firstly, a Poincaré section is selected properly, and a first-return Poincaré map is established. Then, a one-dimensional tensile horseshoe is discovered, which verifies the chaos existence of the system in mathematical view. Finally, the fractional-order chaotic attractor is implemented physically with a field-programmable gate array (FPGA) chip, which is useful in further engineering applications of information encryption and secure communications.

Controllable optical superregular breathers in the femtosecond regime

Yang Ren, Zhan-Ying Yang, Chong Liu, Wen-Li Yang
Chin. Phys. B 2018, 27 (1): 010504;  doi: 10.1088/1674-1056/27/1/010504
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We investigate optical superregular breathers in the femtosecond regime under dispersion management in an inhomogeneous fiber governed by the nonautonomous higher-order nonlinear Schrödinger equation (NLSE). Exact solutions describing the dynamics of superregular breathers are obtained. Furthermore, we discuss the propagation behaviors of controllable superregular breathers, including stabilization and recurrence in an exponential dispersion fiber and a periodic distributed fiber system. Particularly, the nonlinear dynamics of superregular modes evolved from an identical initial small-amplitude modulation is analyzed in detail.

Leader-following consensus of discrete-time fractional-order multi-agent systems

Erfan Shahamatkhah, Mohammad Tabatabaei
Chin. Phys. B 2018, 27 (1): 010701;  doi: 10.1088/1674-1056/27/1/010701
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Leader-following consensus of fractional order multi-agent systems is investigated. The agents are considered as discrete-time fractional order integrators or fractional order double-integrators. Moreover, the interaction between the agents is described with an undirected communication graph with a fixed topology. It is shown that the leader-following consensus problem for the considered agents could be converted to the asymptotic stability analysis of a discrete-time fractional order system. Based on this idea, sufficient conditions to reach the leader-following consensus in terms of the controller parameters are extracted. This leads to an appropriate region in the controller parameters space. Numerical simulations are provided to show the performance of the proposed leader-following consensus approach.
SPECIAL TOPIC—Non-equilibrium phenomena in soft matters
RAPID COMMUNICATION

Spin polarization and dispersion effects in emergence of roaming transition state for nitrobenzene isomerization

Zhi-Yuan Zhang, Wan-Run Jiang, Bo Wang, Yan-Qiang Yang, Zhi-Gang Wang
Chin. Phys. B 2018, 27 (1): 013102;  doi: 10.1088/1674-1056/27/1/013102
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Since roaming was found as a new but common reaction path of isomerization, many of its properties, especially those of roaming transition state (TSR), have been studied on many systems. However, the mechanism of roaming is still not clear at an atomic level. In this work, we use first-principles calculations to illustrate the detailed structure of TSR in an internal isomerization process of nitrobenzene. The calculations distinctively show its nature of antiferromagnetic coupling between two roaming fragments. Moreover, the effect of dispersion is also revealed as an important issue for the stability of the TSR. Our work provides a new insight into the TSR from the view of electronic structure and contributes to the basic understanding of the roaming systems.

Direct observation of melted Mott state evidenced from Raman scattering in 1T-TaS2 single crystal Hot!

Qing Hu, Cong Yin, Leilei Zhang, Li Lei, Zhengshang Wang, Zhiyu Chen, Jun Tang, Ran Ang
Chin. Phys. B 2018, 27 (1): 017104;  doi: 10.1088/1674-1056/27/1/017104
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The evolution of electron correlation and charge density wave (CDW) in 1T-TaS2 single crystal has been investigated by temperature-dependent Raman scattering, which undergoes two obvious peaks of A1g modes about 70.8 cm-1 and 78.7 cm-1 at 80 K, respectively. The former peak at 70.8 cm-1 is accordant with the lower Hubbard band, resulting in the electron-correlation-driven Mott transition. Strikingly, the latter peak at 78.7 cm-1 shifts toward low energy with increasing the temperature, demonstrating the occurrence of nearly commensurate CDW phase (melted Mott phase). In this case, phonon transmission could be strongly coupled to commensurate CDW lattice via Coulomb interaction, which likely induces appearance of hexagonal domains suspended in an interdomain phase, composing the melted Mott phase characterized by a shallow electron pocket. Combining electronic structure, atomic structure, transport properties with Raman scattering, these findings provide a novel dimension in understanding the relationship between electronic correlation, charge order, and phonon dynamics.

Measurement of the bulk and surface bands in Dirac line-node semimetal ZrSiS Hot!

Guang-Hao Hong, Cheng-Wei Wang, Juan Jiang, Cheng Chen, Sheng-Tao Cui, Hai-Feng Yang, Ai-Ji Liang, Shuai Liu, Yang-Yang Lv, Jian Zhou, Yan-Bin Chen, Shu-Hua Yao, Ming-Hui Lu, Yan-Feng Chen, Mei-Xiao Wang, Le-Xian Yang, Zhong-Kai Liu, Yu-Lin Chen
Chin. Phys. B 2018, 27 (1): 017105;  doi: 10.1088/1674-1056/27/1/017105
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Dirac semimetals are materials in which the conduction and the valence bands have robust crossing points protected by topology or symmetry. Recently, a new type of Dirac semimetals, so called the Dirac line-node semimetals (DLNSs), have attracted a lot of attention, as they host robust Dirac points along the one-dimensional (1D) lines in the Brillouin zone (BZ). In this work, using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations, we systematically investigated the electronic structures of non-symmorphic ZrSiS crystal where we clearly distinguished the surface states from the bulk states. The photon-energy-dependent measurements further prove the existence of Dirac line node along the X-R direction. Remarkably, by in situ surface potassium doping, we clearly observed the different evolutions of the bulk and surface electronic states while proving the robustness of the Dirac line node. Our studies not only reveal the complete electronic structures of ZrSiS, but also demonstrate the method manipulating the electronic structure of the compound.

Two-dimensional transport and strong spin-orbit interaction in SrMnSb2 Hot!

Jiwei Ling, Yanwen Liu, Zhao Jin, Sha Huang, Weiyi Wang, Cheng Zhang, Xiang Yuan, Shanshan Liu, Enze Zhang, Ce Huang, Raman Sankar, Fang-Cheng Chou, Zhengcai Xia, Faxian Xiu
Chin. Phys. B 2018, 27 (1): 017504;  doi: 10.1088/1674-1056/27/1/017504
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We have carried out magneto-transport measurements for single crystal SrMnSb2. Clear Shubnikov-de Haas oscillations were resolved at relatively low magnetic field around 4 T, revealing a quasi-2D Fermi surface. We observed a development of quantized plateaus in Hall resistance (Rxy) at high pulsed fields up to 60 T. Due to the strong 2D confinement and layered properties of the samples, we interpreted the observation as bulk quantum Hall effect that is contributed by the parallel 2D conduction channels. Moreover, the spin degeneracy was lifted leading to Landau level splitting. The presence of anisotropic g factor and the formation of the oscillation beating pattern reveal a strong spin-orbit interaction in the SrMnSb2 system.

Optical study on intermediate-valence compounds Yb1-xLuxAl3 Hot!

J L Lv, J L Luo, N L Wang
Chin. Phys. B 2018, 27 (1): 017803;  doi: 10.1088/1674-1056/27/1/017803
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We report an optical spectroscopy study on intermediate valence system Yb1-xLuxAl3 with x=0, 0.25, 0.5, 0.75, and 1. The Kondo temperature in the system is known to increase with increasing Lu concentration. Therefore, it is expected that the energy scale of the hybridization gap should increase with increasing Lu concentration based on the periodic Anderson model. On the contrary, we find that the spectral structure associated with the hybridization effect shifts monotonically to lower energy. Furthermore, the Lu substitution results in a substantial increase of the free carrier spectral weight and less pronounced plasma frequency reduction upon lowering temperature. We attribute the effect to the disruption of the Kondo lattice periodicity by the random substitution of Yb by Lu. The work highlights the importance of the lattice periodicity of the rare earth element for understanding the Kondo lattice phenomena.

ATOMIC AND MOLECULAR PHYSICS

A new fully quantum-mechanical method used to calculate the collisional broadening coefficients and shift coefficients of Rb D1 lines perturbed by noble gases He and Ar

Wei Zhang, Yanchao Shi, Bitao Hu, Yi Zhang
Chin. Phys. B 2018, 27 (1): 013201;  doi: 10.1088/1674-1056/27/1/013201
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In this work, a new full quantum method is proposed to calculate the broadening and shift coefficients of the D1 line in neutral collision. Based on the variable phase approach and Baranger theory, this method calculates the scattering phase shift instead of scattering matrix elements in order to simplify the calculation. As an illustration, this method is used to calculate the broadening and shift coefficients of the absorption lines of alkali metal atom Rb, as it collides with buffer gas He and Ar, in a temperature range from 150 K to 800 K. With a comparison with other calculations and experiment measurements, the reasonable agreements in all cases demonstrate the validity and simplicity of this method.

