#### Current Issue

Volume 37 Issue 8
Online Date:
GENERAL
 Select Search for 2D Ferromagnets: Molecular Beam Epitaxy is a Critical Tool Matthias Batzill Chin. Phys. Lett. 2020, 37 (8): 080101 .   DOI: 10.1088/0256-307X/37/8/080101 Abstract   PDF(pc) (277KB) ( 359 ) PDF(mobile)(263KB) ( 18 ) HTML ( 121 )
 Select Meter-Level Optical Delay Line on a Low-Loss Lithium Niobate Nanophotonics Chip Shining Zhu Chin. Phys. Lett. 2020, 37 (8): 080102 .   DOI: 10.1088/0256-307X/37/8/080102 Abstract   PDF(pc) (284KB) ( 271 ) PDF(mobile)(270KB) ( 6 ) HTML ( 87 )
 Select Strong Anisotropy of 3D Diffusion in Living Cells Xiaosong Chen Chin. Phys. Lett. 2020, 37 (8): 080103 .   DOI: 10.1088/0256-307X/37/8/080103 Abstract   PDF(pc) (269KB) ( 122 ) PDF(mobile)(264KB) ( 4 ) HTML ( 59 )
 Select Direct Strong Measurement of a High-Dimensional Quantum State Chen-Rui Zhang, Meng-Jun Hu, Guo-Yong Xiang, Yong-Sheng Zhang, Chuan-Feng Li, and Guang-Can Guo Chin. Phys. Lett. 2020, 37 (8): 080301 .   DOI: 10.1088/0256-307X/37/8/080301 Abstract   PDF(pc) (3985KB) ( 64 ) PDF(mobile)(3983KB) ( 6 ) HTML ( 68 )  It is of great importance to determine an unknown quantum state for fundamental studies of quantum mechanics, yet it is still difficult to characterize systems of large dimensions in practice. Although the scan-free direct measurement approach based on a weak measurement scheme was proposed to measure a high-dimensional photonic state, how weak the interaction should be to give a correct estimation remains unclear. Here we propose and experimentally demonstrate a technique that measures a high-dimensional quantum state with the combination of scan-free measurement and direct strong measurement. The procedure involves sequential strong measurement, in which case no approximation is made similarly to the conventional direct weak measurement. We use this method to measure a transverse state of a photon with effective dimensionality of $65000$ without the time-consumed scanning process. Furthermore, the high fidelity of the result and the simplicity of the experimental apparatus show that our approach can be readily used to measure the complex field of a beam in diverse applications such as wavefront sensing and quantitative phase imaging.
 Select Machine Learning for Many-Body Localization Transition Wen-Jia Rao Chin. Phys. Lett. 2020, 37 (8): 080501 .   DOI: 10.1088/0256-307X/37/8/080501 Abstract   PDF(pc) (770KB) ( 90 ) PDF(mobile)(752KB) ( 9 ) HTML ( 40 )  We employ the methods of machine learning to study the many-body localization (MBL) transition in a 1D random spin system. By using the raw energy spectrum without pre-processing as training data, it is shown that the MBL transition point is correctly predicted by the machine. The structure of the neural network reveals the nature of this dynamical phase transition that involves all energy levels, while the bandwidth of the spectrum and nearest level spacing are the two dominant patterns and the latter stands out to classify phases. We further use a comparative unsupervised learning method, i.e., principal component analysis, to confirm these results.
 Select Negative Thermal Transport in Conduction and Advection Liujun Xu and Jiping Huang Chin. Phys. Lett. 2020, 37 (8): 080502 .   DOI: 10.1088/0256-307X/37/8/080502 Abstract   PDF(pc) (1064KB) ( 423 ) PDF(mobile)(1067KB) ( 45 ) HTML ( 423 )  Negative refractive index has drawn a great deal of attention due to its unique properties and practical applications in wave systems. To promote the related physics in thermotics, here we manage to coin a complex thermal conductivity whose imaginary part corresponds to the real part of complex refractive index. Therefore, the thermal counterpart of negative refractive index is just negative imaginary thermal conductivity, which is featured by the opposite directions of energy flow and wave vector in thermal conduction and advection, thus called negative thermal transport herein. To avoid violating causality, we design an open system with energy exchange and explore three different cases to reveal negative thermal transport. We further provide experimental suggestions with a solid ring structure. All finite-element simulations agree with theoretical analyses, indicating that negative thermal transport is physically feasible. These results have potential applications such as designing the inverse Doppler effect in thermal conduction and advection.
 Select Pressure Generation above 35 GPa in a Walker-Type Large-Volume Press Yu-Chen Shang, Fang-Ren Shen, Xu-Yuan Hou, Lu-Yao Chen, Kuo Hu, Xin Li, Ran Liu, Qiang Tao, Pin-Wen Zhu, Zhao-Dong Liu, Ming-Guang Yao, Qiang Zhou, Tian Cui, and Bing-Bing Liu Chin. Phys. Lett. 2020, 37 (8): 080701 .   DOI: 10.1088/0256-307X/37/8/080701 Abstract   PDF(pc) (3565KB) ( 263 ) PDF(mobile)(3556KB) ( 31 ) HTML ( 191 )  Pressure generation to a higher pressure range in a large-volume press (LVP) denotes our ability to explore more functional materials and deeper Earth's interior. Pressure generated by normal tungsten carbide (WC) anvils in a commercial way is mostly limited to 25 GPa in LVPs due to the limitation of their hardness and design of cell assemblies. We adopt three newly developed WC anvils for ultrahigh pressure generation in a Walker-type LVP with a maximum press load of 1000 ton. The hardest ZK01F WC anvils exhibit the highest efficiency of pressure generation than ZK10F and ZK20F WC anvils, which is related to their performances of plastic deformations. Pressure up to 35 GPa at room temperature is achieved at a relatively low press load of 4.5 MN by adopting the hardest ZK01F WC anvils with three tapering surfaces in conjunction with an optimized cell assembly, while pressure above 35 GPa at 1700 K is achieved at a higher press load of 7.5 MN. Temperature above 2000 K can be generated by our cell assemblies at pressure below 30 GPa. We adopt such high-pressure and high-temperature techniques to fabricate several high-quality and well-sintered polycrystalline minerals for practical use. The present development of high-pressure techniques expands the pressure and temperature ranges in Walker-type LVPs and has wide applications in physics, materials, chemistry, and Earth science.
