中国物理B

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Second order conformal multi-symplectic method for the damped Korteweg-de Vries equation

郭峰

2019 (5): 50201-050201. doi: 10.1088/1674-1056/28/5/050201

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A conformal multi-symplectic method has been proposed for the damped Korteweg-de Vries (DKdV) equation, which is based on the conformal multi-symplectic structure. By using the Strang-splitting method and the Preissmann box scheme, we obtain a conformal multi-symplectic scheme for multi-symplectic partial differential equations (PDEs) with added dissipation. Applying it to the DKdV equation, we construct a conformal multi-symplectic algorithm for it, which is of second order accuracy in time. Numerical experiments demonstrate that the proposed method not only preserves the dissipation rate of mass exactly with periodic boundary conditions, but also has excellent long-time numerical behavior.

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General way to define tunneling time

舒正, 郝小雷, 李卫东, 陈京

2019 (5): 50301-050301. doi: 10.1088/1674-1056/28/5/050301

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With the development of attosecond science, tunneling time can now be measured experimentally with the attoclock technique. However, there are many different theoretical definitions of tunneling time and no consensus has been achieved. Here, we bridge the relationship between different definitions of tunneling time based on a quantum travel time in one-dimensional rectangular barrier tunneling problem. We find that the real quantum travel time t_Re is equal to the Bohmian time t_Bohmian, which is related to the resonance lifetime of a bound state. The total quantum travel time τ_t can perfectly retrieve the transversal time t_x and the Büttiker-Landauer time τ_BL in two opposite limits, regardless of the particle energy.

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Dynamics of two levitated nanospheres nonlinearly coupling with non-Markovian environment

李逊, 熊标, 晁石磊, 金家森, 周玲

2019 (5): 50302-050302. doi: 10.1088/1674-1056/28/5/050302

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The dynamics of two nanospheres nonlinearly coupling with non-Markovian reservoir is investigated. A master equation of the two nanospheres is derived by employing quantum state diffusion method. It is shown that the nonlinear coupling can improve the non-Markovianity. Due to the sharing of the common non-Markovian environment, the state transfer between the two nanospheres can be realized. The entanglement and the squeezing of the individual mode, as well as the jointed two-mode are analyzed. The present system can be realized by trapping two nanospheres in a wideband cavity, which might provide a method to study adjustable non-Markovian dynamics of mechanical motion.

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Successive lag cluster consensus on multi-agent systems via delay-dependent impulsive control

邱小芬, 张银星, 李科赞

2019 (5): 50501-050501. doi: 10.1088/1674-1056/28/5/050501

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We introduce a new consensus pattern, named a successive lag cluster consensus (SLCC), which is a generalized pattern of successive lag consensus (SLC). By applying delay-dependent impulsive control, the SLCC of first-order and second-order multi-agent systems is discussed. Furthermore, based on graph theory and stability theory, some sufficient conditions for the stability of SLCC on multi-agent systems are obtained. Finally, several numerical examples are given to verify the correctness of our theoretical results.

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Unifying quantum heat transfer and superradiant signature in a nonequilibrium collective-qubit system:A polaron-transformed Redfield approach

陈许敏, 王晨

2019 (5): 50502-050502. doi: 10.1088/1674-1056/28/5/050502

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We investigate full counting statistics of quantum heat transfer in a collective-qubit system constructed by multi-qubits interacting with two thermal baths. The nonequilibrium polaron-transformed Redfield approach embedded with an auxiliary counting field is applied to obtain the steady state heat current and fluctuations, which enables us to study the impact of the qubit-bath interaction in a wide regime. The heat current, current noise, and skewness are all found to clearly unify the limiting results in the weak and strong couplings. Moreover, the superradiant heat transfer is clarified as a system-size-dependent effect, and large number of qubits dramatically suppress the nonequilibrium superradiant signature.

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Turing pattern in the fractional Gierer-Meinhardt model

王语, 张荣培, 王震, 韩子健

2019 (5): 50503-050503. doi: 10.1088/1674-1056/28/5/050503

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It is well-known that reaction-diffusion systems are used to describe the pattern formation models. In this paper, we will investigate the pattern formation generated by the fractional reaction-diffusion systems. We first explore the mathematical mechanism of the pattern by applying the linear stability analysis for the fractional Gierer-Meinhardt system. Then, an efficient high-precision numerical scheme is used in the numerical simulation. The proposed method is based on an exponential time differencing Runge-Kutta method in temporal direction and a Fourier spectral method in spatial direction. This method has the advantages of high precision, better stability, and less storage. Numerical simulations show that the system control parameters and fractional order exponent have decisive influence on the generation of patterns. Our numerical results verify our theoretical results.

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Molecule opacities of X²Σ⁺, A²Π, and B²Σ⁺ states of CS⁺

林晓贺, 梁桂颖, 王建国, 彭裔耕, 邵彬, 李瑞, 吴勇

2019 (5): 53101-053101. doi: 10.1088/1674-1056/28/5/053101

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Carbon sulfide cation (CS⁺) plays a dominant role in some astrophysical atmosphere environments. In this work, the rovibrational transition lines are computed for the lowest three electronic states, in which the internally contracted multireference configuration interaction approach (MRCI) with Davison size-extensivity correction (+Q) is employed to calculate the potential curves and dipole moments, and then the vibrational energies and spectroscopic constants are extracted. The Frank-Condon factors are calculated for the bands of X²Σ⁺-A²Π and X²Σ⁺-B²Σ⁺ systems, and the band of X²Σ⁺-A²Π is in good agreement with the available experimental results. Transition dipole moments and the radiative lifetimes of the low-lying three states are evaluated. The opacities of the CS⁺ molecule are computed at different temperatures under the pressure of 100 atms. It is found that as temperature increases, the band systems associated with different transitions for the three states become dim because of the increased population on the vibrational states and excited electronic states at high temperature.

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Tensile-strain induced phonon splitting in diamond

蒲梅芳, 张峰, 刘珊, 入舩徹男, 雷力

2019 (5): 53102-053102. doi: 10.1088/1674-1056/28/5/053102

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The first-order Raman spectroscopy of diamond exhibits splitting and redshift after the burst of high-pressure (160-200 GPa) and high-temperature (~2000 K). The observed longitudinal optical (LO) and the transverse optical (TO) splitting of Raman phonon is related to the tensile-strain induced activation of the forbidden or silent Raman modes that arise in the proximity of the Brillouin zone center.

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Dependence of photoelectron-momentum distribution of H₂⁺ molecule on orientation angle and laser ellipticity

张宏丹, 张思琪, 纪磊, 甄琪, 郭静, 刘学深

2019 (5): 53201-053201. doi: 10.1088/1674-1056/28/5/053201

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By numerically solving the two-dimensional time-dependent Schrödinger equation under the frozen-nuclei approximation, we are able to study the molecular photoelectron-momentum distribution (MPMD) of H₂⁺ with different orientation angles driven by elliptically polarized laser pulse with varying ellipticities. Our numerical results show that the MPMD is sensitive to the orientation angle and the laser ellipticity, which can be explained by the attosecond perturbation ionization theory and the exactly solvable photoionization model. When the ellipticity ε=0, the final MPMD of x-oriented H₂⁺ shows a distinct six-lobe pattern that is different from that with ε=0.5 and ε=1. The evolutions of electron wave packet (EWP) and MPMD with x-oriented H₂⁺ are presented to interpret this distinct pattern.