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation

Da-Long Qi, Ye Zheng, Wen-Jing Cheng, Yun-Hua Yao, Lian-Zhong Deng, Dong-Hai Feng, Tian-Qing Jia, Zhen-Rong Sun, Shi-An Zhang
Chin. Phys. B 2018, 27 (1): 013202;  doi: 10.1088/1674-1056/27/1/013202
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Improving the up-conversion luminescence efficiency of rare-earth ions via the multi-photon absorption process is crucial in several related application areas. In this work, we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er3+ ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components. So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. Moreover, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width (or laser spectral bandwidth), final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earth ions.

Photoelectron longitudinal momentum distributions containing nondipole effects

Yi-Ning Huo, Jian Li, Feng-Cai Ma
Chin. Phys. B 2018, 27 (1): 013203;  doi: 10.1088/1674-1056/27/1/013203
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This paper proposes a modified strong field approximation model for evaluating nondipole effects on the ionization of an atom in an intense laser field. The photoelectron longitudinal momentum distributions (PLMD) of a hydrogen-like atom exposed to a mid-infrared laser field is calculated. The theoretical results indicate an obvious asymmetry in the PLMD, and an offset of the PLMD peak appears in the opposite direction of the beam propagation due to nondipole effects. The peak offsets of the PLMD increased with the laser intensity, imposed by the initial state of the hydrogen-like atom.

Attosecond transient absorption spectroscopy: Comparative study based on three-level modeling

Zeng-Qiang Yang, Di-Fa Ye, Li-Bin Fu
Chin. Phys. B 2018, 27 (1): 013301;  doi: 10.1088/1674-1056/27/1/013301
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In the present paper, the time-resolved transient absorption spectroscopy of helium atoms is investigated based on the three-level modeling. The helium atoms are subjected to an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared (IR) few-cycle laser field. The odd excited state are populated from the ground state by the XUV pulse due to the dipole selection rule, and probed by the time-delayed IR laser. The time-resolved transient absorption spectroscopy based on the different coupling mechanism demonstrate some different features, the photoabsorption spectrum based on three-level model with rotating wave approximation (RWA) cannot repeat the fast oscillation and the sideband structure which have been observed in the previous experimental investigation. The dressing effect of IR laser pulse on the ground state can contribute new interference structures in the photoabsorption spectrum.

Plasma-screening effects on positronium formation

Jia Ma, Yuan-Cheng Wang, Ya-Jun Zhou, Heng Wang
Chin. Phys. B 2018, 27 (1): 013401;  doi: 10.1088/1674-1056/27/1/013401
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Plasma-screening effects on positronium (Ps) formation for positron-hydrogen collisions in a Debye plasma environment is further investigated using the screening approximation model with the inclusion of the modified structure of Ps. More accurate Ps formation cross sections (n=1, 2) are obtained for various Debye lengths from the Ps formation thresholds to 50 eV. The influence of considering modified bound-state wave functions and eigenenergies for the Ps is found to result in the reduction of the Ps formation cross sections at low energies, whereas it cannot counteract the enhancement of the Ps formation by the Debye screening.

Comparison of the sensitivities for atom interferometers in two different operation methods

Xiao-Chun Duan, De-Kai Mao, Xiao-Bing Deng, Min-Kang Zhou, Cheng-Gang Shao, Zhu Zhu, Zhong-Kun Hu
Chin. Phys. B 2018, 27 (1): 013701;  doi: 10.1088/1674-1056/27/1/013701
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We investigated the sensitivities of atom interferometers in the usual fringe-scanning method (FSM) versus the fringe-locking method (FLM). The theoretical analysis shows that for typical noises in atom interferometers, the FSM will degrade the sensitivity while the FLM does not. The sensitivity-improvement factor of the FLM over the FSM depends on the type of noises, which is validated by numerical simulations. The detailed quantitative analysis on this fundamental issue is presented, and our analysis is readily extendable to other kinds of noises as well as other fringe shapes in addition to a cosine one.

ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

A broadband cross-polarization conversion anisotropic metasurface based on multiple plasmon resonances

M Ismail Khan, Farooq A Tahir
Chin. Phys. B 2018, 27 (1): 014101;  doi: 10.1088/1674-1056/27/1/014101
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A compact broadband cross-polarization conversion metasurface functioning in the microwave regime is realized and experimentally demonstrated. The metasurface consists of a two-dimensional periodic arrangement of anisotropic double-slit split-ring-resonator-based unit cells printed on top of a dielectric substrate, backed by metallic cladding. The proposed metasurface converts an x-or y-polarized wave into its orthogonal polarization over a fractional bandwidth of 100% from 5-15 GHz, both for normal as well as oblique incidence. Moreover, the sub-wavelength unit-cell size, thin dielectric substrate, and unique unit-cell design collectively make the response of the metasurface same for both polarizations and insensitive to the incidence angle. The designed structure is fabricated and tested. The measurement and simulation results are found to be consistent with each other.

Birefringence via Doppler broadening and prevention of information hacking

Humayun Khan, Muhammad Haneef, Bakhtawar
Chin. Phys. B 2018, 27 (1): 014201;  doi: 10.1088/1674-1056/27/1/014201
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We propose a new scheme for the coherent control of birefringent light pulses propagation in a four-level atomic medium. We modify the splitting of a light pulse by controlling the electric and magnetic responses. The Doppler broadening effect is also noted on the propagation of the birefringent pulses. The dispersions of the birefringence beams are oppositely manipulated for delay and advancement of time at a Doppler width of 10γ. A time gap is created between the birefringence beams, which protects from hacking of information. The time gap is then closed to restore the pulse into the original form by a reverse manipulation of the dispersion of the birefringence beams, i.e., introducing another medium whose transfer function is the complex conjugate of that of the original medium. The results are useful for secure communication technology.

Aberration correction of conformal dome based on rotated cylindrical lenses for ultra-wide field of regard

Linyao Yu, Yongfeng Hong, Zhifeng Cheng, Bao Zhang
Chin. Phys. B 2018, 27 (1): 014202;  doi: 10.1088/1674-1056/27/1/014202
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A new compact conformal dome optical system was designed, and the aberration characteristics of the dome were investigated using Zernike aberration theory. The aberrations induced by the conformal dome at different fields of regard (FORs) from 0^o to 90^o were effectively balanced by a pair of rotating cylindrical lenses. A design method was introduced and the optimization results were analyzed in detail. The results showed that the Zernike aberrations produced by the conformal dome were decreased dramatically. Also, a complete conformal optical system was designed to further illustrate the aberration correction effect of the rotating cylindrical lenses. Using a pair of rotating cylindrical lenses not only provided an ultra-wide FOR, but also perduced a better image quality of the optical system.

Super-sensitive phase estimation with coherent boosted light using parity measurements

Lan Xu, Qing-Shou Tan
Chin. Phys. B 2018, 27 (1): 014203;  doi: 10.1088/1674-1056/27/1/014203
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We consider a passive and active hybrid interferometer for phase estimation, which can reach the sub-shot-noise limit in phase sensitivity with only the cheapest coherent sources. This scheme is formed by adding an optical parametric amplifier before a Mach-Zehnder interferometer. It is shown that our hybrid protocol can obtain a better quantum Cramer-Rao bound than the pure active (e.g., SU(1,1)) interferometer, and this precision can be reached by implementing the parity measurements. Furthermore, we also draw a detailed comparison between our scheme and the scheme suggested by Caves[Phys. Rev. D 23 1693 (1981)], and it is found that the optimal phase sensitivity gain obtained in our scheme is always larger than that in Caves' scheme.

Broad bandwidth interference filter-stabilized external cavity diode laser with narrow linewidth below 100 kHz

Guan-Zhong Pan, Bao-Lu Guan, Chen Xu, Peng-Tao Li, Jia-Wei Yang, Zhen-Yang Liu
Chin. Phys. B 2018, 27 (1): 014204;  doi: 10.1088/1674-1056/27/1/014204
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Interference filter-stabilized external cavity diode lasers (ECDLs) have properties of simple configurations, high stabilities, and narrow linewidths. However, the interference filter used in common ECDL designs requires an ultra-narrow bandwidth (about 0.3 nm) to achieve mode selection, that is considerably expensive and not yet available for a wide range of wavelengths. In this paper, a robust ECDL using an available broad bandwidth (about 4 nm) interference filter as the wavelength discriminator is constructed and tested. The ECDL demonstrated a narrow Lorentzian fitted linewidth of 95 kHz and a spectral purity of 2.9 MHz. The long-term frequency stability of the ECDL reaches 5.59×10-12.