 Select Characterization of Scanning SQUID Probes Based on 3D Nano-Bridge Junctions in Magnetic Field Yin-Ping Pan, Yue Wang, Ruo-Ting Yang, Yan Tang, Xiao-Yu Liu, Hua Jin, Lin-Xian Ma, Yi-Shi Lin, Zhen Wang, Jie Ren, Yi-Hua Wang, and Lei Chen Chin. Phys. Lett. 2020, 37 (8): 080702 .   DOI: 10.1088/0256-307X/37/8/080702 Abstract   PDF(pc) (803KB) ( 40 ) PDF(mobile)(798KB) ( 2 ) HTML ( 29 )  We develop superconducting quantum interference device (SQUID) probes based on 3D nano-bridge junctions for the scanning SQUID microscopy. The use of these nano-bridge junctions enables imaging in the presence of a high magnetic field. Conventionally, a superconducting ground layer has been employed for better magnetic shielding. In our study, we prepare a number of scanning SQUID probes with and without a ground layer to evaluate their performance in external magnetic fields. The devices show the improved magnetic modulation up to 1.4 T. It is found that the ground layer reduces the inductance, and increases the modulation depth and symmetricity of the gradiometer design in the absence of the field. However, the layer is not compatible with the use of the scanning SQUID probe in the field because it decreases its working field range. Moreover, by adding the layer, the mutual inductance between the feedback coil and the SQUID also decreases linearly as a function of the field.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
 Select $\mathcal{PT}$ Symmetry of a Square-Wave Modulated Two-Level System Liwei Duan, Yan-Zhi Wang, and Qing-Hu Chen Chin. Phys. Lett. 2020, 37 (8): 081101 .   DOI: 10.1088/0256-307X/37/8/081101 Abstract   PDF(pc) (787KB) ( 84 ) PDF(mobile)(770KB) ( 6 ) HTML ( 99 )  We study a non-Hermitian two-level system with square-wave modulated dissipation and coupling. Based on the Floquet theory, we achieve an effective Hamiltonian from which the boundaries of the $\mathcal{PT}$ phase diagram are captured exactly. Two kinds of $\mathcal{PT}$ symmetry broken phases are found, whose effective Hamiltonians differ by a constant $\omega / 2$. For the time-periodic dissipation, a vanishingly small dissipation strength can lead to the $\mathcal{PT}$ symmetry breaking in the $(2k-1)$-photon resonance ($\varDelta = (2k-1) \omega$), with $k=1,2,3\dots$ It is worth noting that such a phenomenon can also happen in $2k$-photon resonance ($\varDelta = 2k \omega$), as long as the dissipation strengths or the driving times are imbalanced, namely $\gamma_0 \ne - \gamma_1$ or $T_0 \ne T_1$. For the time-periodic coupling, the weak dissipation induced $\mathcal{PT}$ symmetry breaking occurs at $\varDelta_{\rm eff}=k\omega$, where $\varDelta_{\rm eff}=(\varDelta_0 T_0 + \varDelta_1 T_1)/T$. In the high frequency limit, the phase boundary is given by a simple relation $\gamma_{\rm eff}=\pm\varDelta_{\rm eff}$.
FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)
 Select Electro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical Etching Jun-xia Zhou, Ren-hong Gao, Jintian Lin, Min Wang, Wei Chu, Wen-bo Li, Di-feng Yin, Li Deng, Zhi-wei Fang, Jian-hao Zhang, Rong-bo Wuand Ya Cheng Chin. Phys. Lett. 2020, 37 (8): 084201 .   DOI: 10.1088/0256-307X/37/8/084201 Abstract   PDF(pc) (1221KB) ( 941 ) PDF(mobile)(1224KB) ( 73 ) HTML ( 1209 )  Optical true delay lines (OTDLs) of low propagation losses, small footprints and high tuning speeds and efficiencies are of critical importance for various photonic applications. Here, we report fabrication of electro-optically switchable OTDLs on lithium niobate on insulator using photolithography assisted chemo-mechanical etching. Our device consists of several low-loss optical waveguides of different lengths which are consecutively connected by electro-optical switches to generate different amounts of time delay. The fabricated OTLDs show an ultra-low propagation loss of $\sim 0.03$ dB/cm for waveguide lengths well above 100 cm.
 Select Multi-Core Conformal Lenses Xinghong Zhu, Pengfei Zhao, and Huanyang Chen Chin. Phys. Lett. 2020, 37 (8): 084202 .   DOI: 10.1088/0256-307X/37/8/084202 Abstract   PDF(pc) (1320KB) ( 71 ) PDF(mobile)(1310KB) ( 3 ) HTML ( 55 )  We propose multi-core conformal lenses by combining conformal transformation optics with absolute instruments. Depending on the cores and incident angles, the conformal lenses have tunable functionalities like focusing, reflection, and transparency, thereby providing a feasible general method for designing multi-functional devices.