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Rydberg electromagnetically induced transparency and Autler-Townes splitting in a weak radio-frequency electric field

郝丽萍, 薛咏梅, 樊佳蓓, 焦月春, 赵建明, 贾锁堂

2019 (5): 53202-053202. doi: 10.1088/1674-1056/28/5/053202

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We utilize an electromagnetically induced transparency (EIT) of a three-level cascade system involving Rydberg state in a room-temperature cell, formed with a cesium 6S_1/2-6P_3/2-66S_1/2 scheme, to investigate the Autler-Townes (AT) splitting resulting from a 15.21-GHz radio-frequency (RF) field that couples the|66S_1/2>→|65P_1/2> Rydberg transition. The radio-frequency electric field induced AT splitting, γ_AT, is defined as the peak-to-peak distance of an EIT-AT spectrum. The dependence of AT splitting γ_AT on the probe and coupling Rabi frequency, Ω_p and Ω_c, is investigated. It is found that the EIT-AT splitting strongly depends on the EIT linewidth that is related to the probe and coupling Rabi frequency in a weak RF-field regime. Using a narrow linewidth EIT spectrum would decrease the uncertainty of the RF field measurements. This work provides new experimental evidence for the theoretical framework in[J. Appl. Phys. 121, 233106 (2017)].

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Effect of laser polarization on strong-field ionization and fragmentation of nitrous oxide molecules

王瑞, 张世文, 刘洋, 孙添, 吕航, 徐海峰

2019 (5): 53301-053301. doi: 10.1088/1674-1056/28/5/053301

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Ionization of molecules in femtosecond laser fields is the most fundamental and important step of various strong-field physical processes. In this study, we experimentally investigate strong field ionization of linear N₂O molecules using a time-of-flight mass spectrometer in 800-nm laser fields. Yields of the parent ion and different fragment ions are measured as a function of laser intensity in the range of 2.0×10¹³ W/cm² to 3.6×10¹⁴ W/cm². We also investigate the dependence of strong field ionization and dissociation of N₂O on laser ellipticity and polarization direction. The significant role of laser induced electron re-collision in the formation of highly charged fragment ions is proved. The physical mechanism of strong field ionization and fragmentation is discussed, based on our experimental results.

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Isotope effect and Coriolis coupling effect forthe Li + H(D)Cl→LiCl + H(D) reaction

翟红生, 梁广雷, 丁俊霞, 刘玉芳

2019 (5): 53401-053401. doi: 10.1088/1674-1056/28/5/053401

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A time-dependent quantum wave packet method is used to investigate the dynamics of the Li+ H(D)Cl reaction based on a new potential energy surface (J. Chem. Phys. 146 164305 (2017)). The reaction probabilities of the Coriolis coupled (CC) and centrifugal sudden (CS) calculations, the integral cross sections, the reaction rate constants are obtained. The rate constants of the Li+ HCl reaction are within the error bounds at low temperature. A comparison of the CC and CS results reveals that the Coriolis coupling plays an important role in the Li+ H(D)Cl reaction. The CC cross sections are larger than the CS results within the entire energy range, demonstrating that the Coriolis coupling effect can more effectively promote the Li+ DCl reaction than the Li+ HCl reaction. It is found that the isotope effect has a great influence on the title reaction.

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Generation of high-energy-resolved NH₃ molecular beam by a Stark decelerator with 179 stages

魏斌, 侯顺永, 郭恒娇, 纪亚兵, 李胜强, 印建平

2019 (5): 53701-053701. doi: 10.1088/1674-1056/28/5/053701

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We demonstrate the production of cold, slow NH₃ molecules from a supersonic NH₃ molecular beam using our electrostatic Stark decelerator consisting of 179 slowing stages. By using this long Stark decelerator, a supersonic NH₃ molecular beam can be easily decelerated to trappable velocities. Here we present two modes for operating the Stark decelerator to slow the supersonic NH₃ molecules. The first is the normal mode, where all 179 stages are used to decelerate molecules, and it allows decelerating the NH₃ molecular beam from 333 m/s to 18 m/s, with a final temperature of 29.2 mK. The second is the deceleration-bunch mode, which allows us to decelerate the supersonic NH₃ beam from 333 m/s to 24 m/s, with a final temperature of 2.9 mK. It is clear that the second mode promises to produce colder (high-energy-resolution) molecular samples than the normal mode. Three-dimensional Monte Carlo simulations are also performed for the experiments and they show a good agreement with the observed results. The deceleration-bunch operation mode presented here can find applications in the fields of cold collisions, high-resolution spectroscopy, and precision measurements.

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Loss induced negative refraction and super-prism effect at highly absorptive interface

吴坚, 王涛, 侯天悦, 阚雪芬, 殷澄, 周朴, 曹庄琪

2019 (5): 54201-054201. doi: 10.1088/1674-1056/28/5/054201

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It is well-established that waves are inhomogeneous in a lossy isotropic medium, and the validation of the classical Snell's law is still questionable for light refraction at the dissipative and dispersive interface. With high absorption, direct experimental investigation is rather difficult due to the extremely short penetration depth; i.e., the skin depth. In this paper, a simple and unified description of this issue is proposed, which can be applied to both materials with anomalous dispersion and in the Drude region. The gradient ▽_kω is found to be incident angle θ_i-dependent, and the direction of the group velocity may deviate significantly from the phase velocity due to the loss induced permittivity structure. The physics behind the negative refraction effect is explained, and a novel loss induced super-prism effect is also predicted.

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Dynamics of Airy beams in parity-time symmetric optical lattices

陈睿弘, 洪伟毅

2019 (5): 54202-054202. doi: 10.1088/1674-1056/28/5/054202

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We investigate the dynamics of airy beams propagating in the parity-time (PT) symmetric optical lattices in linear and nonlinear regimes, respectively. For the linear propagation, the position of the channel guided by the PT lattice can be shifted by tuning the lattice frequency. The underlying physical mechanism of this phenomenon is also discussed. An interesting phenomenon is found in the nonlinear regime in that the airy beam becomes a tilt channel with several Rayleigh lengths. These findings create new opportunities for optical steering and manipulations.

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Electrically triggered dual-band tunable terahertz metamaterial band-pass filter based on Si₃N₄-VO₂-Si₃N₄ sandwich

赵帅, 胡放荣, 徐新龙, 江明珠, 张文涛, 银珊, 姜文英

2019 (5): 54203-054203. doi: 10.1088/1674-1056/28/5/054203

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We experimentally demonstrate an electrically triggered terahertz (THz) dual-band tunable band-pass filter based on Si₃N₄-VO₂-Si₃N₄ sandwich-structured hybrid metamaterials. The insulator-metal phase transition of VO₂ film is induced by the Joule thermal effect of the top metal layer. The finite-integration-time-domain (FITD) method and finite element method (FEM) are used for numerical simulations. The sample is fabricated using a surface micromachining process, and characterized by a THz time-domain-spectrometer (TDS). When the bias current is 0.225 A, the intensity modulation depths at two central frequencies of 0.56 THz and 0.91 THz are about 81.7% and 81.3%, respectively. This novel design can achieve dynamically electric-thermo-optic modulation in the THz region, and has potential applications in the fields of THz communications, imaging, sensing, and astronomy exploration.

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Optically manipulated nanomechanics of semiconductor nanowires

宋晨之, 杨是赜, 李晓敏, 李晓, 冯济, 潘安练, 王文龙, 许智, 白雪冬

2019 (5): 54204-054204. doi: 10.1088/1674-1056/28/5/054204

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Opto-electromechanical coupling at the nanoscale is an important topic in new scientific studies and technical applications. In this work, the optically manipulated electromechanical behaviors of individual cadmium sulfide (CdS) nanowires are investigated by a customer-built optical holder inside transmission electron microscope, wherein in situ electromechanical resonance took place in conjunction with photo excitation. It is found that the natural resonance frequency of the nanowire under illumination becomes considerably lower than that under darkness. This redshift effect is closely related to the wavelength of the applied light and the diameter of the nanowires. Density functional theory (DFT) calculation shows that the photoexcitation leads to the softening of CdS nanowires and thus the redshift of natural frequency, which is in agreement with the experimental results.

