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The 45K Onset Superconductivity and the Suppression of the Nematic Order in FeSe by Electrolyte Gating
Wei-Ke Wang, Yan Liu, Ji-Yong Yang, Hai-Feng Du, Wei Ning, Lang-Sheng Ling, Wei Tong, Zhe Qu, Zhao-Rong Yang, Ming-Liang Tian, Yu-Heng Zhang
Chin. Phys. Lett.    2016, 33 (05): 057401 .   DOI: 10.1088/0256-307X/33/5/057401
Abstract   HTML   PDF (896KB)
The electronic doping effect on both the superconductivity and the nematic order in the FeSe nanoflake are investigated by using the electric-double-layer transistor configuration. The superconductivity can be effectively controlled by electronic doping, and the onset superconducting transition temperature $T_{\rm c}$ reaches as high as 45 K at a gate voltage of $V_{\rm g}=4$ V. Meanwhile, the nematic phase is gradually suppressed with the increase of electronic doping (or $V_{\rm g}$). The results provide an effective method with variable charge doping for investigation of the rich physics in the FeSe superconductor.
Large Storage Window in a-SiNx/nc-Si/a-SiNx Sandwiched Structure for Nanocrystalline Silicon Floating Gate Memory Application
WANG Xiang, HUANG Jian, DING Hong-Lin, ZHANG Xian-Gao, YU Lin-Wei, HUANG Xin-Fan, LI Wei, CHEN Kun-Ji
Chin. Phys. Lett.    2008, 25 (7): 2690-2693 .  
Abstract   PDF (1137KB)
An a-SiNx/nanocrystalline silicon [(nc-Si)/a-SiNx] sandwiched structure is fabricated in a plasma enhanced chemical vapour deposition (PECVD) system at low temperature (250°C). The nc-Si layer is fabricated from a hydrogen-diluted silane mixture gas by using a layer-by-layer deposition technique. Atom force microscopy measurement shows that the density of nc-Si is about 2×1011cm-2. By the pretreatment of plasma nitridation, low density of interface states and high-quality interface between the Si substrate and a-SiNx insulator layer are obtained. The density of interface state at the midgap is calculated to be 1×1010cm-2eV-1 from the quasistatic and high frequency C-V data. The charging and discharging property of nc-Si quantum dots is studied by capacitance-voltage (C-V) measurement at room temperature. An
ultra-large hysteresis is observed in the C-V characteristics, which is attributed to storage of the electrons and holes into the nc-Si dots. The long-term charge-loss process is studied and ascribed to low density of interface states at SiNx/Si substrate.
Resonant Quantum Search with Monitor Qubits
Frank Wilczek, Hong-Ye Hu, Biao Wu
Chin. Phys. Lett.    2020, 37 (5): 050304 .   DOI: 10.1088/0256-307X/37/5/050304
Abstract   HTML   PDF (582KB)
We present an algorithm for the generalized search problem (searching $k$ marked items among $N$ items) based on a continuous Hamiltonian and exploiting resonance. This resonant algorithm has the same time complexity $O(\sqrt{N/k})$ as the Grover algorithm. A natural extension of the algorithm, incorporating auxiliary "monitor" qubits, can determine $k$ precisely, if it is unknown. The time complexity of our counting algorithm is $O(\sqrt{N})$, similar to the best quantum approximate counting algorithm, or better, given appropriate physical resources.
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   HTML   PDF (1221KB)
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.
Pressure-Dependent Point-Contact Spectroscopy of Superconducting PbTaSe$_2$ Single Crystals
Hai Zi, Ling-Xiao Zhao, Xing-Yuan Hou, Lei Shan, Zhian Ren, Gen-Fu Chen, and Cong Ren
Chin. Phys. Lett.    2020, 37 (9): 097403 .   DOI: 10.1088/0256-307X/37/9/097403
Abstract   HTML   PDF (5795KB)
We develop an experimental tool to investigate the order parameter of superconductors by combining point-contact spectroscopy measurement with high-pressure technique. It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe$_2$ single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar. Under such a high pressure, the normal-state contact resistance is sensitive to the applied pressure, reflecting the underlying variation of contact transparency upon pressures. In a superconducting state, the pressure dependence of the energy gap $\varDelta_0$ and the critical temperature $T_{\rm c}$ for gap opening/closing are extracted based on a generalized Blond–Tinkham–Klapwijk model. The gap ratio $2\varDelta_0/k_{_{\rm B}}T_{\rm c}$ indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe$_2$. Our experimental results show the accessibility and validity of high-pressure point-contact spectroscopy, offering rich information about high-pressure superconductivity.
Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn$_2$Bi$_2$Te$_5$
Jinlong Zhang, Dinghui Wang, Minji Shi, Tongshuai Zhu, Haijun Zhang, Jing Wang
Chin. Phys. Lett.    2020, 37 (7): 077304 .   DOI: 10.1088/0256-307X/37/7/077304
Abstract   HTML   PDF (2348KB)
The dynamical axion field is a new state of quantum matter where the magnetoelectric response couples strongly to its low-energy magnetic fluctuations. It is fundamentally different from an axion insulator with a static quantized magnetoelectric response. The dynamical axion field exhibits many exotic phenomena such as axionic polariton and axion instability. However, these effects have not been experimentally confirmed due to the lack of proper topological magnetic materials. Combining analytic models and first-principles calculations, here we predict a series of van der Waals layered Mn$_2$Bi$_2$Te$_5$-related topological antiferromagnetic materials that could host the long-sought dynamical axion field with a topological origin. We also show that a large dynamical axion field can be achieved in antiferromagnetic insulating states close to the topological phase transition. We further propose the optical and transport experiments to detect such a dynamical axion field. Our results could directly aid and facilitate the search for topological-origin large dynamical axion field in realistic materials.
Pressure-Induced Topological and Structural Phase Transitions in an Antiferromagnetic Topological Insulator
Cuiying Pei, Yunyouyou Xia, Jiazhen Wu, Yi Zhao, Lingling Gao, Tianping Ying, Bo Gao, Nana Li, Wenge Yang, Dongzhou Zhang, Huiyang Gou, Yulin Chen, Hideo Hosono, Gang Li, Yanpeng Qi
Chin. Phys. Lett.    2020, 37 (6): 066401 .   DOI: 10.1088/0256-307X/37/6/066401
Abstract   HTML   PDF (5392KB)
Recently, natural van der Waals heterostructures of (MnBi$_{2}$Te$_{4}$)$_{m}$(Bi$_{2}$Te$_{3}$)$_{n}$ have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states. We systematically investigate both the structural and electronic responses of MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ to external pressure. In addition to the suppression of antiferromagnetic order, MnBi$_{2}$Te$_{4}$ is found to undergo a metal–semiconductor–metal transition upon compression. The resistivity of MnBi$_{4}$Te$_{7}$ changes dramatically under high pressure and a non-monotonic evolution of $\rho (T)$ is observed. The nontrivial topology is proved to persist before the structural phase transition observed in the high-pressure regime. We find that the bulk and surface states respond differently to pressure, which is consistent with the non-monotonic change of the resistivity. Interestingly, a pressure-induced amorphous state is observed in MnBi$_{2}$Te$_{4}$, while two high-pressure phase transitions are revealed in MnBi$_{4}$Te$_{7}$. Our combined theoretical and experimental research establishes MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ as highly tunable magnetic topological insulators, in which phase transitions and new ground states emerge upon compression.
X-Ray Diffraction Pattern of Graphite Oxide
MU Shi-Jia, SU Yu-Chang, XIAO Li-Hua, LIU Si-Dong, HU Te, TANG Hong-Bo
Chin. Phys. Lett.    2013, 30 (9): 096101 .   DOI: 10.1088/0256-307X/30/9/096101
Abstract   PDF (633KB)
X-ray diffraction patterns of graphite oxide (GO) are theoretically simulated as a function of the displacements of carbon atoms using the Debye–Waller factor in terms of the Warren–Bodenstein equation. The results demonstrate that GO has the turbostratically stacked structure. The high order (00l) peaks gradually disappear with the increase in atomic thermal vibrations along c-axis while the (hk0) ones weaken for the vibrations along a-axis. When the displacement deviation ua=0.015 nm and uc=0.100 nm the computed result is consistent with the experimental measurements.
Experimental Realization of an Intrinsic Magnetic Topological Insulator
Yan Gong, Jingwen Guo, Jiaheng Li, Kejing Zhu, Menghan Liao, Xiaozhi Liu, Qinghua Zhang, Lin Gu, Lin Tang, Xiao Feng, Ding Zhang, Wei Li, Canli Song, Lili Wang, Pu Yu, Xi Chen, Yayu Wang, Hong Yao, Wenhui Duan, Yong Xu, Shou-Cheng Zhang, Xucun Ma, Qi-Kun Xue, Ke He
Chin. Phys. Lett.    2019, 36 (7): 076801 .   DOI: 10.1088/0256-307X/36/7/076801
Abstract   HTML   PDF (7918KB)
An intrinsic magnetic topological insulator (TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remained elusive experimentally for a long time. Here we report the experimental realization of thin films of an intrinsic magnetic TI, MnBi$_{2}$Te$_{4}$, by alternate growth of a Bi$_{2}$Te$_{3}$ quintuple layer and a MnTe bilayer with molecular beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topological insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calculations. The unique magnetic and topological electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temperature and in a well-controlled way.