 Select Photon Coalescence in a Lossy Non-Hermitian Beam Splitter Zhiqiang Ren , Rong Wen , and J. F. Chen Chin. Phys. Lett. 2020, 37 (8): 084203 .   DOI: 10.1088/0256-307X/37/8/084203 Abstract   PDF(pc) (530KB) ( 51 ) PDF(mobile)(525KB) ( 4 ) HTML ( 31 )  We investigate photon coalescence in a lossy non-Hermitian system and study a dynamic device modeled by a beam splitter with an extra intrinsic phase term added in the transformation matrix, with which the device is a lossy non-Hermitian linear system. The two-photon interference behavior is altered accordingly since this extra intrinsic phase affects the unitary of transformation and the coalescence of the incoming photons. We calculate the coincidence between two single-photon pulses, considering the interferometric phase between two pulses and the extra intrinsic phase as the tunable parameters. The extra phase turns the famous Hong–Ou–Mandel dip into a bump, with the visibility dependent on both the interferometric phase and the extra phase.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
 Select Temperature Gradient, Toroidal and Ion FLR Effects on Drift-Tearing Modes Hao Shi, Wenlu Zhang, Chao Dong, Jian Bao, Zhihong Lin, Jintao Cao, and Ding Li Chin. Phys. Lett. 2020, 37 (8): 085201 .   DOI: 10.1088/0256-307X/37/8/085201 Abstract   PDF(pc) (1601KB) ( 42 ) PDF(mobile)(1600KB) ( 1 ) HTML ( 24 )  The influences of the temperature gradient and toroidal effects on drift-tearing modes have been studied using the Gyrokinetic Toroidal code. After the thermal force term is introduced into the parallel electron force balance equation, the equilibrium temperature gradient can cause a significant increase in the growth rate of the drift-tearing mode and a broadening of the mode structure. The simulation results show that the toroidal effects increase the growth rate of the drift-tearing mode, and the contours of the perturbation field “squeeze” toward the stronger field side in the poloidal section. Finally, the hybrid model for fluid electrons and kinetic ions has been studied briefly, and the dispersion relation of the drift-tearing mode under the influence of ion finite Larmor radius effects is obtained. Compared with the dispersion relation under the fluid model, a stabilizing effect of the ion finite Larmor radius is observed.
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES
 Select Coupling between Particle Shape and Long-Range Interaction in the High-Density Regime Can-can Zhou, Hongchuan Shen, Hua Tong, Ning Xu, and Peng Tan Chin. Phys. Lett. 2020, 37 (8): 086301 .   DOI: 10.1088/0256-307X/37/8/086301 Abstract   PDF(pc) (1414KB) ( 97 ) PDF(mobile)(1404KB) ( 6 ) HTML ( 96 )  We experimentally probe the coupling between particle shape and long-range interaction, using long-range interacting polygons. For two typical space-filling polygons, square and triangle, we find two types of coupling modes that predominantly control the structure formation. Specifically, the rotational ordering of squares brings a lattice deformation that produces a hexagonal-to-rhombic transition in the high density regime, whereas the alignment of triangles introduces a large geometric frustration that causes an order-to-disorder transition. Moreover, the two coupling modes lead to small and large “internal roughness” of the two systems, and thus predominantly control their structure relaxations. Our study thus provides a physical picture to the coupling between long-range interaction effect and short-range shape effect in the high-density regime unexplored before.
 Select A New Cu-Based Metallic Glass Composite with Excellent Mechanical Properties Dong-Mei Li, Lan-Sheng Chen, Peng Yu, Ding Ding, and Lei Xia Chin. Phys. Lett. 2020, 37 (8): 086401 .   DOI: 10.1088/0256-307X/37/8/086401 Abstract   PDF(pc) (1441KB) ( 93 ) PDF(mobile)(1434KB) ( 3 ) HTML ( 54 )  A new Cu-based bulk metallic glass composite of nominal composition (at.%) Cu$_{41}$Ni$_{27}$Ti$_{25}$Al$_{7}$ with excellent plasticity and a strong work-hardening behavior is fabricated. Strength above 1859 MPa and plasticity more than 11% are achieved under compression and tension modes. The deformation mechanism is proposed to the structural heterogeneities of the composite that promotes multiple shear bands meanwhile inhibits their free propagation, which results in the macroscopically plastic strain and work hardening. The alloy contains relatively cheap metals and has a low cost, which is beneficial to industrial applications.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
 Select de Haas–van Alphen Quantum Oscillations in BaSn$_{3}$ Superconductor with Multiple Dirac Fermions Gaoning Zhang, Xianbiao Shi, Xiaolei Liu, Wei Xia, Hao Su, Leiming Chen, Xia Wang, Na Yu, Zhiqiang Zou, Weiwei Zhao, and Yanfeng Guo Chin. Phys. Lett. 2020, 37 (8): 087101 .   DOI: 10.1088/0256-307X/37/8/087101 Abstract   PDF(pc) (2436KB) ( 122 ) PDF(mobile)(2419KB) ( 6 ) HTML ( 29 )  Characterization of Fermi surface of the BaSn$_{3}$ superconductor ($T_{\rm c} \sim 4.4$ K) by de Haas–van Alphen (dHvA) effect measurement reveals its non-trivial topological properties. Analysis of non-zero Berry phase is supported by the ab initio calculations, which reveals a type-II Dirac point setting and tilting along the high symmetric $K$–$H$ line of the Brillouin zone, about 0.13 eV above the Fermi level, and other two type-I Dirac points on the high symmetric $\varGamma$–$A$ direction, but slightly far below the Fermi level. The results demonstrate BaSn$_{3}$ as an excellent example hosting multiple Dirac fermions and an outstanding platform for studying the interplay between nontrivial topological states and superconductivity.