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High power diode-pumped passively mode-locked Nd:YVO₄ laser at repetition rate of 3.2 GHz

程梦尧, 王兆华, 曹艳芳, 孟祥昊, 朱江峰, 王军利, 魏志义

2019 (5): 54205-054205. doi: 10.1088/1674-1056/28/5/054205

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A compact high power diode-pumped passively mode-locked Nd:YVO₄ laser with high repetition rate is realized. Using an Nd:YVO₄ crystal and a semiconductor saturable absorber mirror (SESAM) in the oscillator, the picosecond pulse output with an average power of 1.38 W, a repetition rate of 3.24 GHz, and a pulse duration of 11.4 ps is achieved. After one stage of amplification, the final output power reaches 11.34 W, corresponding to a total optical-to-optical efficiency of about 32%. The root mean square (RMS) value of power fluctuation is demonstrated to be less than 0.6% in 24 hours, showing a superior stability with the compact configuration.

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Mid-infrared supercontinuum generation and its application on all-optical quantization with different input pulses

李妍, 桑新柱

2019 (5): 54206-054206. doi: 10.1088/1674-1056/28/5/054206

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Supercontinuum generation (SCG) and its application on all-optical quantization of all-optical analog-to-digital conversions (AOADCs) at the mid-infrared region in an AlGaAs strip waveguide are investigated numerically. The simulation results show that when the parabolic pulse is input, not only broader and higher-coherence SCG is obtained and a higher effective number of bits (ENOB) can be achieved, compared with the input pulse with hyperbolic-secant and Gaussian shaping. A four-bit quantization resolution is achieved along with a signal-to-noise ratio of 24.02 dB and an ENOB of 3.99 bit, and the required input peak power is 760 mW.

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Influence of warm eddies on sound propagation in the Gulf of Mexico

肖瑶, 李整林, 李鋆, 刘佳琪, Karim G Sabra

2019 (5): 54301-054301. doi: 10.1088/1674-1056/28/5/054301

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An automatic detection method is employed to identify and track eddies in the Gulf of Mexico. The physical parameters of the eddies, such as lifespan, radius, and distribution position are first examined and used to determine the spatio-temporal evolution of a strong warm eddy separated from the Mexico current. Then, the influence of this strong warm eddy on sound propagation during its lifespan are comprehensively analyzed with the parabolic equation and explained by using the normal mode and ray theories. Additionally, the influence of mesoscale eddies on the redistribution of total depth-integrated energy among the normal modes in the deep water is also discussed. The variation of arrival angle is investigated to explain the spreading acoustic energy caused by eddies. Overall, the results show that warm eddies can change the propagation paths and cause the convergence zone to broaden and approach the sound source. Moreover, the warm eddy can disperse sound energy and cause the total depth-integrated energy to incline to a lower normal mode. Throughout the whole of these three periods (eddy generating, eddy maturing, and eddy terminating), the fluctuation in the transmission loss is up to 30 dB (depending on the relative location of eddy center to the source).

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A passive source ranging method based on the frequency warping transform of the vertical intensity flux in shallow water

戚聿波, 周士弘, 于梦枭, 杜淑媛, 孙梅, 张仁和

2019 (5): 54302-054302. doi: 10.1088/1674-1056/28/5/054302

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The phase of cross-correlation function of two different normal modes contains source range information, which can be extracted by warping transform due to the dispersive characteristics of the shallow water waveguide. The autocorrelation function of the received pressure or particle velocity contains both modal autocorrelation component (MAC) and modal cross-correlation component (MCC), with the former part usually treated as interference for source ranging. Because the real part of the vertical intensity flux (RPVIF) only contains MCC, a passive impulsive source ranging method based on the frequency warping transform of RPVIF with a single vector receiver in shallow water is presented. Using a waveguide-invariant-based frequency warping operator, the cross-correlation components of two different modes in the vertical intensity flux are warped into separable impulsive sequences, the time delays of which are subsequently used for source ranging. The advantages of source ranging based on warping the vertical intensity flux compared with warping the pressure autocorrelation function are pointed out, and the experiment results are also presented.

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A novel multi-cavity Helmholtz muffler

邵瀚波, 何欢, 陈岩, 陈国平

2019 (5): 54303-054303. doi: 10.1088/1674-1056/28/5/054303

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A novel multi-cavity Helmholtz muffler is proposed. The multi-cavity Helmholtz muffler is composed of steel structures and silicone membranes. With suitable construction, the Helmholtz muffler can be designed to exhibit negative mass density in low frequency, and the muffling frequency can be adjusted when we change the internal structure of the cavity, which will be very attractive for noise control. In this paper, we investigate the influence of the membranes and the cavities on noise reduction characteristics with theoretical calculations and simulations. The results show that the numbers of membranes and the volumes of the cavities can have a great effect on the position of the muffling frequency. The number of cavities can have a great effect on the width of the muffling frequency (reduce the noise by 10 dB). With different combinations of the membranes and cavities, we can get different muffling frequencies, which can meet different muffling demands in practical applications and is more flexible than the traditional Helmholtz cavity.

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An experimental method for quantitative analysis of real contact area based on the total reflection optical principle

罗治军, 宋保江, 韩靖宇, 阎绍泽

2019 (5): 54601-054601. doi: 10.1088/1674-1056/28/5/054601

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The simulation of real contact area between materials is foundationally important for the contact mechanics of mechanical structures. The Greenwood and Williamson (GW) model and the Majumdar (MB) model are the basic models in this field, which are widely accepted and proven to be valid in many experiments and engineering. Although the contact models have evolved considerably in recent years, the verifications of the models are most based on the indirect methods such as electrical conductivity and contact stiffness, because of the lack of effective methods to directly measure the variation of contact surface. In this paper, the total reflection (TR) method is introduced into the verification of contact models. An experiment system based on TR method is constructed to measure the real contact area of two PMMA specimens. The comparison analysis between the results of experiment and models suggests that the experiment result has the same trend with simulation, the MB model has better agreement with the experimental result because this method can take into account the variation of radius and the merging of asperities, while the GW model has a huge deviation because of the dependence on resolution and the lack of considering the variation of radius and asperity's merging process. Taking the interaction of asperities into account could give a better result that is closer to the experiment. Our results and analysis prove that the experimental methods in this paper could be used as a more direct and valid method to quantitatively measure the real contact area and to verify the contact models.

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Direct numerical simulation on relevance of fluctuating velocities and drag reduction in turbulent channel flow with spanwise space-dependent electromagnetic force

江代文, 张辉, 范宝春, 王安华

2019 (5): 54701-054701. doi: 10.1088/1674-1056/28/5/054701

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Based on the Fourier-Chebyshev spectral method, the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation (DNS) methods for different Reynolds numbers. A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient. With the application of electromagnetic force, the in-depth relations among the fluctuating velocities near the wall, Reynolds stress, and the effect of drag reduction for different Reynolds numbers are discussed. The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers. The fluctuating velocities along the streamwise and normal directions are suppressed significantly, while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force. However, the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction. Therefore, the significant effect of drag reduction is obtained. Moreover, the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.

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On-node lattices construction using partial Gauss-Hermite quadrature for the lattice Boltzmann method

叶欢锋, 干则成, 匡波, 杨燕华

2019 (5): 54702-054702. doi: 10.1088/1674-1056/28/5/054702

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A concise theoretical framework, the partial Gauss-Hermite quadrature (pGHQ), is established to construct on-node lattices of the lattice Boltzmann (LB) method under a Cartesian coordinate system. Compared with the existing approaches, the pGHQ scheme has the following advantages:extremely concise algorithm, unifies the constructing procedure for symmetric and asymmetric on-node lattices, and covers a full-range quadrature degree of a given discrete velocity set. We employ the pGHQ scheme to search the local optimal and asymmetric lattices for {n=3,4,5,6,7} moment degree equilibrium distribution discretization on the range [-10,10]. The search reveals a surprising abundance of available lattices. Through a brief analysis, the discrete velocity set shows a significant influence on the positivity of equilibrium distributions, which is considered as one of the major impacts of the numerical stability of the LB method. Hence, the results of the pGHQ scheme lay a foundation for further investigations to improve the numerical stability of the LB method by modifying the discrete velocity set. It is also worth noting that pGHQ can be extended into the entropic LB model, even though it was proposed for the Hermite polynomial expansion LB theory.