Imaginary Time Crystal of Thermal Quantum Matter
Zi Cai, Yizhen Huang, W. Vincent Liu
Chin. Phys. Lett.    2020, 37 (5): 050503 .   DOI: 10.1088/0256-307X/37/5/050503
Abstract   HTML   PDF (2767KB)
Temperature is a fundamental thermodynamic variable for matter. Physical observables are often found to either increase or decrease with it, or show a non-monotonic dependence with peaks signaling underlying phase transitions or anomalies. Statistical field theory has established connection between temperature and time: a quantum ensemble with inverse temperature $\beta$ is formally equivalent to a dynamic system evolving along an imaginary time from 0 to $i\beta$ in the space one dimension higher. Here we report that a gas of hard-core bosons interacting with a thermal bath manifests an unexpected temperature-periodic oscillation of its macroscopic observables, arising from the microscopic origin of space-time locked translational symmetry breaking and crystalline ordering. Such a temperature crystal, supported by quantum Monte Carlo simulation, generalizes the concept of purely spatial density-wave order to the imaginary time axis for Euclidean action.
A Ubiquitous Thermal Conductivity Formula for Liquids, Polymer Glass, and Amorphous Solids
Qing Xi, Jinxin Zhong, Jixiong He, Xiangfan Xu, Tsuneyoshi Nakayama, Yuanyuan Wang, Jun Liu, Jun Zhou, and Baowen Li
Chin. Phys. Lett.    2020, 37 (10): 104401 .   DOI: 10.1088/0256-307X/37/10/104401
Abstract   HTML   PDF (1069KB)
The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time. There have been several approaches to explain the thermal conductivities in these systems, for example, Bridgman's formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers. Here, we present a ubiquitous formula to calculate the thermal conductivities of liquids and amorphous solids in a unified way, and compare it with previous ones. The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data. Our formula not only provides a detailed microscopic mechanism of heat transfer in these systems, but also resolves the discrepancies between existing formulae and experimental data.
Characterization and Magnetic Properties of Nickel Ferrite Nanoparticles Prepared by Ball Milling Technique
G. Nabiyouni**, M. Jafari Fesharaki, M. Mozafari, J. Amighian
Chin. Phys. Lett.    2010, 27 (12): 126401 .   DOI: 10.1088/0256-307X/27/12/126401
Abstract   PDF (676KB)
Nickel ferrite nanoparicles with various grain sizes are synthesized using annealing treatment followed by ball milling of its bulk component materials. Commercially available nickel and iron oxide powders are first mixed, and then annealed at 1100°C in an oxygen environment furnace and for 3 h. The samples are then milled for different times in an SPEX mill. X-ray diffraction pattern indicates that in this stage the sample is single phase. The average grain size is estimated by scanning electron microscopy (SEM) and x-ray diffraction techniques. Magnetic behavior of the sample at room temperature is studied using a superconducting quantum interference device (SQUID). The Curie temperature of the powders is measured by an LCR–meter unit. The x-ray diffraction patterns clearly indicate that increasing the milling time leads to a decrease in the grain size and consequently leads to a decrease in the saturation magnetization as well as the Curie temperatures. This result is attributed to the spin-glass-like surface layer on the nanocrystalline nickel ferrite with a ferrimagnetically aligned core.
Grain Size Effect on Electrical Conductivity and Giant Magnetoresistance of Bulk Magnetic Polycrystals
LUO Wei, ZHU Lin-Li, ZHENG Xiao-Jing
Chin. Phys. Lett.    2009, 26 (11): 117502 .   DOI: 10.1088/0256-307X/26/11/117502
Abstract   PDF (384KB)
By solving the Boltzmann transport equation and considering the spin-dependent grain boundary scattering, the distribution of electrons in grains and the electrical transport properties in the applied magnetic field are studied. With regard to the dominant influence of grain boundary scattering which is taken as a boundary condition for the electrical transport, the grain size-dependent electrical conductivity is investigated. In addition, the reorientation of the relative magnetization between grains brings the change of the electron spin when the magnetonanocrystalline material is subjected to the magnetic field, resulting in the remarkable giant magnetoresistance effect.
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   HTML   PDF (3117KB)
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.