 Select Experimental Observation of Electronic Structures of Kagome Metal YCr$_{6}$Ge$_{6}$ Pengdong Wang, Yihao Wang, Bo Zhang, Yuliang Li, Sheng Wang, Yunbo Wu, Hongen Zhu, Yi Liu, Guobin Zhang, Dayong Liu, Yimin Xiong, and Zhe Sun Chin. Phys. Lett. 2020, 37 (8): 087102 .   DOI: 10.1088/0256-307X/37/8/087102 Abstract   PDF(pc) (1217KB) ( 129 ) PDF(mobile)(1203KB) ( 2 ) HTML ( 48 )  Using angle-resolved photoemission spectroscopy, we study electronic structures of a Kagome metal YCr$_{6}$Ge$_{6}$. Band dispersions along $k_{z}$ direction are significant, suggesting a remarkable interlayer coupling between neighboring Kagome planes. Comparing ARPES data with first-principles calculations, we find a moderate electron correlation in this material, since band calculations must be compressed in the energy scale to reach an excellent agreement between experimental data and theoretical calculations. Moreover, as indicated by band calculations, there is a flat band in the vicinity of the Fermi level at the $\varGamma$–$M$–$K$ plane in the momentum space, which could be responsible for the unusual transport behavior in YCr$_{6}$Ge$_{6}$.
 Select Type-II Dirac Semimetal State in a Superconductor Tantalum Carbide Zhihai Cui, Yuting Qian, Wei Zhang, Hongming Weng, and Zhong Fang Chin. Phys. Lett. 2020, 37 (8): 087103 .   DOI: 10.1088/0256-307X/37/8/087103 Abstract   PDF(pc) (1021KB) ( 65 ) PDF(mobile)(1006KB) ( 4 ) HTML ( 34 )  The exploration of topological Dirac semimetals with intrinsic superconductivity can be a most plausible way to discover topological superconductors. We propose that type-II Dirac semimetal states exist in the band structure of TaC, a well-known s-wave superconductor, by using the first-principles calculations and the ${\boldsymbol{k} \cdot {\boldsymbol p}}$ effective model. The tilted gapless Dirac cones, which are composed of Ta $d$ and C $p$ orbitals and are protected by $C_{4v}$ symmetry, are found to be below the Fermi level. The bands from Ta $d$ orbitals are greatly coupled with the acoustic modes around the zone boundary, indicating their significant contribution to the superconductivity. The relatively high transition temperature $\sim$10.5 K is estimated to be consistent with the experimental data. To bring the type-II Dirac points close to chemical potential, hole doping is needed. This seems to decrease the transition temperature a lot, making the realization of topological superconductivity impossible.
 Select Predicting the Potential Performance in P-Type SnS Crystals via Utilizing the Weighted Mobility and Quality Factor Wenke He , Bingchao Qin , and Li-Dong Zhao Chin. Phys. Lett. 2020, 37 (8): 087104 .   DOI: 10.1088/0256-307X/37/8/087104 Abstract   PDF(pc) (774KB) ( 468 ) PDF(mobile)(772KB) ( 52 ) HTML ( 458 )  The figure of merit $ZT$ is the direct embodiment of thermoelectric performance for a given material. However, as an indicator of performance improvement, the only $ZT$ value is not good enough to identify its outstanding inherent properties, which are highly sought in thermoelectric community. Here, we utilize one powerful parameter to reveal the outstanding properties of a given material. The weighted mobility is used to estimate the carrier transports of p-type SnS crystals, including the differences in doping level, carrier scattering and electronic band structure. We analyze the difference in carrier scattering mechanism for different crystal forms with the same doping level, then evaluate and confirm the temperature-dependent evolution of electronic band structures in SnS. Finally, we calculate the quality factor $B$ based on the weighted mobility, and establish the relationship between $ZT$ and $B$ to further predict the potential performance in p-type SnS crystals with low cost and earth abundance, which can be realized through taking advantage of the inherent material property, thus improving $B$ factor to achieve optimal thermoelectric level.
 Select Symmetry-Assisted Protection and Compensation of Hidden Spin Polarization in Centrosymmetric Systems Yingjie Zhang, Pengfei Liu, Hongyi Sun, Shixuan Zhao, Hu Xu, and Qihang Liu Chin. Phys. Lett. 2020, 37 (8): 087105 .   DOI: 10.1088/0256-307X/37/8/087105 Abstract   PDF(pc) (3117KB) ( 491 ) PDF(mobile)(4499KB) ( 156 ) HTML ( 415 )  It was recently noted that in certain nonmagnetic centrosymmetric compounds, spin–orbit interactions couple each local sector that lacks inversion symmetry, leading to visible spin polarization effects in the real space, dubbed “hidden spin polarization (HSP)”. However, observable spin polarization of a given local sector suffers interference from its inversion partner, impeding material realization and potential applications of HSP. Starting from a single-orbital tight-binding model, we propose a nontrivial way to obtain strong sector-projected spin texture through the vanishing hybridization between inversion partners protected by nonsymmorphic symmetry. The HSP effect is generally compensated by inversion partners near the ${\varGamma}$ point but immune from the hopping effect around the boundary of the Brillouin zone. We further summarize 17 layer groups that support such symmetry-assisted HSP and identify hundreds of quasi-2D materials from the existing databases by first-principle calculations, among which a group of rare-earth compounds LnIO (Ln = Pr, Nd, Ho, Tm, and Lu) serves as great candidates showing strong Rashba- and Dresselhaus-type HSP. Our findings expand the material pool for potential spintronic applications and shed light on controlling HSP properties for emergent quantum phenomena.