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Crystalline order and disorder in dusty plasmas investigated by nonequilibrium molecular dynamics simulations

Aamir Shahzad, Maogang He, Sheeba Ghani, Muhammad Kashif, Tariq Munir, Fang Yang

2019 (5): 55201-055201. doi: 10.1088/1674-1056/28/5/055201

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The particle structure of a complex system has been explored through a unique Evans's homogenous nonequilibrium molecular dynamics (HNEMD) simulation technique. The crystalline order-disorder structures (OD-structures) and the corresponding energies of three-dimensional (3D) nonideal complex systems (NICSs) have been measured over a wide range of plasma states (Γ, κ) for a body-centered cubic (BCC) structure. The projected technique provides accurate OD-structures with fast convergence and applicable to very small size effect for different temperatures (≡1/Γ) and constant force field (F^*) values. The OD-structure obtained through HNEMD approach is found to be reasonable agreement and more reliable than those earlier identified by simulation approaches and experimental data of NICSs. New simulations of OD-structures show that dusty plasma remains in crystalline (strongly coupled) state at lower temperature and constant F^* values, for the whole simulation runs. Our investigations show that the crystalline structure is changed and the particle structure switches from intermediate to disorder (nonideal gaseous) state with an increase of the system's temperature. It has been shown that the long range order shifts toward lower temperature with increasing κ. The presented technique exhibits that the potential energy has a maximum value when the dusty plasma remains in crystalline states (low temperatures), which confirms earlier 3D simulation results.

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Temporal and spatial evolution of air-spark switch plasmainvestigated by the Mach-Zehnder interferometer

黄杰, 杨林, 张宏超, 陈磊, 吴先映

2019 (5): 55202-055202. doi: 10.1088/1674-1056/28/5/055202

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An air-spark switch plasma was diagnosed by the Mach-Zehnder laser interferometer with ultra-high spatial and temporal resolution. The interferograms containing plasma phase shift information at different time were obtained. The phase shift distributions of the plasma were extracted by numerically processing the interferograms. The three-dimensional (3D) electron density distributions of the air-spark switch plasma were then obtained. The working process of the air-spark switch was described by analyzing the temporal and spatial evolution of the plasma electron density.

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Crystal structures and decomposing of B-P compounds under pressure

张蝶, 靳锡联, 庄全, 李颖, 杨淑涵, 宋丽莹, 刘冰冰, 崔田

2019 (5): 56101-056101. doi: 10.1088/1674-1056/28/5/056101

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We have systematically studied the structures, electronic properties, and lattice dynamics of B-P compounds at high pressures. BP and B₆P are found to be thermodynamically stable below 100 GPa, and other stoichiometries are decomposable under pressure. The predicted structures of F-43m BP and R-3m B₆P are in good agreement with the experimental results by comparing the powder diffraction file (PDF) standard cards with our simulated x-ray diffractions. The bonding properties of BP and B₆P have also been analyzed by electronic localization functions, charge density difference, and Bader charge analysis. Our results show that BP and B₆P decompose into B and P under high pressure, which is proven to be dominated by the volumes of them. Furthermore, the infrared and Raman spectra of F-43m and R-3m are investigated at selected pressures and will provide useful information for future experimental studies about B-P compounds.

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Crystallographic and magnetic properties of van der Waals layered FePS₃ crystal

解其云, 吴敏, 陈丽敏, 白刚, 邹文琴, 王伟, 何亮

2019 (5): 56102-056102. doi: 10.1088/1674-1056/28/5/056102

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The crystallographic and magnetic properties are presented for van der Waals antiferromagnetic FePS₃. High-quality single crystals of millimeter size have been successfully synthesized through the chemical vapor transport method. The layered structure and cleavability of the compound are apparent, which are beneficial for a potential exploration of the interesting low dimensional magnetism, as well as for incorporation of FePS₃ into van der Waals heterostructures. For the sake of completeness, we have measured both direct current (dc) and alternating current (ac) magnetic susceptibility. The paramagnetic to antiferromagnetic transition occurs at approximately T_N~115 K. The effective moment is larger than the spin-only effective moment, suggesting that an orbital contribution to the total angular momentum of the Fe²⁺ could be present. The ac susceptibility is independent of frequency, which means that the spin freezing effect is excluded. Strong anisotropy of out-of-plane and in-plane susceptibility has been shown, demonstrating the Ising-type magnetic order in FePS₃ system.

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Low temperature Pmmm and C2/m phases in Sr₂CuO_3+δ high temperature superconductor

王海波, 罗震林, 杨远俊, 刘清青, 胡思侠, 杨蒙蒙, 靳常青, 高琛

2019 (5): 56103-056103. doi: 10.1088/1674-1056/28/5/056103

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A new low temperature Pmmm (120 K) phase was found in high temperature superconductor Sr₂CuO_3+δ, which was indicated as a pure electronic phase by resonant x-ray diffraction at Cu K-edge. As shown by x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) at Cu K-edge, the strong charge density redistribution and local lattice fluctuations around Cu site at the onset of phase transition were due to the occurrence of superconductive coherence, the redistribution and fluctuation finished at T_c. Finally, the electron-lattice interaction was mainly elaborated to understand the superconductivity of Sr₂CuO_3+δ.

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Structural and electrical properties of Ga-Te systems under high pressure

王友春, 田夫波, 李达, 段德芳, 谢慧, 刘冰冰, 周强, 崔田

2019 (5): 56104-056104. doi: 10.1088/1674-1056/28/5/056104

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First-principles evolutionary calculation was performed to search for all probable stable Ga-Te compounds at extreme pressure. In addition to the well-known structures of P6₃/mmc and Fm-3m GaTe and I4/m Ga₂Te₅, several new structures were uncovered at high pressure, namely, orthorhombic I4/mmm GaTe₂ and monoclinic C2/m GaTe₃, and all the Ga-Te structures stabilize up to a maximum pressure of 80 GPa. The calculation of the electronic energy band indicated that the high-pressure phases of the Ga-Te system are metallic, whereas the low-pressure phases are semiconductors. The electronic localization functions (ELFs) of the Ga-Te system were also calculated to explore the bond characteristics. The results showed that a covalent bond is formed at low pressure, however, this bond disappears at high pressure, and an ionic bond is formed at extreme pressure.

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A revised jump-diffusion and rotation-diffusion model

李华, 陈昱沆, 唐宾泽

2019 (5): 56105-056105. doi: 10.1088/1674-1056/28/5/056105

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Quasi-elastic neutron scattering (QENS) has many applications that are directly related to the development of high-performance functional materials and biological macromolecules, especially those containing some water. The analysis method of QENS spectra data is important to obtain parameters that can explain the structure of materials and the dynamics of water. In this paper, we present a revised jump-diffusion and rotation-diffusion model (rJRM) used for QENS spectra data analysis. By the rJRM, the QENS spectra from a pure magnesium-silicate-hydrate (MSH) sample are fitted well for the Q range from 0.3 Å^-1 to 1.9 Å^-1 and temperatures from 210 K up to 280 K. The fitted parameters can be divided into two kinds. The first kind describes the structure of the MSH sample, including the ratio of immobile water (or bound water) C and the confining radius of mobile water a₀. The second kind describes the dynamics of confined water in pores contained in the MSH sample, including the translational diffusion coefficient D_t, the average translational residence time τ₀, the rotational diffusion coefficient D_r, and the mean squared displacement (MSD) <u²>. The rJRM is a new practical method suitable to fit QENS spectra from porous materials, where hydrogen atoms appear in both solid and liquid phases.