Design of photonic crystal fiber with elliptical air-holes to achieve simultaneous high birefringence and nonlinearity

Min Liu, Jingyun Hou, Xu Yang, Bingyue Zhao, Ping Shum
Chin. Phys. B 2018, 27 (1): 014206;  doi: 10.1088/1674-1056/27/1/014206
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A new type of V-shaped photonic crystal fiber with elliptical air-holes is proposed to realize simultaneous high birefringence and nonlinearity at a wavelength of 1.55 μm. The full vector finite element method was adopted to investigate its characteristics, including birefringence, nonlinearity, and dispersion. The PCF exhibited a very high birefringence of 2.89×10-2 and very high nonlinear coefficient of 102.69 W-1·km-1. In particular, there were two zero-dispersion wavelengths (ZDWs) in the visible (X:640-720 nm and Y:730-760 nm) and near-infrared regions (X:1050-1606 nm and Y:850-1500 nm). The combination of high birefringence and nonlinearity allowed the PCF to maintain the polarization state and generate a broadband super continuum, with potential applications in nonlinear optics.

Temperature rise induced by an annular focused transducer with a wide aperture angle in multi-layer tissue

Meng Qi, Jiehui Liu, Yiwei Mao, Xiaozhou Liu
Chin. Phys. B 2018, 27 (1): 014301;  doi: 10.1088/1674-1056/27/1/014301
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In order to improve the operability and accuracy of high-intensity focused ultrasound (HIFU), an annular focused transducer, whereby a B-ultrasound probe is placed in its center, is used to realize the real time monitoring and control of the treatment. In this paper, the spheroidal beam equation (SBE) was used to calculate the sound field by an annular focused transducer with a wide aperture angle to first derive the heat deposition and the Pennes equation was used to calculate the temperature field in multi-layer tissue. We studied the effect of different parameters on the temperature of the tissues. The result shows that the focal length has a significant influence on both maximum liver temperature rise and skin temperature rise, and both increase with the increase in the focal length. When the frequency increases, the temperature rise first undergoes a rapid increase before gradually reaching a maximum, and then finally decreasing. The temperature rise increases while the inner radius decreases or the sound pressure increases. By choosing suitable parameters, the proper temperature rise both on the target tissue and skin via an annular focused transducer with a wide aperture angle can be obtained.

Acoustic radiation force induced by two Airy-Gaussian beams on a cylindrical particle

Sha Gao, Yiwei Mao, Jiehui Liu, Xiaozhou Liu
Chin. Phys. B 2018, 27 (1): 014302;  doi: 10.1088/1674-1056/27/1/014302
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Based on the angular spectrum decomposition and partial-wave series expansion methods, we investigate the radiation force functions of two Airy-Gaussian (AiG) beams on a cylindrical particle and the motion trajectory of the particle. The simulations show that the particle can be pulled or propelled into either the positive or negative transverse direction by turning the phase difference between the two AiG beams appropriately; and the larger the beam widths of the two AiG beams are, the bigger the radiation force can be obtained to control the particle. In addition, the direction of the accelerated particle can be controlled while the dimensionless frequency bandwidth changes. The results indicate that the phase plays an important role in controlling the direction of the particle, which may provide a theoretical basis for the design of acoustical tweezers and the development of drug delivery.

Envelope solitary waves and their reflection and transmission due to impurities in a granular material

Wen-Qing Du, Jian-An Sun, Yang-Yang Yang, Wen-Shan Duan
Chin. Phys. B 2018, 27 (1): 014501;  doi: 10.1088/1674-1056/27/1/014501
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A nonlinear Schrödinger equation in one-dimensional bead chain is first obtained and an envelope solitary wave of the system is verified numerically in this system. The reflection and the transmission of an incident envelope solitary wave due to impurities has also been investigated. It is found that the magnitudes of both the reflection and the transmission not only depend on the characters of impurity materials, the wave number, the incident wave amplitude, but also on the impurity number. This can be used to detect the character and the number of the impurity materials in the bead chain by measuring the reflection and the transmission of an incident pulse.
COMMENT

Comment on “Band gaps structure and semi-Dirac point of two-dimensional function photonic crystals” by Si-Qi Zhang et al.

Hai-Feng Zhang
Chin. Phys. B 2018, 27 (1): 014205;  doi: 10.1088/1674-1056/27/1/014205
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Recently, Zhang et al. (Chin. Phys. B 26 024208 (2017)) investigated the band gap structures and semi-Dirac point of two-dimensional function photonic crystals, and the equations for the plane wave expansion method were induced to obtain the band structures. That report shows the band diagrams with the effects of function coefficient k and medium column ra under TE and TM waves. The proposed results look correct at first glance, but the authors made some mistakes in their report. Thus, the calculated results in their paper are incorrect. According to our calculations, the errors in their report are corrected, and the correct band structures also are presented in this paper.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Excitation of chorus-like waves by temperature anisotropy in dipole research experiment (DREX): A numerical study

Hua Huang, Zhi-Bin Wang, Xiao-Gang Wang, Xin Tao
Chin. Phys. B 2018, 27 (1): 015201;  doi: 10.1088/1674-1056/27/1/015201
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Due to their significant roles in the radiation belts dynamics, chorus waves are widely investigated in observations, experiments, and simulations. In this paper, numerical studies for the generation of chorus-like waves in a launching device, dipole research experiment (DREX), are carried out by a hybrid code. The DREX plasma is generated by electron cyclotron resonance (ECR), which leads to an intrinsic temperature anisotropy of energetic electrons. Thus the whistler instability can be excited in the device. We then investigate the effects of three parameters, i.e., the cold plasma density nc, the hot plasma density nh, and the parallel thermal velocity of energetic electrons, on the generation of chorus-like waves under the DREX design parameters. It is obtained that a larger temperature anisotropy is needed to excite chorus-like waves with a high nc with other parameters fixed. Then we fix the plasma density and parallel thermal velocity, while varying the hot plasma density. It is found that with the increase of nh, the spectrum of the generated waves changes from no chorus elements, to that with several chorus elements, and then further to broad-band hiss-like waves. Besides, different structures of chorus-like waves, such as rising-tone and/or falling-tone structures, can be generated at different parallel thermal velocities in the DREX parameter range.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Bandgap engineering to tune the optical properties of BexMg1-xX (X=S, Se, Te) alloys

B Sabir, N A Noor, M Rashid, Fasih Ud Din, Shahid M Ramay, Asif Mahmood
Chin. Phys. B 2018, 27 (1): 016101;  doi: 10.1088/1674-1056/27/1/016101
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Structural, electronic, and optical properties of alloys BexMg1-xX(X=S, Se, Te) in the assortment 0 < x < 1 were theoretically reported for the first time in zinc-blende (ZB) phase. The calculations were carried out by using full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) formalism contained by the framework of density functional theory (DFT). Wu-Cohen (WC) generalized gradient approximation (GGA), based on optimization energy, has been applied to calculate these theoretical results. In addition, we used Becke and Johnson (mBJ-GGA) potential, modified form of GGA functional, to calculate electronic structural properties up to a high precision degree. The alloys were composed with the concentrations x=0.25, 0.5, and 0.75 in pursuance of ‘special quasi-random structures’ (SQS) approach of Zunger for the restoration of disorder around the observed site of alloys in the first few shells. The structural parameters have been predicted by minimizing the total energy in correspondence of unit cell volume. Our alloys established direct band gap at different concentrations that make their importance in optically active materials. Furthermore, density of states was discussed in terms of the contribution of Be and Mg s and chalcogen (S, Se, and Te) s and p states and observed charge density helped us to investigate the bonding nature. By taking into consideration of immense importance in optoelectronics of these materials, the complex dielectric function was calculated for incident photon energy in the range 0-15 eV.

Synthesized few-layers hexagonal boron nitride nanosheets

Zhao-Jun Mo, Zhi-Hong Hao, Xiao-Jie Ping, Li-Na Kong, Hui Yang, Jia-Lin Cheng, Jun-Kai Zhang, Yong-Hao Jin, Lan Li
Chin. Phys. B 2018, 27 (1): 016102;  doi: 10.1088/1674-1056/27/1/016102
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The hexagonal boron nitrides (BNs) with different morphologies are synthesized on a large scale by a simple route using a two-step synthetic process. The morphology of h-BN can be easily controlled by changing the heat-treatment atmosphere. The whiskers with 0.5-10 μm in diameter and 50-100 μm in length consist of few-layers nanosheets in the NH3 gas. The BN nanosheets can be dissociated from the whiskers by ultrasonic treatment, which are less than 5 nm in thickness and even only two layers thick. The concentration and activity of N play an important role, and abundant N and higher activity are conducive for refining grain in reaction. The H3BO3 and C3N6H6 molecules form a layer-like morphology with the interlinked planar triangle by a hydrogen-bonded structure.