 Select Structure-Spin-Transport Anomaly in Quasi-One-Dimensional Ba$_{9}$Fe$_{3}$Te$_{15}$ under High Pressure Jun Zhang, Mei-Ling Jin, Xiang Li, Xian-Cheng Wang, Jian-Fa Zhao, Ying Liu, Lei Duan, Wen-Min Li, Li-Peng Cao, Bi-Juan Chen, Li-Juan Wang, Fei Sun, Yong-Gang Wang, Liu-Xiang Yang, Yu-Ming Xiao, Zheng Deng, Shao-Min Feng, Chang-Qing Jin, and Jin-Long Zhu Chin. Phys. Lett. 2020, 37 (8): 087106 .   DOI: 10.1088/0256-307X/37/8/087106 Abstract   PDF(pc) (2065KB) ( 305 ) PDF(mobile)(2160KB) ( 14 ) HTML ( 166 )  Recently, a series of novel compounds Ba$_{3}$MX$_{5}$ (M =  Fe, Ti, V; X = Se, Te) with hexagonal crystal structures composed of quasi-1-dimensional (1D) magnetic chains has been synthesized by our research team using high-pressure and high-temperature methods. The initial hexagonal phases persist to the maximum achievable pressure, while spin configurations and magnetic interactions may change dramatically as a result of considerable reductions in interchain separations upon pressurization. These compounds therefore offer unique possibilities for studying the evolution of intrinsic electronic structures in quasi-1D magnetic systems. Here we present a systematic investigation of Ba$_{9}$Fe$_{3}$Te$_{15}$, in which the interchain separations between trimerized 1D chains ($\sim$10.2 Å) can be effectively modulated by external high pressure. The crystal structure especially along the 1D chains exhibits an abnormal expansion at $\sim$5 GPa, which accompanies trimerization entangled anomalous mixed-high-low spin transition. An insulator-metal transition has been observed under high pressure as a result of charge-transfer gap closing. Pressure-induced superconductivity emerges at 26 GPa, where the charge-transfer gap fully closes, 3D electronic configuration forms and local spin fully collapses.
 Select Magnetization Reversal of Single-Molecular Magnets by a Spin-Polarized Current Chao Yang, Zheng-Chuan Wang, and Gang Su Chin. Phys. Lett. 2020, 37 (8): 087201 .   DOI: 10.1088/0256-307X/37/8/087201 Abstract   PDF(pc) (775KB) ( 37 ) PDF(mobile)(770KB) ( 2 ) HTML ( 37 )  We study the magnetization reversal of single-molecular magnets by a spin-polarized current in the framework of the spinor Boltzmann equation. Because of the spin–orbit coupling, the spin-polarized current will impose a non-zero spin transfer torque on the single-molecular magnets, which will induce the magnetization switching of the latter. Via the $s$–$d$ exchange interaction between the conducting electrons and single-molecular magnets, we can investigate the magnetization dynamics of single-molecular magnets. We demonstrate the dynamics of the magnetization based on the spin diffusion equation and the Heisenberg-like equation. The results show that when the current is large enough, the magnetization of the single-molecular magnets can be reversed. We also calculate the critical current density required for the magnetization reversal under different anisotropy and external magnetic fields, which is helpful for the corresponding experimental design.
 Select Anisotropy Properties of Mn$_{2}$P Single Crystals with Antiferromagnetic Transition Shi-Hang Na, Wei Wu, and Jian-Lin Luo Chin. Phys. Lett. 2020, 37 (8): 087301 .   DOI: 10.1088/0256-307X/37/8/087301 Abstract   PDF(pc) (1064KB) ( 145 ) PDF(mobile)(1059KB) ( 10 ) HTML ( 237 )  Single crystals of hexagonal structure Mn$_{2}$P are synthesized by Sn flux for the first time. Transport and magnetic properties have been performed on the single crystals, which is an antiferromagnet with Neel temperature 103 K. Obvious anisotropy of resistivity is observed below the Neel temperature, which is manifested by metallic behavior with a current along the $c$-axis and semiconducting behavior with a current along the $a$-axis. The negative slope of temperature-dependent resistivity is observed above the Neel temperature in both $a$ and $c$ directions. Strong anisotropy of magnetic susceptibility is also evident from the magnetization measurements. A weak metamagnetic transition is observed only in $a$-axis plane at high magnetic field near 50–60 K compared to the $c$-axis. We believe these strong anisotropies of magnetic and transport properties are due to the anisotropy of spin arrangement. Mn$_{2}$P could be a candidate for exploration of possible superconductivity due to the low spin state.
 Select Zirconium Aided Epitaxial Growth of In$_{x}$Se$_{y}$ on InP(111) Substrates Cheng Zheng, Dapeng Zhao, Xinqiang Cai, Wantong Huang, Fanqi Meng, Qinghua Zhang, Lin Tang, Xiaopeng Hu, Lin Gu, Shuai-Hua Ji, Xi Chen Chin. Phys. Lett. 2020, 37 (8): 087401 .   DOI: 10.1088/0256-307X/37/8/087401 Abstract   PDF(pc) (1147KB) ( 151 ) PDF(mobile)(1138KB) ( 6 ) HTML ( 105 )  Layered material indium selenide (In$_{x}$Se$_{y}$) is a promising candidate for building next-generation electronic and photonic devices. We report a zirconium aided MBE growth of this van der Waals material. When co-depositing zirconium and selenium onto an indium phosphide substrate with a substrate temperature of 400℃ at a constant zirconium flux rate of 0.01 ML/min, the polymorphic In$_{x}$Se$_{y}$ layer emerges on top of the insulating ZrSe$_{2}$ layer. Different archetypes, such as InSe, $\alpha$-In$_{2}$Se$_{3}$ and $\beta$-In$_{2}$Se$_{3}$, are found in the In$_{x}$Se$_{y}$ layers. A negative magnetoresistance of 40% at 2 K under 9 T magnetic field is observed. Such an In$_{x}$Se$_{y}$/ZrSe$_{2}$ heterostructure with good lattice-matching may serve as a candidate for device applications.