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Studying the charge carrier properties in CuInS₂ films via femtosecond transient absorption and nanosecond transient photocurrents

谭铭瑞, 刘庆辉, 隋宁, 康智慧, 张里荃, 张汉壮, 王文全, 周强, 王英惠

2019 (5): 56106-056106. doi: 10.1088/1674-1056/28/5/056106

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The carrier behavior in CuInS₂ thin films at femtosecond and microsecond time scales is discussed in detail. Transient absorption data suggests that the photo-generated carriers relax rapidly accompanied by a change in energy. The photo-generated charge carriers are extracted by a bias electric field E in the nanosecond transient photocurrent system. An applied E improves the efficiency of photon conversion to charge carriers and enhances the velocity of the extracted charge carriers. In addition, there exists a threshold of illumination intensity in the extraction process of charge carriers in the CuInS₂ thin film, above which carrier recombination occurs. The corresponding loss further increases with illumination intensity and the recombination rate is almost independent of E. Our results provide useful insights into the characteristics of carriers in the CuInS₂ thin film and are important for the operation of optoelectronic devices realized with these films.

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Low-temperature growth of large-scale, single-crystalline graphene on Ir(111)

郭辉, 陈辉, 阙炎德, 郑琦, 张余洋, 鲍丽宏, 黄立, 王业亮, 杜世萱, 高鸿钧

2019 (5): 56107-056107. doi: 10.1088/1674-1056/28/5/056107

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Iridium is a promising substrate for self-limiting growth of graphene. However, single-crystalline graphene can only be fabricated over 1120 K. The weak interaction between graphene and Ir makes it challenging to grow graphene with a single orientation at a relatively low temperature. Here, we report the growth of large-scale, single-crystalline graphene on Ir(111) substrate at a temperature as low as 800 K using an oxygen-etching assisted epitaxial growth method. We firstly grow polycrystalline graphene on Ir. The subsequent exposure of oxygen leads to etching of the misaligned domains. Additional growth cycle, in which the leftover aligned domain serves as a nucleation center, results in a large-scale and single-crystalline graphene layer on Ir(111). Low-energy electron diffraction, scanning tunneling microscopy, and Raman spectroscopy experiments confirm the successful growth of large-scale and single-crystalline graphene. In addition, the fabricated single-crystalline graphene is transferred onto a SiO₂/Si substrate. Transport measurements on the transferred graphene show a carrier mobility of about 3300 cm²·V^-1·s^-1. This work provides a way for the synthesis of large-scale, high-quality graphene on weak-coupled metal substrates.

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Raman scattering study of magnetic layered MPS₃ crystals (M=Mn, Fe, Ni)

王艺朦, 张建丰, 李承贺, 马肖莉, 籍建葶, 金峰, 雷和畅, 刘凯, 张玮璐, 张清明

2019 (5): 56301-056301. doi: 10.1088/1674-1056/28/5/056301

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We report a comprehensive Raman scattering study on layered MPS₃ (M=Mn, Fe, Ni), a two-dimensional magnetic compound with weak van der Waals interlayer coupling. The observed Raman phonon modes have been well assigned by the combination of first-principles calculations and the polarization-resolved spectra. Careful symmetry analysis on the angle-dependent spectra demonstrates that the crystal symmetry is strictly described by C_2h but can be simplified to D_3d with good accuracy. Interestingly, the three compounds share exactly the same lattice structure but show distinct magnetic structures. This provides us with a unique opportunity to study the effect of different magnetic orders on lattice dynamics in MPS₃. Our results reveal that the in-plane Néel antiferromagnetic (AF) order in MnPS₃ favors a spin-phonon coupling compared to the in-plane zig-zag AF in NiPS₃ and FePS₃. We have discussed the mechanism in terms of the folding of magnetic Brillouin zones. Our results provide insights into the relation between lattice dynamics and magnetism in the layered MPX₃ (M=transition metal, X=S, Se) family and shed light on the magnetism of monolayer MPX₃ materials.

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Variational and diffusion Monte Carlo simulations of a hydrogen molecular ion in a spherical box

肖学会, 包括, 王友春, 谢慧, 段德芳, 田夫波, 崔田

2019 (5): 56401-056401. doi: 10.1088/1674-1056/28/5/056401

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The variational and diffusion Monte Carlo approaches are used to study the ground-state properties of a hydrogen molecular ion in a spheroidal box. In this work, we successfully treat the zero-point motion of protons in the same formalism with as of electrons and avoid the Born-Oppenheimer approximation in density function theory. The study shows that the total energy increases with the decrease in volume, and that the distance between protons decreases as the pressure increases. Considering the motion of protons, the kinetic energy of the electron is higher than that of the fixed model under the same conditions and increases by 5%. The kinetic energy of the proton is found to be small under high pressure, which is only a fraction of the kinetic energy of the electron.

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Surface stabilized cubic phase of CsPbI₃ and CsPbBr₃ at room temperature

杨凤, 王聪, 潘宇浩, 周谐宇, 孔祥华, 季威

2019 (5): 56402-056402. doi: 10.1088/1674-1056/28/5/056402

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Inorganic halide perovskites CsPbX₃ (X=I, Br) have attracted tremendous attention in solar cell applications. However, the bulk form of the cubic phase CsPbX₃, which offers moderate direct bandgaps, is metastable at room temperature and tends to transform into a tetragonal or orthorhombic phase. Here, our density functional theory calculation results found that the surface energies of the cubic phase are smaller than those of the orthorhombic phase, although the bulk counterpart of the cubic phase is less stable than that of the orthorhombic phase. These results suggest a surface stabilization strategy to maintain the stability of the cubic phase at room temperature that an enlarged portion of surfaces shall change the relative stability of the two phases in nanostructured CsPbX₃. This strategy, which may potentially solve the long-standing stability issue of cubic CsPbX₃, was demonstrated to be feasible by our calculations in zero-, one-, and two-dimensional nanostructures. In particular, confined sizes from few to tens of nanometers could keep the cubic phase as the most thermally favored form at room temperature. Our predicted values in particular cases, such as the zero-dimensional form of CsPbI₃, are highly consistent with experimental values, suggesting that our model is reasonable and our results are reliable. These predicted critical sizes give the upper and lower limits of the confined sizes, which may guide experimentalists to synthesize these nanostructures and promote likely practical applications such as solar cells and flexible displays using CsPbX₃ nanostructures.

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Conductive property of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ with low thermal expansion

何小可, 戚恒, 徐启, 刘献省, 许磊, 袁保合

2019 (5): 56501-056501. doi: 10.1088/1674-1056/28/5/056501

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Low thermal expansion materials are mostly ceramics with low conductive property, which limits their applications in electronic devices. The poor conductive property of ceramic ZrV₂O₇ could be improved by bi-substitution of Fe and Mo for Zr and V, accompanied with low thermal expansion. Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ has electrical conductivity of 8.2×10^-5 S/cm and 9.41×10^-4 S/cm at 291 K and 623 K, respectively. From 291 K to 413 K, thermal excitation leads to the increase of carrier concentration, which causes the rapid decrease of resistance. At 413-533 K, the conductivity is unchanged due to high scattering probability and a slowing increase of carrier concentration. The conductivity rapidly increases again from 533 K to 623 K due to the intrinsic thermal excitation. The thermal expansion coefficient of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ is as low as 0.72×10^-6 K^-1 at 140-700 K from the dilatometer measurement. These properties suggest that Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ has attractive application in electronic components.

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Spatiotemporal Bloch states of a spin-orbit coupled Bose-Einstein condensate in an optical lattice

魏娅雯, 孔超, 海文华

2019 (5): 56701-056701. doi: 10.1088/1674-1056/28/5/056701

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We study the spatiotemporal Bloch states of a high-frequency driven two-component Bose-Einstein condensate (BEC) with spin-orbit coupling (SOC) in an optical lattice. By adopting the rotating-wave approximation (RWA) and applying an exact trial-solution to the corresponding quasistationary system, we establish a different method for tuning SOC via external field such that the existence conditions of the exact particular solutions are fitted. Several novel features related to the exact states are demonstrated; for example, SOC leads to spin-motion entanglement for the spatiotemporal Bloch states, SOC increases the population imbalance of the two-component BEC, and SOC can be applied to manipulate the stable atomic flow which is conducive to control quantum transport of the BEC for different application purposes.