Bulk and surface damages in complementary bipolar junction transistors produced by high dose irradiation

J Assaf
Chin. Phys. B 2018, 27 (1): 016103;  doi: 10.1088/1674-1056/27/1/016103
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Two complementary types NPN and PNP of bipolar junction transistors (BJTs) were exposed to high dose of neutrons and gamma rays. The change in the base and collector currents, minority carriers lifetime, and current gain factor β with respect to the dose were analyzed. The contributions of the base current according to the defect types were also reported. It was declared that the radiation effect of neutrons was almost similar between the two transistor types, this effect at high dose may decrease the value of β to less than one. The Messenger-Spratt equation was used to describe the experimental results in this case. However, the experimental data demonstrated that the effect of gamma rays was generally higher on NPN than PNP transistors. This is mainly attributed to the difference in the behavior of the trapped positive charges in the SiO2 layers. Meanwhile, this difference tends to be small for high gamma dose.

First-principles calculations on elastic, magnetoelastic, and phonon properties of Ni2FeGa magnetic shape memory alloys

Wangqiang He, Houbing Huang, Zhuhong Liu, Xingqiao Ma
Chin. Phys. B 2018, 27 (1): 016201;  doi: 10.1088/1674-1056/27/1/016201
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The elastic, magnetoelastic, and phonon properties of Ni2 FeGa were investigated through first-principles calculations. The obtained elastic and phonon dispersion curves for the austenite and martensite phases agree well with available theoretical and experimental results. The isotropic elastic moduli are also predicted along with the polycrystalline aggregate properties including the bulk modulus, shear modulus, Young's modulus, and Poisson's ratio. The Pugh ratio indicates that Ni2 FeGa shows ductility, especially the austenite phase, which is consistent with the experimental results. The Debye temperatures of the Ni2 FeGa in the austenite and martensite phases are 344 K and 392 K, respectively. It is predicted that the magnetoelastic coefficient is -5.3×106 J/m3 and magnetostriction coefficient is between 135 and 55 ppm in the Ni2 FeGa austenite phase.

Ejecta from periodic grooved Sn surface under unsupported shocks

Wen-Bin Liu, Dong-Jun Ma, An-Min He, Pei Wang
Chin. Phys. B 2018, 27 (1): 016202;  doi: 10.1088/1674-1056/27/1/016202
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Dynamic failure and ejection characteristics of a periodic grooved Sn surface under unsupported shock loading are studied using a smoothed particle hydrodynamics method. An “ Tower” spatial structure is observed, which is composed of high-speed jet tip, high-density jet slug, longitudinal tensile sparse zone, and complex broken zone between grooves. It is very different from the spike-bubble structure under supported shocks, and has been validated by detonation loading experiments. In comparison with that under supported shocks at the same peak pressure, the high-speed ejecta decreases obviously, whereas the truncated location of ejecta moves towards the interior of the sample and the total mass of ejecta increases due to the vast existence of low-speed broken materials. The shock wave profile determines mainly the total ejection amount, while the variation of V-groove angle will significantly alter the distribution of middle-and high-speed ejecta, and the maximum ejecta velocity has a linear correlation with the groove angle.

Magnetism and piezoelectricity of hexagonal boron nitride with triangular vacancy

Lu-Si Zhao, Chun-Ping Chen, Lin-Lin Liu, Hong-Xia Yu, Yi Chen, Xiao-Chun Wang
Chin. Phys. B 2018, 27 (1): 016301;  doi: 10.1088/1674-1056/27/1/016301
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First-principle calculations reveal that the configuration system of hexagonal boron nitride (h-BN) monolayer with triangular vacancy can induce obvious magnetism, contrary to that of the nonmagnetic pristine boron nitride monolayer. Interestingly, the h-BN with boron atom vacancy (VB-BN) displays metallic behavior with a total magnetic moment being 0.46μB per cell, while the h-BN with nitrogen atom vacancy (VN-BN) presents a half-metallic characteristic with a total magnetic moment being 1.0μB per cell. Remarkably, piezoelectric stress coefficient e11 of the VN-BN is about 1.5 times larger than that of pristine h-BN. Furthermore, piezoelectric strain coefficient d11 (12.42 pm/V) of the VN-BN is 20 times larger than that of pristine h-BN and also one order of magnitude larger than the value for the h-MoS2 monolayer, which is mainly due to the spin-down electronic state in the VN-BN system. Our study demonstrates that the nitrogen atom vacancies can be an efficient route to tailoring the magnetic and piezoelectric properties of h-BN monolayer, which have promising performances for potential applications in nano-electromechanical systems (NEMS) and nanoscale electronics devices.

Transport properties of mixing conduction in CaF2 nanocrystals under high pressure

Ting-Jing Hu, Xiao-Yan Cui, Jing-Shu Wang, Jun-Kai Zhang, Xue-Fei Li, Jing-Hai Yang, Chun-Xiao Gao
Chin. Phys. B 2018, 27 (1): 016401;  doi: 10.1088/1674-1056/27/1/016401
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We report on the intriguing electrical transport properties of compressed CaF2 nanocrystals. The diffusion coefficient, grain and grain boundary resistances vary abnormally at about 14.37 GPa and 20.91 GPa, corresponding to the beginning and completion of the Fm3m-Pnma structural transition. Electron conduction and ion conduction coexist in the transport process and the electron conduction is dominant. The electron transference number of the Fm3m and Pnma phases increases with pressure increasing. As the pressure rises, the F- ion diffusion and electronic transport processes in the Fm3m and Pnma phases become more difficult. Defects at grains play a dominant role in the electronic transport process.

Spin-dependent balance equations in spintronics

Zheng-Chuan Wang
Chin. Phys. B 2018, 27 (1): 016701;  doi: 10.1088/1674-1056/27/1/016701
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It is commonly known that the hydrodynamic equations can be derived from the Boltzmann equation. In this paper, we derive similar spin-dependent balance equations based on the spinor Boltzmann equation. Besides the usual charge current, heat current, and pressure tensor, we also explore the characteristic spin accumulation and spin current as well as the spin-dependent pressure tensor and heat current in spintronics. The numerical results of these physical quantities are demonstrated using an example of spin-polarized transport through a mesoscopic ferromagnet.

Two types of ground-state bright solitons in a coupled harmonically trapped pseudo-spin polarization Bose–Einstein condensate

T F Xu
Chin. Phys. B 2018, 27 (1): 016702;  doi: 10.1088/1674-1056/27/1/016702
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We study two types of bright solitons in an attractive Bose-Einstein condensate with a spin-orbit interaction. By solving the coupled nonlinear Schrödinger equations with the variational method and the imaginary time evolution method, fundamental properties of solitons are carefully investigated in different parameter regimes. It is shown that the detuning between the Raman beam and energy states of the atoms dominates the ground state type and spin polarization strength. The soliton dynamics is also studied for various moving velocities for zero and nonzero detuning cases. We find that the shape of individual component solitons can be maintained when the moving speed of solitons is low and the detuning is small in the coupled harmonically trapped pseudo-spin polarization Bose-Einstein condensate.

A combined system for generating a uniform magnetic field and its application in the investigation of Efimov physics

Rui Yao, Zhen-Dong Sun, Shu-Yu Zhou, Ying Wang, Yu-Zhu Wang
Chin. Phys. B 2018, 27 (1): 016703;  doi: 10.1088/1674-1056/27/1/016703
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We propose a scheme to produce a uniform magnetic field with a system comprising a pair of coils and an atom chip. After optimizing the parameters of the chip wires, we improve the homogeneity of the magnetic field by two orders of magnitude. We exhibit that this method can be applied in the investigation of Efimov physics in 87Rb-40K mixture.