 Select Electronic Evolution from the Parent Mott Insulator to a Superconductor in Lightly Hole-Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Qiang Gao, Lin Zhao, Cheng Hu, Hongtao Yan, Hao Chen, Yongqing Cai, Cong Li, Ping Ai, Jing Liu, Jianwei Huang, Hongtao Rong, Chunyao Song, Chaohui Yin, Qingyan Wang, Yuan Huang, Guo-Dong Liu, Zu-Yan Xu, and Xing-Jiang Zhou Chin. Phys. Lett. 2020, 37 (8): 087402 .   DOI: 10.1088/0256-307X/37/8/087402 Abstract   PDF(pc) (3827KB) ( 218 ) PDF(mobile)(3805KB) ( 39 ) HTML ( 368 )  High temperature superconductivity in cuprates is realized by doping the Mott insulator with charge carriers. A central issue is how such an insulating state can evolve into a conducting or superconducting state when charge carriers are introduced. Here, by in situ vacuum annealing and Rb deposition on the Bi$_2$Sr$_2$Ca$_{0.6}$Dy$_{0.4}$Cu$_2$O$_{8+\delta}$ (Bi2212) sample surface to push its doping level continuously from deeply underdoped ($T_{\rm c}=25$ K, doping level $p\sim0.066$) to the near-zero doping parent Mott insulator, angle-resolved photoemission spectroscopy measurements are carried out to observe the detailed electronic structure evolution in the lightly hole-doped region for the first time. Our results indicate that the chemical potential lies at about l eV above the charge transfer band for the parent state at zero doping, which is quite close to the upper Hubbard band. With increasing hole doping, the chemical potential moves continuously towards the charge transfer band and the band structure evolution exhibits a rigid band shift-like behavior. When the chemical potential approaches the charge transfer band at a doping level of $\sim$0.05, the nodal spectral weight near the Fermi level increases, followed by the emergence of the coherent quasiparticle peak and the insulator–superconductor transition. Our observations provide key insights in understanding the insulator–superconductor transition in doping the parent cuprate compound and for establishing related theories.
 Select An Origin of Dzyaloshinskii–Moriya Interaction at Graphene-Ferromagnet Interfaces Due to the Intralayer RKKY/BR Interaction Jin Yang, Jian Li, Liangzhong Lin, and Jia-Ji Zhu Chin. Phys. Lett. 2020, 37 (8): 087501 .   DOI: 10.1088/0256-307X/37/8/087501 Abstract   PDF(pc) (729KB) ( 284 ) PDF(mobile)(721KB) ( 11 ) HTML ( 222 )  We present a theory of both the itinerant carrier-mediated RKKY interaction and the virtual excitations-mediated Bloembergen–Rowland (BR) interaction between magnetic moments in graphene induced by proximity effect with a ferromagnetic film. It is shown that the RKKY/BR interaction consists of the Heisenberg, Ising, and Dzyaloshinskii–Moriya (DM) terms. In the case of the nearest distance, we estimate the DM term from the RKKY/BR interaction is about $0.13$ meV for the graphene/Co interface, which is consistent with the experimental result of DM interaction $0.16\pm0.05$ meV. Our calculations indicate that the intralayer RKKY/BR interaction may be a possible physical origin of the DM interaction in the graphene-ferromagnet interface. This work provides a new perspective to comprehend the DM interaction in graphene/ferromagnet systems.
 Select A Nonlinear Theoretical Model of Magnetization and Magnetostriction for Ferromagnetic Materials under Applied Stress and Magnetic Fields Pengpeng Shi Chin. Phys. Lett. 2020, 37 (8): 087502 .   DOI: 10.1088/0256-307X/37/8/087502 Abstract   PDF(pc) (723KB) ( 71 ) PDF(mobile)(711KB) ( 14 ) HTML ( 51 )  A thermodynamic and micro-statistical model is proposed to explain the magnetization and magnetostriction mechanisms for isotropic ferromagnetic materials. Here a nonlinear magnetostrictive expression enhances the characterization of the nonlinear magnetic-mechanical effect, and the Brillouin function makes it possible to describe the relationship between the equivalent field and magnetization for various types of materials. Through detailed comparisons with the recent models of Wu et al. [Appl. Phys. Lett. 115 (2019) 162406] and Daniel [Eur. Phys. J.: Appl. Phys. 83 (2018) 30904], it is confirmed that the proposed model can provide greater physical insight and a more accurate description of the complex magnetostriction and magnetization behaviors, especially the complex nonlinearity of stress effects.
 Select Mode Structures and Damping of Quantized Spin Waves in Ferromagnetic Nanowires Qingwei Fu, Yong Li, Lina Chen, Fusheng Ma, Haotian Li, Yongbing Xu, Bo Liu, Ronghua Liu, and Youwei Du Chin. Phys. Lett. 2020, 37 (8): 087503 .   DOI: 10.1088/0256-307X/37/8/087503 Abstract   PDF(pc) (760KB) ( 82 ) PDF(mobile)(751KB) ( 4 ) HTML ( 76 )  Magnonic devices based on spin waves are considered as a new generation of energy-efficient and high-speed devices for storage and processing of information. Here we experimentally demonstrate that three distinct dominated magneto-dynamic modes are excited simultaneously and coexist in a transversely magnetized ferromagnetic wire by the ferromagnetic resonance (FMR) technique. Besides the uniform FMR mode, the spin-wave well mode, the backward volume magnetostatic spin-wave mode, and the perpendicular standing spin-wave mode are experimentally observed and further confirmed with more detailed spatial profiles by micromagnetic simulation. Furthermore, our experimental approach can also access and reveal damping coefficients of these spin-wave modes, which provides essential information for development of magnonic devices in the future.