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Epitaxial fabrication of two-dimensional TiTe₂ monolayer on Au(111) substrate with Te as buffer layer

宋志朋, 雷宝, 曹云, 戚竞, 彭浩, 汪琴, 黄立, 路红亮, 林晓, 王业亮, 杜世萱, 高鸿钧

2019 (5): 56801-056801. doi: 10.1088/1674-1056/28/5/056801

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Two-dimensional (2D) materials provide a platform to exploit the novel physical properties of functional nanodevices. Here, we report on the formation of a new 2D layered material, a well-ordered monolayer TiTe₂, on a Au(111) surface by molecular beam epitaxy (MBE). Low-energy electron diffraction (LEED) measurements of the samples indicate that the TiTe₂ film forms (√3×√7) superlattice with respect to the Au(111) substrate, which has three different orientations. Scanning tunneling microscopy (STM) measurements clearly show three ordered domains consistent with the LEED patterns. Density functional theory (DFT) calculations further confirm the formation of 2H-TiTe₂ monolayer on the Au(111) surface with Te as buffer layer. The fabrication of this 2D layered heterostructure expands 2D material database, which may bring new physical properties for future applications.

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Mott transition in ruby lattice Hubbard model

保安

2019 (5): 57101-057101. doi: 10.1088/1674-1056/28/5/057101

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Mott transition in a ruby lattice with fermions described by the Hubbard model including on-site repulsive interaction is investigated by combining the cellular dynamical mean-field theory and the continuous-time quantum Monte Carlo algorithm. The effect of temperature and on-site repulsive interaction on the metallic-insulating phase transition in ruby lattice with fermions is discussed based on the density of states and double occupancy. In addition, the magnetic property of each phase is discussed by defining certain magnetic order parameters. Our results show that the antiferromagnetic metal is found at the low temperature and weak interaction region and the antiferromagnetic insulating phase is found at the low temperature and strong interaction region. The paramagnetic metal appears in whole on-site repulsive interaction region when the temperature is higher than a certain value and the paramagnetic insulator appears at the middle scale of temperature and on-site repulsive interaction.

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Mechanism of free electron concentration saturation phenomenon in Te-GaSb single crystal

余丁, 沈桂英, 谢辉, 刘京明, 孙静, 赵有文

2019 (5): 57102-057102. doi: 10.1088/1674-1056/28/5/057102

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Te-doped GaSb single crystal grown by the liquid encapsulated Czochralski (LEC) method exhibits a lag of compensating progress and a maximum carrier concentration around 8×10¹⁷ cm^-3. The reason for this phenomenon has been investigated by a quantity concentration evaluation of the Te donor and native acceptor. The results of glow discharge mass spectrometry (GDMS) and Hall measurement suggest that the acceptor concentration increases with the increase of Te doping concentration, resulting in the enhancement of electrical compensation and free electron concentration reduction. The acceptor concentration variation is further demonstrated by photoluminescence spectra and explained by the principle of Fermi level dependent defect formation energy.

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Effects of interface bound states on the shot noise in normal metal-low-dimensional Rashba semiconductor tunnel junctions with induced s-wave pairing potential

陈文祥, 王瑞强, 胡梁宾

2019 (5): 57201-057201. doi: 10.1088/1674-1056/28/5/057201

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We consider the effects of interface bound states on the electrical shot noise in tunnel junctions formed between normal metals and one-dimensional (1D) or two-dimensional (2D) Rashba semiconductors with proximity-induced s-wave pairing potential. We investigate how the shot noise properties vary as the interface bound state is evolved from a non-zero energy bound state to a zero-energy bound state. We show that in both 1D and 2D tunnel junctions, the ratio of the noise power to the charge current in the vicinity of zero bias voltage may be enhanced significantly due to the induction of the midgap interface bound state. But as the interface bound state evolves from a non-zero energy bound state to a zero-energy bound state, this ratio tends to vanish completely at zero bias voltage in 1D tunnel junctions, while in 2D tunnel junctions it decreases smoothly to the usual classical Schottky value for the normal state. Some other important aspects of the shot noise properties in such tunnel junctions are also clarified.

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Tunable superconductivity in parent cuprate Pr₂CuO_4±δ thin films

魏鑫健, 何格, 胡卫, 张旭, 秦明阳, 袁洁, 朱北沂, 林媛, 金魁

2019 (5): 57401-057401. doi: 10.1088/1674-1056/28/5/057401

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We studied the role of oxygen in Pr₂CuO_4±δ thin films fabricated by the polymer assisted deposition method. The magnetoresistance and Hall resistivity of Pr₂CuO_4±δ samples were systematically investigated. It was found that with decreasing oxygen content, the low-temperature Hall coefficient (R_H) and magnetoresistance changed from negative to positive, similar to those with the increase of Ce-doped concentration in R_2-xCe_xCuO₄ (R=La, Nd, Pr, Sm, Eu). In addition, we observed that the dependence of the superconducting critical temperature T_c with R_H for the Pr_2-xCe_xCuO₄ perfectly overlapped with that of Pr₂CuO_4±δ. These findings point to the fact that the doped electrons induced by the oxygen removal are responsible for the superconductivity of the T'-phase parent compounds.

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Particle-hole fluctuations and possible superconductivity in doped α-RuCl₃

王斌斌, 王巍, 于顺利, 李建新

2019 (5): 57402-057402. doi: 10.1088/1674-1056/28/5/057402

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We study various particle-hole excitations and possible superconducting pairings mediated by these fluctuations in doped α-RuCl₃ by using multi-band Hubbard model with all t_2g orbitals. By performing a random-phase-approximation (RPA) analysis, we find that among all particle-hole excitations, the j_eff=1/2 pseudospin fluctuations are dominant, suggesting the robustness of j_eff=1/2 picture even in the doped systems. We also find that the most favorable superconducting state has a d-wave pairing symmetry.

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Effects of chemical pressure on diluted magnetic semiconductor (Ba,K)(Zn,Mn)₂As₂

彭毅, 于爽, 赵国强, 李文敏, 赵建发, 曹立朋, 望贤成, 刘清青, 张思佳, 于润泽, 邓正, 朱小红, 靳常青

2019 (5): 57501-057501. doi: 10.1088/1674-1056/28/5/057501

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Chemical pressure induced by iso-valent doping has been widely employed to tune physical properties of materials. In this work, we report effects of chemical pressure by substitution of Sb or P into As on a recently discovered diluted magnetic semiconductor (Ba,K)(Zn,Mn)₂As₂, which has the record of reliable Curie temperature of 230 K due to independent charge and spin doping. Sb and P are substituted into As-site to produce negative and positive chemical pressures, respectively. X-ray diffraction results demonstrate the successful chemical solution of dopants. Magnetic properties of both K-under-doped and K-optimal-doped samples are effectively tuned by Sb- and P-doping. The Hall effect measurements do not show decrease in carrier concentrations upon Sb- and P-doping. Impressively, magnetoresistance is significantly improved from 7% to 27% by only 10% P-doping, successfully extending potential application of (Ba,K)(Zn,Mn)₂As₂.