Charge distribution in graphene from quantum calculation

Ze-Fen Liang, Sheng-Ling Ma, Hong-Tao Xue, Fan Ding, Jingbo Louise Liu, Fu-Ling Tang
Chin. Phys. B 2018, 27 (1): 016801;  doi: 10.1088/1674-1056/27/1/016801
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The local charge distributions of different shape graphene sheets are investigated by using the quantum calculations. It is found that the charge distribution on carbon atom is not uniform, strongly depending on its position in the graphene and its local atomic environment condition. The symmetrical characteristic and geometrical structures of graphene also have an important influence on the charge distribution. The charges of atom at the graphene edge are strongly related to their surrounding bonds. It is found that the charges of double-bonded atom at the zigzag edge are closely related to the bond angle, but the charges of double-bonded atom at the armchair edge are mainly influenced by the area of triangle. The charges of triple-bonded atom at the edge are mainly affected by the standard deviation of the length of the associated triple bonds.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Density functional theory analysis of electronic structure and optical properties of La-doped Cd2SnO4 transparent conducting oxide

Mei Tang, Jia-Xiang Shang, Yue Zhang
Chin. Phys. B 2018, 27 (1): 017101;  doi: 10.1088/1674-1056/27/1/017101
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The electronic structural, effective masses of carriers, and optical properties of pure and La-doped Cd2SnO4 are calculated by using the first-principles method based on the density functional theory. Using the GGA+U method, we show that Cd2SnO4 is a direct band-gap semiconductor with a band gap of 2.216 eV, the band gap decreases to 2.02 eV and the Fermi energy level moves to the conduction band after La doping. The density of states of Cd2SnO4 shows that the bottom of the conduction band is composed of Cd 5s, Sn 5s, and Sn 5p orbits, the top of the valence band is composed of Cd 4d and O 2p, and the La 5d orbital is hybridized with the O 2p orbital, which plays a key role at the conduction band bottom after La doping. The effective masses at the conduction band bottom of pure and La-doped Cd2SnO4 are 0.18m0 and 0.092m0, respectively, which indicates that the electrical conductivity of Cd2SnO4 after La doping is improved. The calculated optical properties show that the optical transmittance of La-doped Cd2SnO4 is 92%, the optical absorption edge is slightly blue shifted, and the optical band gap is increased to 3.263 eV. All the results indicate that the conductivity and optical transmittance of Cd2SnO4 can be improved by doping La.

Structural, electronic, and mechanical properties of cubic TiO2: A first-principles study

Debashish Dash, Chandan K Pandey, Saurabh Chaudhury, Susanta K Tripathy
Chin. Phys. B 2018, 27 (1): 017102;  doi: 10.1088/1674-1056/27/1/017102
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We present an analysis of structural, electronic, and mechanical properties of cubic titanium dioxide (TiO2) using an all electron orthogonalzed linear combinations of atomic orbitals (OLCAO) basis set under the framework of density functional theory (DFT). The structural property, especially the lattice constant a, and the electronic properties such as the band diagram and density of states (DOS) are studied and analyzed. The mechanical properties such as bulk moduli, shear moduli, Young's Moduli, and Poison's ratio are also investigated thoroughly. The calculations are carried out on shear moduli and anisotropy factor for cubic TiO2. The Vickers hardness is also tested for fluorite and pyrite cubic-structured TiO2. Furthermore, the results are compared with the previous theoretical and experimental results. It is found that DFT-based simulation produces results which are approximation to experimental results, whereas the calculated elastic constants are better than the previous theoretical and experimental values.

Electronic and mechanical properties of half-metallic half-Heusler compounds CoCrZ (Z=S, Se, and Te)

Hai-Ming Huang, Chuan-Kun Zhang, Ze-Dong He, Jun Zhang, Jun-Tao Yang, Shi-Jun Luo
Chin. Phys. B 2018, 27 (1): 017103;  doi: 10.1088/1674-1056/27/1/017103
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The electronic structures, magnetic properties, half-metallicity, and mechanical properties of half-Heulser compounds CoCrZ (Z=S, Se, and Te) were investigated using first-principles calculations within generalized gradient approximation based on the density function theory. The half-Heusler compounds show half-metallic properties with a half-metallic gap of 0.15 eV for CoCrS, 0.10 eV for CoCrSe, and 0.31 eV for CoCrTe at equilibrium lattice constant, respectively. The total magnetic moments are 3.00μB per formula unit, which agrees well with the Slater-Pauling rule. The half-metallicity, elastic constants, bulk modulus, shear modulus, Pough's ratio, Frantesvich ratio, Young's modulus, Poisson's ratio, and Debye temperature at equilibrium lattice constant and versus lattice constants are reported for the first time. The results indicate that the half-Heulser compounds CoCrZ (Z=S, Se, and Te) maintain the perfect half-metallic and mechanical stability within the lattice constants range of 5.18-5.43 Å for CoCrS, 5.09-5.61 Å for CoCrSe, and 5.17-6.42 Å for CoCrTe, respectively.

Optical interaction between one-dimensional fiber photonic crystal microcavity and gold nanorod

Yang Yu, Ting-Hui Xiao, Zhi-Yuan Li
Chin. Phys. B 2018, 27 (1): 017301;  doi: 10.1088/1674-1056/27/1/017301
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Localized surface plasmon resonance (LSPR) has demonstrated its promising capability for biochemical sensing and surface-enhanced spectroscopy applications. However, harnessing LSPR for remote sensing and spectroscopy applications remains a challenge due to the difficulty in realizing a configuration compatible with the current optical communication system. Here, we propose and theoretically investigate a hybrid plasmonic-photonic device comprised of a single gold nanorod and an optical fiber-based one-dimensional photonic crystal microcavity, which can be integrated with the optical communication system without insertion loss. The line width of the LSPR, as a crucial indicator that determines the performances for various applications, is narrowed by the cavity-plasmon coupling in our device. Our device provides a promising alternative to exploit the LSPR for high-performance remote sensing and spectroscopy applications.

Raman spectrum study of δ -doped GaAs/AlAs multiple-quantum wells

Wei-Min Zheng, Wei-Yan Cong, Su-Mei Li, Ai-Fang Wang, Bin Li, Hai-Bei Huang
Chin. Phys. B 2018, 27 (1): 017302;  doi: 10.1088/1674-1056/27/1/017302
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Three samples of GaAs/AlAs multiple-quantum wells with different quantum well widths and δ -doped with Be acceptors at the well center were grown on (100) GaAs substrates by molecular beam epitaxy. Polarized Raman spectra were recorded on the three samples at temperatures in a range of 4-50 K in a backscattering configuration. The two branches of coupled modes due to the interaction of the hole intersubband transitions and the quantum-well longitudinal optical (LO) phonon were observed clearly. The evaluation formalism of the Green function was employed and each lineshape of the Raman spectrum of the coupled modes was simulated. The dependence of the peak position of Raman shifts of the two coupled modes as well as the quantum-well LO phonon on the quantum-well size and measured temperature were given, and the coupling interaction mechanism between the hole subband transitions and the quantum-well LO phonon was researched.

Closed-form internal impedance model and characterization of mixed carbon nanotube bundles for three-dimensional integrated circuits

Qijun Lu, Zhangming Zhu, Yintang Yang, Ruixue Ding, Yuejin Li
Chin. Phys. B 2018, 27 (1): 017303;  doi: 10.1088/1674-1056/27/1/017303
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Based on the complex effective conductivity method, a closed-form expression for the internal impedance of mixed carbon nanotube (CNT) bundles, in which the number of CNTs for a given diameter follows a Gaussian distribution, is proposed in this paper. It can appropriately capture the skin effect as well as the temperature effect of mixed CNT bundles. The results of the closed-form expression and the numerical calculation are compared with various mean diameters, standard deviations, and temperatures. It is shown that the proposed model has very high accuracy in the whole frequency range considered, with maximum errors of 1% and 2.3% for the resistance and the internal inductance, respectively. Moreover, by using the proposed model, the high-frequency electrical characteristics of mixed CNT bundles are deeply analyzed to provide helpful design guidelines for their application in future high-performance three-dimensional integrated circuits.

Electronic states and spin-filter effect in three-dimensional topological insulator Bi2Se3 nanoribbons

Genhua Liu, Pingguo Xiao, Piaorong Xu, Huiying Zhou, Guanghui Zhou
Chin. Phys. B 2018, 27 (1): 017304;  doi: 10.1088/1674-1056/27/1/017304
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We study the electronic band structure, density distribution, and transport of a Bi2Se3 nanoribbon. We find that the density distribution of the surface states is dependent on not only the shape and size of the transverse cross section of the nanoribbon, but also the energy of the electron. We demonstrate that a transverse electric field can eliminate the coupling between surface states on the walls of the nanoribbon, remove the gap of the surface states, and restore the quantum spin Hall effects. In addition, we study the spin-dependent transport property of the surface states transmitting from top and bottom surfaces (x-y plane) to the side surfaces (z-x plane) of a Bi2Se3 nanoribbon. We find that transverse electric fields can open two surface channels for spin-up and -down Dirac electrons, and then switch off one channel for the spin-up Dirac electron. Our results may provide a simple way for the design of a spin filter based on topological insulator nanostructures.