 Select Unexpectedly Strong Diamagnetism of Self-Assembled Aromatic Peptides Haijun Yang, Zixin Wang, Liuhua Mu, Yongshun Song, Jun Hu, Feng Zhang, and Haiping Fang Chin. Phys. Lett. 2020, 37 (8): 087504 .   DOI: 10.1088/0256-307X/37/8/087504 Abstract   PDF(pc) (471KB) ( 339 ) PDF(mobile)(483KB) ( 45 ) HTML ( 189 )  There is a considerable amount of work that shows the biomagnetism of organic components without ferromagnetic components at the molecular level, but it is of great challenge to cover the giant gap of biomagnetism between their experimental and theoretical results. Here we show that the diamagnetism of aromatic peptides is greatly enhanced for about 11 times by self-assembling, reaching two orders of magnitude higher than the mass susceptibility of pure water. The self-assembly of aromatic rings in the peptide molecules plays the key role in such a strong diamagnetism.
 Select Enhancement of Curie Temperature under Built-in Electric Field in Multi-Functional Janus Vanadium Dichalcogenides Shilei Ji , Hong Wu , Shuang Zhou , Wei Niu , Lujun Wei , Xing-Ao Li , Feng Li, and Yong Pu Chin. Phys. Lett. 2020, 37 (8): 087505 .   DOI: 10.1088/0256-307X/37/8/087505 Abstract   PDF(pc) (3633KB) ( 112 ) PDF(mobile)(3928KB) ( 3 ) HTML ( 14 )  Functionalized two-dimensional materials with multiferroicity are highly desired to be next-generation electronic devices. Here we theoretically predict a family of Janus vanadium dichalcogenides VXX' (X/X' = S, Se, Te) monolayers with multiferroic properties, combing ferromagnetism, ferroelasticity and piezoelectricity. Due to the unpaired electrons on the V atom, the Janus VXX' monolayers have intrinsic long-range ferromagnetic orders. Particularly, the Curie temperature of 1T-VSeTe monolayer is up to 100 K, which is greatly higher than 2D 1T-VSe$_{2}$ and 1T-VTe$_{2}$. Furthermore, the six Janus VXX' monolayers have similar crater-like ferroelastic switching curves. Compared to black phosphorus, 2H-VSSe monolayer has the similar ferroelastic switching signal and 4 times lower energy barrier. In addition, the out-of-plane piezoelectricity induced by the structure asymmetry in the vertical direction gives the 2H-VXX' monolayers the potential to be piezoelectric materials. It is found that a built-in electric field in the vertical direction due to the different electronegativity values of chalcogen atoms induces the changes of electronic structures, which leads to the appearance of three different types of band gaps in the three H-phase structures. Recently, the experimental growth of the Janus MoSSe monolayers and the electrochemical exfoliation of ferromagnetic monolayered VSe$_{2}$ make the Janus VXX' monolayers possibly fabricated in experiments.
 Select Large Photoluminescence Enhancement by an Out-of-Plane Magnetic Field in Exfoliated WS$_2$ Flakes Sibai Sun, Jianchen Dang, Xin Xie, Yang Yu, Longlong Yang, Shan Xiao, Shiyao Wu, Kai Peng, Feilong Song, Yunuan Wang, Jingnan Yang, Chenjiang Qian, Zhanchun Zuo, and Xiulai Xu Chin. Phys. Lett. 2020, 37 (8): 087801 .   DOI: 10.1088/0256-307X/37/8/087801 Abstract   PDF(pc) (864KB) ( 103 ) PDF(mobile)(849KB) ( 4 ) HTML ( 82 )  We report an out-of-plane magnetic field induced large photoluminescence enhancement in WS$_2$ flakes at $4$ K, in contrast to the photoluminescence enhancement provided by an in-plane field in general. Two mechanisms for the enhancement are proposed. One is a larger overlap of the electron and hole caused by the magnetic field induced confinement. The other is that the energy difference between $\varLambda$ and $K$ valleys is reduced by magnetic field, and thus enhancing the corresponding indirect-transition trions. Meanwhile, the Landé $g$ factor of the trion is measured to be $-0.8$, whose absolute value is much smaller than normal exciton, which is around $|-4|$. A model for the trion $g$ factor is presented, confirming that the smaller absolute value of the Landé $g$ factor is a behavior of this $\varLambda$–$K$ trion. By extending the valley space, we believe this work provides a further understanding of the valleytronics in monolayer transition metal dichalcogenides.
 Select Effect of Dopant Concentration in a Base Layer on Photocurrent–Voltage Characteristics of Photovoltaic Power Converters Wen-Xue Huo, Ming-Long Zhao, Xian-Sheng Tang, Li-Li Han, Zhen Deng, Yang Jiang, Wen-Xin Wang, Hong Chen, Chun-Hua Du, and Hai-Qiang Jia Chin. Phys. Lett. 2020, 37 (8): 087802 .   DOI: 10.1088/0256-307X/37/8/087802 Abstract   PDF(pc) (570KB) ( 50 ) PDF(mobile)(564KB) ( 1 ) HTML ( 9 )  It is known that the p–n junction of an absorption region is a crucial part for power conversion efficiency of photovoltaic power converters. We fabricate four samples with different dopant concentrations in base layers. The dependences of power conversion efficiency and fill factor on input power are displayed by photocurrent–voltage measurement. Photoluminescence characteristics under open circuit and connected circuit conditions are also studied. It is found that the status of p–n junction matching is the critical factor in affecting the power conversion efficiency. In addition, series resistance of photovoltaic power converters impairs the efficiency especially at high input powers. Both the key factors need to be considered to obtain high efficiency, and this work provides promising guidance on designing photovoltaic power converters.