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Magnetic properties and magnetocaloric effects in (Ho_1-xY_x)₅Pd₂ compounds

武小飞, 郭翠萍, 成钢, 李长荣, 王江, 杜玉松, 饶光辉, 杜振民

2019 (5): 57502-057502. doi: 10.1088/1674-1056/28/5/057502

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The crystal structure, magnetic and magnetocaloric properties of (Ho_1-xY_x)₅Pd₂ (x=0, 0.25, and 0.5) compounds are investigated. All the compounds crystallize in a cubic Dy₅Pd₂-type structure with the space group Fd3m and undergo a second order transition from spin glass (SG) state to paramagnetic (PM) state. The spin glass transition temperatures T_g decrease from 26 K for x=0 to 13 K for x=0.5. In the PM region, the reciprocal susceptibilities for all the compounds obey the Curie-Weiss law. The paramagnetic Curie temperatures (θ_p) for Ho₅Pd₂, (Ho_0.75Y_0.25)₅Pd₂, and (Ho_0.5Y_0.5)₅Pd₂ are determined to be 32 K, 30 K, and 22 K, respectively, and the corresponding effective magnetic moments (μ_eff) are 10.8 μ_B/Ho, 10.3 μ_B/RE, and 7.5 μ_B/RE, respectively. Magnetocaloric effect (MCE) is anticipated according to the Maxwell relation, based on the isothermal magnetization curves. For a magnetic field change of 0-5 T, the maximum values of the isothermal magnetic entropy change -ΔS_M of the (Ho_1-xY_x)₅Pd₂ (x=0, 0.25, and 0.5) compounds are determined to be 11.5 J·kg^-1·K^-1, 11.1 J·kg^-1·K^-1, and 8.9 K J·kg^-1·K^-1, with corresponding refrigerant capacity values of 382.3 J·kg^-1, 336.2 J·kg^-1, and 242.5 J·kg^-1, respectively.

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Enhanced soft magnetic properties of iron powders through coating MnZn ferrite by one-step sol-gel synthesis

刘冬, 高善民, 金仁成, 王峰, 初晓晓, 高太平, 王玉宝

2019 (5): 57503-057503. doi: 10.1088/1674-1056/28/5/057503

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The MnZn ferrite coating formed on the surface of iron-based soft magnetic powders via facile and modified sol-gel process has been fabricated to obtain better magnetic performance due to its higher permeability compared with traditional nonmagnetic insulation coatings. The influence of the MnZn ferrite contents on the magnetic performance of the soft magnetic composites (SMCs) has been studied. As the MnZn insulation content increases, the core loss first experiences a decreasing trend that is followed by progressive increase, while the permeability follows an increasing trend and subsequently degrades. The optimized magnetic performance is achieved with 2.0 wt% MnZn ferrite, which results from the decrement of inter-particle eddy current losses based on loss separation. A uniform and compact coating layer composed of MnZn ferrite and oxides with an average thickness of 0.38 ±0.08 μ is obtained by utilizing ion beam technology, and the interface between the powders and the coating shows satisfied adhesiveness compared with the sample directly prepared by mechanical mixing. The evolution of the coating layers during the calcination process has been presented based on careful analysis of the composition and microstructure.

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Computational study of inverse ferrite spinels

A EL Maazouzi, R Masrour, A Jabar, M Hamedoun

2019 (5): 57504-057504. doi: 10.1088/1674-1056/28/5/057504

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The magnetic properties of inverse ferrite (Fe³⁺) [Fe³⁺Co²⁺]O₄^2-,(Fe³⁺) [Fe³⁺Cu²⁺]O₄^2-,(Fe³⁺) [Fe³⁺Fe²⁺]O₄^2-, and (Fe³⁺) [Fe³⁺Ni²⁺]O₄^2- spinels have been studied using Monte Carlo simulation. We have also calculated the critical and Curie Weiss temperatures from the thermal magnetizations and inverse of magnetic susceptibilities for each system. Magnetic hysteresis cycles have been found for the four systems. Finally, we found the critical exponents associated with magnetization, magnetic susceptibility, and external magnetic field. Our results of critical and Curie Weiss temperatures are similar to those obtained by experiment results. The critical exponents are similar to those of known 3D-Ising model.

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Influence of additives on the magnetic damping constant of CoIr soft magnetic thin films with negative magnetocrystalline anisotropy

马天勇, 罗智, 李志伟, 乔亮, 王涛, 李发伸

2019 (5): 57505-057505. doi: 10.1088/1674-1056/28/5/057505

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The oriented (CoIr)_100-xP_x (P=B, Ni, and SiO₂) soft magnetic films are prepared. Their morphology is measured using transmission electron microscopy (TEM), and reveals that these films exhibit good crystallinity and high degree of the c-axis orientation. The magnetic properties are thoroughly investigated as a function of doping x. Our results show that all of these films possess negative magnetocrystalline anisotropy as required by possible applications. Both the intrinsic and extrinsic contributions are considered to interpret the broadening of the ferromagnetic resonance spectral linewidth. The intrinsic Gilbert damping is identified as the main cause of the linewidth broadening, while the extrinsic part originating from inhomogeneities only plays a minor role. More interestingly, our results show that the damping constant can be controlled by using the doping method.

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Structure instability-induced high dielectric properties in[001]-oriented 0.68Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ crystals

李晓娟, 樊星, 惠增哲, 刘鹏, 龙伟, 方频阳, 南瑞华

2019 (5): 57701-057701. doi: 10.1088/1674-1056/28/5/057701

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The structure evolution and origin of ultrahigh dielectric properties have been investigated in the low temperature range from 300 K to 5 K for[001]-oriented 0.68Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-33PT) crystal. The experimental results reveal that a short-range ordered monoclinic M_A is the dominant phase at ambient temperature. As the temperature drops below 270 K, the M_A transforms into monoclinic M_C, and the M_C remains stable until 5 K. Although no phase transition occurs from 5 K to 245 K, polar nanoregions (PNRs) display visible changes. The instability of PNRs is suggested as responsible for the low temperature relaxation. The ultrahigh dielectric constant at room temperature is associated with the instability of local structure and phase transition. Our research provides an insight into the design of high-performance ferroelectric materials.

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The c-axis complex permittivity and electrical impedance in BaFe₂As₂:Experimental examination on transformation validity

李永强, 杜新哲, 龚冬良, 杨綦睿, 张汶良, 谢涛, 冯波, 陈恺, 罗会仟, 刘俊明, 朱劲松

2019 (5): 57702-057702. doi: 10.1088/1674-1056/28/5/057702

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Complex permittivity and electrical impedance have been measured along the c-axis in single crystals BaFe₂As₂, which are the conductors known as the parent compound of 122-type iron superconductor. The resultant relative errors defined in the study indicate the existence of the transformation between complex permittivity and electrical impedance in the conductors, and these two physics quantities possibly reveal different aspects of the consistent superconductivity-relevant physics picture.

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Tuning the intensity statistics of random speckle patterns

孟凡, 赵乐, 张云佐, 霍磊

2019 (5): 57801-057801. doi: 10.1088/1674-1056/28/5/057801

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Speckle patterns are a fundamental tool in a variety of physical and optical applications. Here, we investigate a method of precisely tuning the intensity statistics of random speckle patterns into a desirable pattern that possesses the same spatial correlation length and similar statistics distribution. This tuning mechanism relies on the derivation of the transform function and transmission matrix, which achieves different contrasts while maintaining the same average value or energy level. The statistics properties of the generated speckle patterns are further investigated by analyzing the standard deviation under different fitting parameters. Precisely tuning the intensity statistics of random speckle patterns could be useful for both fundamental research and practical applications, such as microscopy, imaging, and optical manipulation.

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Temperature and excitation dependence of stimulated emission and spontaneous emission in InGaN epilayer

安雪娥, 商正君, 马传贺, 郑新和, 张翠玲, 孙琳, 越方禹, 李波, 陈晔

2019 (5): 57802-057802. doi: 10.1088/1674-1056/28/5/057802

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Temperature and excitation dependent photoluminescence (PL) of InGaN epilayer grown on c-plane GaN/sapphire template by molecular beam epitaxy (MBE) has been systematically investigated. The emission spectra of the sample consisted of strong multiple peaks associated with one stimulated emission (SE) located at 430 nm and two spontaneous emissions (SPE) centered at about 450 nm and 480 nm, indicating the co-existence of shallow and deep localized states. The peak energy of SE exhibiting weak s-shaped variation with increasing temperature revealed the localization effect of excitons. Moreover, an abnormal increase of the SPE intensity with increasing temperature was also observed, which indicated that the carrier transfer between the shallow and deeper localized states exists. Temperature dependent time-resolved PL (TRPL) demonstrated the carrier transfer processes among the localized states. In addition, a slow thermalization of hot carriers was observed in InGaN film by using TRPL and transient differential reflectivity, which is attributed to the phonon bottleneck effect induced by indium aggregation.