Directional mechanical and thermal properties of single-layer black phosphorus by classical molecular dynamics

Afira Maryam, Ghulam Abbas, Muhammad Rashid, Atif Sattar
Chin. Phys. B 2018, 27 (1): 017401;  doi: 10.1088/1674-1056/27/1/017401
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Black phosphorus (BP) has received attention due to its own higher carrier mobility and layer dependent electronic properties, such as direct band gap. Interestingly, the single layer black phosphorus (SLBP) has had large popularity in applications related to thermoelectric, optoelectronic, and electronic devices. Here, we investigate the phonon spectrum, thermal conductivities, and stress strain effects. Robust anisotropy was mainly observed in the thermal conductivities together with the alongside zigzag (ZZ) direction value, compared to the armchair (AC) directions. We also investigated the attitude of stress that was anisotropic in both directions, and the stress effects were two times greater across the ZZ path than those in the AC direction at a low temperature. We obtained a Young's modulus of 63.77 and 20.74 GPa in the AC and ZZ directions, respectively, for a strain range of 0.01. These results had good agreement with first principle calculations. Our study here is useful and significant for the thermal tuning of phosphorus-based nanoelectronics and thermalelectric applications of phosphorus.

High-pressure synchrotron x-ray diffraction and Raman spectroscopic study of plumbogummite

Duan Kang, Xiang Wu, Guan Yuan, Sheng-Xuan Huang, Jing-Jing Niu, Jing Gao, Shan Qin
Chin. Phys. B 2018, 27 (1): 017402;  doi: 10.1088/1674-1056/27/1/017402
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PbAl3(PO4)2(OH,H2O)6, an important environmental mineral, is in-situ studied by synchrotron x-ray diffraction (XRD) and Raman scattering combined with diamond anvil cells (DACs) at pressures up to~11.0 GPa and room temperature. The XRD results indicate that plumbogummite does not undergo a phase transition between 0 GPa and 10.9 GPa. Moreover, the c axis is more compressible than the a axis, revealing its anisotropic behavior. The pressure-volume data are fitted to the third-order Birch-Murnaghan equation of state to yield the plumbogummite bulk modulus K0 of 68(1) GPa and K'0 of 6.1. The[PO4]3- and[HPO4]2- Raman vibrational modes exhibit scale nearly linearly as a function of pressure. The[PO4]3- stretching modes are generally more sensitive to pressure than the bending modes. The Grüneisen parameters range from -0.07 to 1.19, with an arithmetic mean of approximately 0.39.

Influences of La and Ce doping on giant magnetocaloric effect of EuTiO

Zhao-Jun Mo, Qi-Lei Sun, Jun Shen, Mo Yang, Yu-Jin Li, Lan Li, Guo-Dong Liu, Cheng-Chun Tang, Fan-Bin Meng
Chin. Phys. B 2018, 27 (1): 017501;  doi: 10.1088/1674-1056/27/1/017501
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Giant reversible magnetocaloric effects and magnetic properties in Eu0.9R0.1TiO3 (R=La, Ce) are investigated. The antiferromagnetic ordering of pure EuTiO3 can significantly change to be ferromagnetic as substitution of La (x=0.1) and Ce (x=0.1) ions for Eu2+ ions. The values of -ΔSM and RC are evaluated to be 10.8 J/(kg·K) and 51.8 J/kg for Eu0.9Ce0.1TiO3 and 11 J/(kg·K) and 39.3 J/kg for Eu0.9La0.1TiO3 at a magnetic field change of 10 kOe, respectively. The large low-field enhancements of -ΔSM and RC can be attributed to magnetic phase transition. The giant reversible MCE and large RC suggest that Eu0.9R0.1TiO3 (R=La, Ce) compounds could be promising materials in low temperature and low magnetic field refrigerants.

Strong anti-strain capacity of CoFeB/MgO interface on electronic structure and state coupling

Fei Guo, Yaping Wu, Zhiming Wu, Ting Chen, Heng Li, Chunmiao Zhang, Mingming Fu, Yihong Lu, Junyong Kang
Chin. Phys. B 2018, 27 (1): 017502;  doi: 10.1088/1674-1056/27/1/017502
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Electronic structure and spin-related state coupling at ferromagnetic material (FM)/MgO (FM=Fe, CoFe, CoFeB) interfaces under biaxial strain are evaluated using the first-principles calculations. The CoFeB/MgO interface, which is superior to the Fe/MgO and CoFe/MgO interfaces, can markedly maintain stable and effective coupling channels for majority-spin 1 state under large biaxial strain. Bonding interactions between Fe, Co, and B atoms and the electron transfer between Bloch states are responsible for the redistribution of the majority-spin 1 state, directly influencing the coupling effect for the strained interfaces. Layer-projected wave function of the majority-spin 1 state suggests slower decay rate and more stable transport property in the CoFeB/MgO interface, which is expected to maintain a higher tunneling magnetoresistance (TMR) value under large biaxial strain. This work reveals the internal mechanism for the state coupling at strained FM/MgO interfaces. This study may provide some references to the design and manufacturing of magnetic tunnel junctions with high tunneling magnetoresistance effect.

Multiferroic and enhanced microwave absorption induced by complex oxide interfaces

Cuimei Cao, Chunhui Dong, Jinli Yao, Changjun Jiang
Chin. Phys. B 2018, 27 (1): 017503;  doi: 10.1088/1674-1056/27/1/017503
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NiFe2O4 (NFO)/ZnO composite nanoparticles with different ZnO components were investigated, which were prepared by a simple wet chemical route method. The magnetoelectric coupling between magnetostriction from NFO and piezoelectricity from ZnO was induced by the surface coating NFO nanoparticles of ZnO layer, NFO/ZnO composite showed ferroelectric properties and the remanent electric polarization reached 0.08 μC/cm. Moreover, the changes of resistance at different room temperatures reached about 2% under 3 T magnetic fields comparing with that of zero magnetic fields. Furthermore, multiferroic NFO/ZnO resulted in enhancement of microwave absorption due to magnetoelectric coupling.

A general method for large-scale fabrication of Cu nanoislands/dragonfly wing SERS flexible substrates

Yuhong Wang, Mingli Wang, Lin Shen, Yanying Zhu, Xin Sun, Guochao Shi, Xiaona Xu, Ruifeng Li, Wanli Ma
Chin. Phys. B 2018, 27 (1): 017801;  doi: 10.1088/1674-1056/27/1/017801
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Noble metal nanorough surfaces that support strong surface-enhanced Raman scattering (SERS) is widely applied in the practical detection of organic molecules. A low-cost, large-area, and environment-friendly SERS-active substrate was acquired by sputtering inexpensive copper (Cu) on natural dragonfly wing (DW) with an easily controlled way of magnetron sputtering. By controlling the sputtering time of the fabrication of Cu on the DW, the performance of the SERS substrates was greatly improved. The SERS-active substrates, obtained at the optimal sputtering time (50 min), showed a low detection limit (10-6M) to 4-aminothiophenol (4-ATP), a high average enhancement factor (EF, 1.98×104), excellent signal uniformity, and good reproducibility. In addition, the results of the 3D finite-difference time-domain (3D-FDTD) simulation illustrated that the SERS-active substrates provided high-density “hot spots”, leading to a large SERS enhancement.

Substitution priority of Eu2+ in multi-cation compound Sr0.8Ca0.2Al2Si2O8 and energy transfer

Zhi-Ping Yang, Zhen-Ling Li, Zhi-Jun Wang, Pan-Lai Li, Miao-Miao Tian, Jin-Ge Cheng, Chao Wang
Chin. Phys. B 2018, 27 (1): 017802;  doi: 10.1088/1674-1056/27/1/017802
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A blue phosphor was obtained by doping Eu2+ into a multi-cation host Sr0.8Ca0.2Al2Si2O8 through high temperature solid state reaction. The emission spectra show a continuous red-shift behavior from 413 nm to 435 nm with Eu2+ concentration increasing. The substitution priority of Eu2+ in Sr0.8Ca0.2Al2Si2O8 was investigated via x-ray diffraction (XRD) and temperature properties in detail:the Ca2+ ions are preferentially substituted by Eu2+at lower doping, and with the Eu2+ concentration increasing, the probability of substitution for Sr2+ is greater than that of replacing Ca2+. Accordingly, we propose the underlying method of thermal property to determine the substitution of Eu2+ in the multi-cation hosts. Moreover, the abnormal increase of emission intensity with increasing temperature was studied by the thermoluminescence spectra. The energy transfer mechanism between the Eu2+ ions occupying different cation sites was studied by the lifetime decay curves. A series of warm white light emitting diodes were successfully fabricated using the blue phosphors Sr0.8Ca0.2Al2Si2O8:Eu2+ with commercial red phosphor (Ca Sr)SiAlN3:Eu2+ and green phosphor (Y Lu)3Al5O12:Ce3+, and the luminescent efficiency can reach 45 lm/W.
SPECIAL TOPIC—Soft matter and biological physics (Review)
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Facilitative effect of graphene quantum dots in MoS2 growth process by chemical vapor deposition

Lu Zhang, Yongsheng Wang, Yanfang Dong, Xuan Zhao, Chen Fu, Dawei He
Chin. Phys. B 2018, 27 (1): 018101;  doi: 10.1088/1674-1056/27/1/018101
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The substrate treatment with seeding promoter can promote the two-dimensional material lateral growth in chemical vapor deposition (CVD) process. Herein, graphene quantum dots (GQDs) as a novel seeding promoter were used to obtain uniform large-area MoS2 monolayer. The obtained monolayer MoS2 films were confirmed by optical microscope, scanning electron microscope, Raman and photoluminescence spectra. Raman mapping revealed that the MoS2 monolayer was largely homogeneous.