 Select A Simple Time-Resolved Optical Measurement of Diffusion Transport Dynamics of Photoexcited Carriers and Its Demonstration in Intrinsic GaAs Films Yongyong You , Tianran Jiang , and Tianshu Lai Chin. Phys. Lett. 2020, 37 (8): 087803 .   DOI: 10.1088/0256-307X/37/8/087803 Abstract   PDF(pc) (745KB) ( 62 ) PDF(mobile)(740KB) ( 2 ) HTML ( 7 )  We develop a tightly focused pump-probe absorption technique to study diffusion dynamics of photoexcited carriers. It has many advantages including the simple setup and operations, higher detection sensitivity, an analytic descriptive model and fast data samplings. Diffusion dynamics are measured twice, separately using two different-sized probe spots, instead of many time-delayed diffusion profiles of a carrier pocket measured using spatially probe-spot scanning. An analytic model is derived to describe diffusion dynamics. Diffusion dynamics in GaAs are measured to demonstrate the feasibility of this technique. The diffusion coefficient is obtained and agrees well with the reported experimental and theoretical results.
CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
 Select Pressure-Dependent Phonon Scattering of Layered GaSe Prepared by Mechanical Exfoliation Yu-Lu Zheng , Liang Li, Fang-Fei Li , Qiang Zhou, and Tian Cui  Chin. Phys. Lett. 2020, 37 (8): 088201 .   DOI: 10.1088/0256-307X/37/8/088201 Abstract   PDF(pc) (901KB) ( 44 ) PDF(mobile)(899KB) ( 1 ) HTML ( 10 )  Few-layered gallium selenide (GaSe) is obtained by using the mechanical exfoliation method, and its properties are characterized by photoluminescence and Raman spectroscopy. The pressure-dependent phonon scatterings of bulk, few-layered, oxidized few-layered GaSe are characterized up to 30 GPa by using a diamond anvil cell with inert argon used as the pressure transmission medium. All the GaSe samples processed a phase transition around 28 GPa. A new vibration mode at 250 cm$^{-1}$ is found in oxidized few-layered GaSe by Raman spectra, which is indexed as the Raman vibration mode of $\alpha$-Se.
 Select Polymer-Decorated 2D MoS$_{2}$ Synaptic Transistors for Biological Bipolar Metaplasticities Emulation Yuhang Zhao , Biao Liu , Junliang Yang , Jun He, and Jie Jiang Chin. Phys. Lett. 2020, 37 (8): 088501 .   DOI: 10.1088/0256-307X/37/8/088501 Abstract   PDF(pc) (3448KB) ( 122 ) PDF(mobile)(3434KB) ( 18 ) HTML ( 37 )  Biological bipolar metaplasticities were successfully mimicked in two-dimensional (2D) MoS$_{2}$ transistors via the implementation of two different MoS$_{2}$ surface decorations, poly (vinyl alcohol) (PVA) and chitosan bio-polymers. Interestingly, the depressing metaplasticity was successfully mimicked when the PVA bio-polymer was used as the surface decoration layer, whereas the metaplasticity of long-term potentiation was realized when the chitosan bio-polymer was taken as the surface decoration layer. Furthermore, the electronic band structures of the 2D MoS$_{2}$ devices with different surface decorations were further investigated using first-principles calculations for understanding the underlying mechanisms of such bipolar metaplasticities. These results will deepen our understanding of metaplasticity, and have great potential in neuromorphic computing applications.
 Select Normal Strain-Induced Tunneling Behavior Promotion in van der Waals Heterostructures Yi-Fan He , Lei-Xi Wang , Zhi-Xing Xiao , Ya-Wei Lv, Lei Liao , and Chang-Zhong Jiang  Chin. Phys. Lett. 2020, 37 (8): 088502 .   DOI: 10.1088/0256-307X/37/8/088502 Abstract   PDF(pc) (1631KB) ( 130 ) PDF(mobile)(1616KB) ( 13 ) HTML ( 58 )  Van der Waals heterostructures (vdWHs) realized by vertically stacking of different two-dimensional (2D) materials are a promising candidate for tunneling devices because of their atomically clean and lattice mismatch-free interfaces in which different layers are separated by the vdW gaps. The gaps can provide an ideal electric modulation environment on the vdWH band structures and, on the other hand, can also impede the electron tunneling behavior because of large tunneling widths. Here, through first-principles calculations, we find that the electrically modulated tunneling behavior is immune to the interlayer interaction, keeping a direct band-to-band tunneling manner even the vdWHs have been varied to the indirect semiconductor, which means that the tunneling probability can be promoted through the vdW gap shrinking. Using transition metal dichalcogenide heterostructures as examples and normal strains as the gap reducing strategy, a maximum shrinking of 33% is achieved without changing the direct tunneling manner, resulting in a tunneling probability promotion of more than 45 times. Furthermore, the enhanced interlayer interaction by the strains will boost the stability of the vdWHs at the lateral direction, preventing the interlayer displacement effectively. It is expected that our findings provide perspectives in improving the electric behaviors of the vdWH devices.
 Select Polymorphism and Flexibility of DNA in Alcohols Nan Zhang, Ming-Ru Li, Hui-Ting Xu, and Feng-Shou Zhang Chin. Phys. Lett. 2020, 37 (8): 088701 .   DOI: 10.1088/0256-307X/37/8/088701 Abstract   PDF(pc) (940KB) ( 40 ) PDF(mobile)(931KB) ( 9 ) HTML ( 21 )  Molecular dynamics simulations are performed to investigate the polymorphism and flexibility of DNA in water, ethylene glycol (EG) and ethanol (EA) solutions. DNA in EG resembles the structure of DNA in water exhibiting B-DNA. In contrast, the DNA is an A-DNA state in the EA. We demonstrate that one important cause of these A$\leftrightarrow$B state changes is the competition between hydration and direct cation coupling to the phosphate groups on DNA backbones. To DNA structural polymorphism, it is caused by competition between hydration and cation coupling to the base pairs on grooves. Unlike flexible DNA in water and EA, DNA is immobilized around the canonical structure in EG solution, eliminating the potential biological effects of less common non-canonical DNA sub-states.