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Large-scale control of enhancement and quenching of photoluminescence for ZnSe/ZnS quantum dots and Ag nanoparticles in aqueous solution

殷少轶, 廖李明, 罗松, 张喆, 张晓宇, 鹿建, 陈张海

2019 (5): 57803-057803. doi: 10.1088/1674-1056/28/5/057803

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We investigated the optical properties of hybrid exciton-plasmon coupling ensembles composed of ZnSe/ZnS quantum dots and Ag nanoparticles in aqueous solution. We modulated their average interval by changing the ratio of quantum dots and Ag nanoparticles. The transition from dramatic PL enhancement to PL quenching state was experimentally observed, according to the continuous decrease of the PL lifetime. The PL enhancement rate exceeded 10, with the Purcell factor of 3.5. Meanwhile, the proportion of fast decay increased from 0.3 to 0.6, corresponding to the proportion of slow decay decreased from 0.7 to 0.4. Our experiment is important for the hybrid exciton-plasmon coupling system to be practicable in optoelectronic application.

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Two-step growth of VSe₂ films and their photoelectric properties

曾玉, 张生利, 李秀玲, 敖建平, 孙云, 刘玮, 刘芳芳, 高鹏, 张毅

2019 (5): 58101-058101. doi: 10.1088/1674-1056/28/5/058101

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We put forward a two-step route to synthesize vanadium diselenide (VSe₂), a typical transition metal dichalcogenide (TMD). To obtain the VSe₂ film, we first prepare a vanadium film by electron beam evaporation and we then perform selenization in a vacuum chamber. This method has the advantages of low temperature, is less time-consuming, has a large area, and has a stable performance. At 400 ^circC selenization temperature, we successfully prepare VSe₂ films on both glass and Mo substrates. The prepared VSe₂ has the characteristic of preferential growth along the c-axis, with low transmittance. It is found that the contact between Al and VSe₂/Mo is ohmic contact. Compared to Mo substrate, lower square resistance and higher carrier concentration of the VSe₂/Mo sample reveal that the VSe₂ film may be a potential material for thin film solar cells or other semiconductor devices. The new synthetic strategy that is developed here paves a sustainable way to the application of VSe₂ in photovoltaic devices.

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Equivalent electromagnetic parameters for microwave metamaterial absorber using a new symmetry model

张峻铭, 何东霖, 王国武, 王鹏, 乔亮, 王涛, 李发伸

2019 (5): 58401-058401. doi: 10.1088/1674-1056/28/5/058401

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Transmission line theory uses the complex nature of permeability and permittivity of a conventional magnetic absorber to evaluate its absorption properties and mechanism. However, because there is no method to obtain the electromagnetic parameters of a metamaterial-absorber integrated layer (composed of a medium layer and a periodic metal array), this theory is seldom used to study the absorption properties of the metamaterial absorber. We propose a symmetry model to achieve an equivalent complex permittivity and permeability model for the integrated layer, which can be combined with the transmission line theory to calculate metamaterial absorption properties. The calculation results derived from both the transmission line theory and the high-frequency structure simulator are in good agreement. This method will be beneficial in practical investigations of the absorption mechanism of a metamaterial absorber.

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Responsivity and noise characteristics of AlGaN/GaN-HEMT terahertz detectors at elevated temperatures

田志锋, 徐鹏, 余耀, 孙建东, 冯伟, 丁青峰, 孟占伟, 李想, 蔡金华, 郑中信, 李欣幸, 靳琳, 秦华, 孙云飞

2019 (5): 58501-058501. doi: 10.1088/1674-1056/28/5/058501

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The responsivity and the noise of a detector determine the sensitivity. Thermal energy usually affects both the responsivity and the noise spectral density. In this work, the noise characteristics and responsivity of an antenna-coupled AlGaN/GaN high-electron-mobility-transistor (HEMT) terahertz detector are evaluated at temperatures elevated from 300 K to 473 K. Noise spectrum measurement and a simultaneous measurement of the source-drain conductance and the terahertz photocurrent allow for detailed analysis of the electrical characteristics, the photoresponse, and the noise behavior. The responsivity is reduced from 59 mA/W to 11 mA/W by increasing the detector temperature from 300 K to 473 K. However, the noise spectral density maintains rather constantly around 1-2 pA/Hz^1/2 at temperatures below 448 K, above which the noise spectrum abruptly shifts from Johnson-noise type into flicker-noise type and the noise density is increased up to one order of magnitude. The noise-equivalent power (NEP) is increased from 22 pW/Hz^1/2 at 300 K to 60 pW/Hz^1/2 at 448 K mainly due to the reduction in mobility. Above 448 K, the NEP is increased up to 1000 pW/Hz^1/2 due to the strongly enhanced noise. The sensitivity can be recovered by cooling the detector back to room temperature.

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Enhanced performance of AlGaN-based ultraviolet light-emitting diodes with linearly graded AlGaN inserting layer in electron blocking layer

李光, 王林媛, 宋伟东, 姜健, 罗幸君, 郭佳琦, 贺龙飞, 张康, 吴启保, 李述体

2019 (5): 58502-058502. doi: 10.1088/1674-1056/28/5/058502

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The conventional stationary Al content AlGaN electron blocking layer (EBL) in ultraviolet light-emitting diode (UV LED) is optimized by employing a linearly graded AlGaN inserting layer which is 2.0 nm Al_0.3Ga_0.7N/5.0 nm Al_xGa_1-xN/8.0 nm Al_0.3Ga_0.7N with decreasing value of x. The results indicate that the internal quantum efficiency is significantly improved and the efficiency droop is mitigated by using the proposed structure. These improvements are attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes, which result in an increased hole injection efficiency and a decreased electron leakage into the p-type region. In addition, the linearly graded AlGaN inserting layer can generate more holes in EBL due to the polarization-induced hole doping and a tunneling effect probably occurs to enhance the hole transportation to the active regions, which will be beneficial to the radiative recombination.

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Modeling electric field of power metal-oxide-semiconductor field-effect transistor with dielectric trench based on Schwarz-Christoffel transformation

汪志刚, 廖涛, 王亚南

2019 (5): 58503-058503. doi: 10.1088/1674-1056/28/5/058503

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A power metal-oxide-semiconductor field-effect transistor (MOSFET) with dielectric trench is investigated to enhance the reversed blocking capability. The dielectric trench with a low permittivity to reduce the electric field at reversed blocking state has been studied. To analyze the electric field, the drift region is segmented into four regions, where the conformal mapping method based on Schwarz-Christoffel transformation has been applied. According to the analysis, the improvement in the electric field for using the low permittivity trench is mainly due to the two electric field peaks generated in the drift region around this dielectric trench. The analytical results of the electric field and the potential models are in good agreement with the simulation results.

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Photoinduced phase transitions in two-dimensional charge-density-wave 1T-TaS₂

文雯, 党春鹤, 谢黎明

2019 (5): 58504-058504. doi: 10.1088/1674-1056/28/5/058504

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Charge-density-wave (CDW) materials with strongly correlated electrons have broadband light absorption and ultrafast response to light irradiation, and hence hold great potential in photodetection. 1T-TaS₂ is a typical CDW material with various thermodynamically CDW ground states at different temperatures and fertile out-of-equilibrium intermediate/hidden states. In particular, the light pulses can trigger melting of CDW ordering and also forms hidden states, which exhibits strikingly different electrical conductivity compared to the ground phase. Here, we review the recent research on phase transitions in 1T-TaS₂ and their potential applications in photodetection. We also discuss the ultrafast melting of CDW ordering by ultrafast laser irradiation and the out-of-equilibrium intermediate/hidden states by optical/electrical pulse. For photodetection, demonstrations of photoconductors and bolometers are introduced. Finally, we discuss some of the challenges that remain.

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