A novel multi-scroll chaotic generator: Analysis, simulation, and implementation

Gui-Tao Zhang, Fa-Qiang Wang
Chin. Phys. B 2018, 27 (1): 018201;  doi: 10.1088/1674-1056/27/1/018201
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Based on a new three-dimensional autonomous linear system and designing a specific form of saturated function series and a sign function with two variables of system, which are employed to increase saddle-focus equilibrium points with index 2, a novel multi-scroll chaotic system is proposed and its typical dynamical characteristics including bifurcation diagram, Poincare map, and the stability of equilibrium points are analyzed. The hardware circuit is designed and the experimental results are presented for confirmation.

Pressure-induced structural evolution of apatite-type La9.33Si6O26

Guangchao Yin, Hong Yin, Meiling Sun, Wei Gao
Chin. Phys. B 2018, 27 (1): 018202;  doi: 10.1088/1674-1056/27/1/018202
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The pressure-induced structural evolution of apatite-type La9.33Si6O26 was systematically studied using in situ synchrotron x-ray diffraction (XRD). The XRD spectra indicated that a subtly reversible phase transition from P63/m to P63 symmetry occurred at~13.6 GPa because of the tilting of the SiO4 tetrahedra under compression. Furthermore, the La9.33Si6O26 exhibited a higher axial compression ratio for the a-axis than the c-axis, owing to the different axial arrangement of the SiO4 tetrahedra. Interestingly, the high-pressure phase showed compressibility unusually higher than that of the initial phase, suggesting that the low P63 symmetry provided more degrees of freedom. Moreover, the La9.33Si6O26 exhibited a lower phase transition pressure (PT) and a higher lattice compression than La10Si6O27. Comparisons between La9.33Si6O26 and La10Si6O27 provided a deeper understanding of the effect of interstitial oxygen atoms on the structural evolution of apatite-type lanthanum silicates (ATLSs).

Photon-counting chirped amplitude modulation lidar system using superconducting nanowire single-photon detector at 1550-nm wavelength

Hui Zhou, Yu-Hao He, Chao-Lin Lü, Li-Xing You, Zhao-Hui Li, Guang Wu, Wei-Jun Zhang, Lu Zhang, Xiao-Yu Liu, Xiao-Yan Yang, Zhen Wang
Chin. Phys. B 2018, 27 (1): 018501;  doi: 10.1088/1674-1056/27/1/018501
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We demonstrate a photon-counting chirped amplitude modulation (CAM) light detection and ranging (lidar) system incorporating a superconducting nanowire single-photon detector (SNSPD) and operated at a wavelength of 1550 nm. The distance accuracy of the lidar system was determined by the CAM bandwidth and signal-to-noise ratio (SNR) of an intermediate frequency (IF) signal. Owing to a short dead time (10 ns) and negligible dark count rate (70 Hz) of the SNSPD, the obtained IF signal attained an SNR of 42 dB and the direct distance accuracy was improved to 3 mm when the modulation bandwidth of the CAM signal was 240 MHz and the modulation period was 1 ms.

Improved performance of Ge n+/p diode by combining laser annealing and epitaxial Si passivation

Chen Wang, Yihong Xu, Cheng Li, Haijun Lin
Chin. Phys. B 2018, 27 (1): 018502;  doi: 10.1088/1674-1056/27/1/018502
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A method to improve Ge n+/p junction diode performance by excimer laser annealing (ELA) and epitaxial Si passivation under a low ion implantation dose is demonstrated. The epitaxial Si passivation layer can unpin the Fermi level of the contact of Al/n-Ge to some extent and reduce the contact resistance. In addition, the fabricated Ge n+/p junction diode by ELA plus epitaxial Si passivation exhibits a decreased reverse current density and an increased forward current density, resulting in a rectification ratio of about 6.5×106 beyond two orders magnitude larger than that by ELA alone. The reduced specific contact resistivity of metal on n-doped germanium and well-behaved germanium n+/p diode are beneficial for the performance improvement of Ge n-MOSFETs and other opto-electronic devices.

Excessive levitation for the efficient loading of large-volume optical dipole traps

Xiaoqing Wang, Yuqing Li, Guosheng Feng, Jizhou Wu, Jie Ma, Liantuan Xiao, Suotang Jia
Chin. Phys. B 2018, 27 (1): 018702;  doi: 10.1088/1674-1056/27/1/018702
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We study the excessive levitation effect in the magnetically levitated loading process of ultracold Cs atoms into a large-volume crossed optical dipole trap. We analyze the motion of atoms with a non-zero combined gravito-magnetic force during the loading, where the magnetically levitated force catches up with and surpasses the gravity. We present the theoretical variations of both acceleration and velocity with levitation time and magnetic field gradient. We measure the evolution of the number of trapped atoms with the excessive levitation time at different magnetic field gradients. The dependence of the number of atoms on the magnetic field gradient is also measured for different excessive levitation times. The theoretical analysis shows reasonable agreement with the experimental results. Our investigation illustrates that the excessive levitation can be used to reduce the heating effect of atoms in the magnetically levitated loading process, and to improve the loading rate of a large-volume optical dipole trap.

Efficient design of perovskite solar cell using mixed halide and copper oxide

Navneet kour, Rajesh Mehra, Chandni
Chin. Phys. B 2018, 27 (1): 018801;  doi: 10.1088/1674-1056/27/1/018801
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Solar cells based on perovskites have emerged as a transpiring technology in the field of photovoltaic. These cells exhibit high power conversion efficiency. The perovskite material is observed to have good absorption in the entire visible spectrum which can be well illustrated by the quantum efficiency curve. In this paper, theoretical analysis has been done through device simulation for designing solar cell based on mixed halide perovskite. Various parameters have efficacy on the solar cell efficiency such as defect density, layer thickness, doping concentration, band offsets, etc. The use of copper oxide as the hole transport material has been analyzed. The analysis divulges that due to its mobility of charge carriers, it can be used as an alternative to spiro-OMeTAD. With the help of simulations, reasonable materials have been employed for the optimal design of solar cell based on perovskite material. With the integration of copper oxide into the solar cell structure, the results obtained are competent enough. The simulations have shown that with the use of copper oxide as hole transport material with mixed halide perovskite as absorber, the power conversion efficiency has improved by 6%. The open circuit voltage has shown an increase of 0.09 V, short circuit current density has increased by 2.32 mA/cm2, and improvement in fill factor is 8.75%.

A compact and high-power silicon-wafer solar strip-cells-array module integrated with an array concentrator

Jie Lin, Mengxia Chen, Yongqi Ke, Caiying Ren, Zesheng Xu, Yaoju Zhang, Chaolong Fang
Chin. Phys. B 2018, 27 (1): 018802;  doi: 10.1088/1674-1056/27/1/018802
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A compact, low-cost and high-output-power silicon-wafer solar strip-cells-array module (SCAM) was experimentally demonstrated. The proposed SCAM consisted mainly of a silicon-wafer strip-cell sparse array and low-concentration-ratio array concentrator based on an epoxy resin polymer (ERP) cylindrical plano-convex lens. A polymer replication process based on a polydimethylsiloxane mold was used to fabricate the ERP lens array concentrator. The results show that 46.94% of the silicon-wafer cell was saved in the designed SCAM. Moreover, the output power of the SCAM with a low concentration ratio of 8 suns was improved by 8.6%, compared with a whole piece of a conventional silicon-wafer solar cell with the same area as the module. The proposed method encapsulating solar cells provides a means to reduce the usage of silicon cells in modules as well as improving the output power of modules.

SPECIAL TOPIC—Soft matter and biological physics

To what extent of ion neutralization can multivalent ion distributions around RNA-like macroions be described by Poisson-Boltzmann theory?

Gui Xiong, Kun Xi, Xi Zhang, Zhi-Jie Tan
Chin. Phys. B 2018, 27 (1): 018203;  doi: 10.1088/1674-1056/27/1/018203
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Nucleic acids are negatively charged biomolecules, and metal ions in solutions are important to their folding structures and thermodynamics, especially multivalent ions. However, it has been suggested that the binding of multivalent ions to nucleic acids cannot be quantitatively described by the well-established Poisson-Boltzmann (PB) theory. In this work, we made extensive calculations of ion distributions around various RNA-like macroions in divalent and trivalent salt solutions by PB theory and Monte Carlo (MC) simulations. Our calculations show that PB theory appears to underestimate multivalent ion distributions around RNA-like macroions while can reliably predict monovalent ion distributions. Our extensive comparisons between PB theory and MC simulations indicate that when an RNA-like macroion gets ion neutralization beyond a “critical” value, the multivalent ion distribution around that macroion can be approximately described by PB theory. Furthermore, an empirical formula was obtained to approximately quantify the critical ion neutralization for various RNA-like macroions in multivalent salt solutions, and this empirical formula was shown to work well for various real nucleic acids including RNAs and DNAs.

A network of conformational transitions in an unfolding process of HP-35 revealed by high-temperature MD simulation and a Markov state model

Dandan Shao, Kaifu Gao
Chin. Phys. B 2018, 27 (1): 018701;  doi: 10.1088/1674-1056/27/1/018701
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An understanding of protein folding/unfolding processes has important implications for all biological processes, including protein degradation, protein translocation, aging, and diseases. All-atom molecular dynamics (MD) simulations are uniquely suitable for it because of their atomic level resolution and accuracy. However, limited by computational capabilities, nowadays even for small and fast-folding proteins, all-atom MD simulations of protein folding still presents a great challenge. An alternative way is to study unfolding process using MD simulations at high temperature. High temperature provides more energy to overcome energetic barriers to unfolding, and information obtained from studying unfolding can shed light on the mechanism of folding. In the present study, a 1000-ns MD simulation at high temperature (500 K) was performed to investigate the unfolding process of a small protein, chicken villin headpiece (HP-35). To infer the folding mechanism, a Markov state model was also built from our simulation, which maps out six macrostates during the folding/unfolding process as well as critical transitions between them, revealing the folding mechanism unambiguously.

Improved data analysis method of single-molecule experiments based on probability optimization

Weili Zhai, Guohua Yuan, Chao Liu, Hu Chen
Chin. Phys. B 2018, 27 (1): 018703;  doi: 10.1088/1674-1056/27/1/018703
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To extract the dynamic parameters from single molecule manipulation experiments, usually lots of data at different forces need to be recorded. But the measuring time of a single molecule is limited due to breakage of the tether or degradation of the molecule. Here we propose a data analysis method based on probability maximization of the recorded time trace to extract the dynamic parameters from a single measurement. The feasibility of this method was verified by dealing with the simulation data of a two-state system. We also applied this method to estimate the parameters of DNA hairpin folding and unfolding dynamics measured by a magnetic tweezers experiment.
TOPICAL REVIEW—New generation solar cells

Interfaces of high-efficiency kesterite Cu2ZnSnS(e)4 thin film solar cells

Shoushuai Gao, Zhenwu Jiang, Li Wu, Jianping Ao, Yu Zeng, Yun Sun, Yi Zhang
Chin. Phys. B 2018, 27 (1): 018803;  doi: 10.1088/1674-1056/27/1/018803
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Cu2ZnSnS(e)4 (CZTS(e)) solar cells have attracted much attention due to the elemental abundance and the non-toxicity. However, the record efficiency of 12.6% for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is much lower than that of Cu(In,Ga)Se2 (CIGS) solar cells. One crucial reason is the recombination at interfaces. In recent years, large amount investigations have been done to analyze the interfacial problems and improve the interfacial properties via a variety of methods. This paper gives a review of progresses on interfaces of CZTS(e) solar cells, including:(i) the band alignment optimization at buffer/CZTS(e) interface, (ii) tailoring the thickness of MoS(e)2 interfacial layers between CZTS(e) absorber and Mo back contact, (iii) the passivation of rear interface, (iv) the passivation of front interface, and (v) the etching of secondary phases.

Promise of commercialization: Carbon materials for low-cost perovskite solar cells

Yu Cai, Lusheng Liang, Peng Gao
Chin. Phys. B 2018, 27 (1): 018805;  doi: 10.1088/1674-1056/27/1/018805
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Perovskite solar cells (PVSCs) have attracted extensive studies due to their high power conversion efficiency (PCE) with low-cost in both raw material and processes. However, there remain obstacles that hinder the way to their commercialization. Among many drawbacks in PVSCs, we note the problems brought by the use of noble metal counter electrodes (CEs) such as gold and silver. The costly Au and Ag need high energy-consumption thermal evaporation process which can be made only with expensive evaporation equipment under vacuum. All the factors elevate the threshold of PVSCs' commercialization. Carbon material, on the other hand, is a readily available electrode candidate for the application as CE in the PVSCs. In this review, endeavors on PVSCs with low-cost carbon materials will be comprehensively discussed based on different device structures and carbon compositions. We believe that the PVSCs with carbon-based CE hold the promise of commercialization of this new technology.

Theoretical study on the kesterite solar cells based on Cu2ZnSn(S,Se)4 and related photovoltaic semiconductors

Dingrong Liu, Dan Han, Menglin Huang, Xian Zhang, Tao Zhang, Chenmin Dai, Shiyou Chen
Chin. Phys. B 2018, 27 (1): 018806;  doi: 10.1088/1674-1056/27/1/018806
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The kesterite thin film solar cells based on the quaternary Cu2ZnSnS4 and Cu2ZnSnSe4 and their alloys Cu2ZnSn(S,Se)4 have been considered as environment-friendly and non-toxic alternatives to the currently commercialized CdTe and Cu(In,Ga)Se2 thin film solar cells. From the theoretical point of view, we will review how the group I2-Ⅱ-IV-VI4 quaternary compound semiconductors are derived from the binary CdTe and the ternary CuInSe2 or CuGaSe2 through the cation mutation, and how the crystal structure and electronic band structure evolve as the component elements change. The increased structural and chemical freedom in these quaternary semiconductors opens up new possibility for the tailoring of material properties and design of new light-absorber semiconductors. However, the increased freedom also makes the development of high-efficiency solar cells more challenging because much more intrinsic point defects, secondary phases, surfaces, and grain-boundaries can exist in the thin films and influence the photovoltaic performance in a way different from that in the conventional CdTe and Cu(In,Ga)Se2 solar cells. The experimental characterization of the properties of defects, secondary phase, and grain-boundaries is currently not very efficient and direct, especially for these quaternary compounds. First-principles calculations have been successfully used in the past decade for studying these properties. Here we will review the theoretical progress in the study of the mixed-cation and mixed-anion alloys of the group I2-Ⅱ-IV-VI4 semiconductors, defects, alkaline dopants, and grain boundaries, which provided very important information for the optimization of the kesterite solar cell performance.

Recent progress of colloidal quantum dot based solar cells

Huiyun Wei, Dongmei Li, Xinhe Zheng, Qingbo Meng
Chin. Phys. B 2018, 27 (1): 018808;  doi: 10.1088/1674-1056/27/1/018808
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Colloidal quantum dot (CQD) solar cells have attracted great interest due to their low cost and superior photo-electric properties. Remarkable improvements in cell performances of both quantum dot sensitized solar cells (QDSCs) and PbX (X=S, Se) based CQD solar cells have been achieved in recent years, and the power conversion efficiencies (PCEs) exceeding 12% were reported so far. In this review, we will focus on the recent progress in CQD solar cells. We firstly summarize the advance of CQD sensitizer materials and the strategies for enhancing carrier collection efficiency in QDSCs, including developing multi-component alloyed CQDs and core-shell structured CQDs, as well as various methods to suppress interfacial carrier recombination. Then, we discuss the device architecture development of PbX CQD based solar cells and surface/interface passivation methods to increase light absorption and carrier extraction efficiencies. Finally, a short summary, challenge, and perspective are given.

Editorial

Qingbo Meng
Chin. Phys. B 2018, 27 (1): 018811;  doi: 10.1088/1674-1056/27/1/018811
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