Investigation of Oxygen Vacancy and Interstitial Oxygen Defects in ZnO Films by Photoluminescence and X-Ray Photoelectron Spectroscopy
FAN Hai-Bo, YANG Shao-Yan, ZHANG Pan-Feng, WEI Hong-Yuan, LIU Xiang-Lin, JIAO Chun-Mei, ZHU Qin-Sheng, CHEN Yong-Hai, WANG Zhan-Guo
Chin. Phys. Lett.    2007, 24 (7): 2108-2111 .  
Abstract   PDF (341KB)
ZnO films prepared at different temperatures and annealed at 900°C in
oxygen are studied by photoluminescence (PL) and x-ray photoelectron
spectroscopy (XPS). It is observed that in the PL of the as-grown films the green luminescence (GL) and the yellow luminescence (YL) are related, and after annealing the GL is restrained and the YL is enhanced. The O 1s XPS results also show the coexistence of oxygen vacancy (VO) and interstitial oxygen (Oi) before annealing and the quenching of the VO after annealing. By combining the two results it is deduced that the GL and YL are related to the VO and Oi defects, respectively.
Classical-Noise-Free Sensing Based on Quantum Correlation Measurement
Ping Wang , Chong Chen , and Ren-Bao Liu
Chin. Phys. Lett.    2021, 38 (1): 010301 .   DOI: 10.1088/0256-307X/38/1/010301
Abstract   HTML   PDF (791KB)
Quantum sensing, using quantum properties of sensors, can enhance resolution, precision, and sensitivity of imaging, spectroscopy, and detection. An intriguing question is: Can the quantum nature (quantumness) of sensors and targets be exploited to enable schemes that are not possible for classical probes or classical targets? Here we show that measurement of the quantum correlations of a quantum target indeed allows for sensing schemes that have no classical counterparts. As a concrete example, in the case that the second-order classical correlation of a quantum target could be totally concealed by non-stationary classical noise, the higher-order quantum correlations can single out a quantum target from the classical noise background, regardless of the spectrum, statistics, or intensity of the noise. Hence a classical-noise-free sensing scheme is proposed. This finding suggests that the quantumness of sensors and targets is still to be explored to realize the full potential of quantum sensing. New opportunities include sensitivity beyond classical approaches, non-classical correlations as a new approach to quantum many-body physics, loophole-free tests of the quantum foundation, et cetera.
Mechanism of Strain Rate Effect Based on Dislocation Theory
QIN Kun, YANG Li-Ming, HU Shi-Sheng
Chin. Phys. Lett.    2009, 26 (3): 036103 .   DOI: 10.1088/0256-307X/26/3/036103
Abstract   PDF (253KB)
Based on dislocation theory, we investigate the mechanism of strain rate effect. Strain rate effect and dislocation motion are bridged by Orowan's relationship, and the stress dependence of dislocation velocity is considered as the dynamics relationship of dislocation motion. The mechanism of strain rate effect is then investigated qualitatively by using these two relationships although the kinematics relationship of dislocation motion is absent due to complicated styles of dislocation motion. The process of strain rate effect is interpreted and some details of strain rate effect are adequately discussed. The present analyses agree with the existing experimental results. Based on the analyses, we propose that strain rate criteria rather than stress criteria should be satisfied when a metal is fully yielded at a given strain rate.
A New Path to Improve High $\beta_{\rm p}$ Plasma Performance on EAST for Steady-State Tokamak Fusion Reactor
Baonian Wan and the EAST team
Chin. Phys. Lett.    2020, 37 (4): 045202 .   DOI: 10.1088/0256-307X/37/4/045202
Abstract   HTML   PDF (573KB)
High $\beta_{\rm p}$ scenario is foreseen to be a promising candidate operational mode for steady-state tokamak fusion reactors. Dedicated experiments on EAST and data analysis find that density gradient $\nabla n$ is a control knob to improve energy confinement in high $\beta_{\rm p}$ plasmas at low toroidal rotation as projected for a fusion reactor. Different from previously known turbulent stabilization mechanisms such as ${\boldsymbol E} \times {\boldsymbol B}$ shear and Shafranov shift, high density gradient can enhance the Shafranov shift stabilizing effect significantly in high $\beta_{\rm p}$ regime, giving that a higher density gradient is readily accessible in future fusion reactors with lower collisionality. This new finding is of great importance for the next-step fusion development because it may open a new path towards even higher energy confinement in the high $\beta_{\rm p}$ scenario. It has been demonstrated in the recent EAST experiments, i.e., a fully non-inductive high $\beta_{\rm p}$ ($\sim $2) H-mode plasma ($H_{98y2}\ge 1.3$) has been obtained for a duration over 100 current diffusion times, which sets another new world record of long-pulse high-performance tokamak plasma operation with the normalized performance approaching the ITER and CFETR regimes.
Giant Spin Transfer Torque in Atomically Thin Magnetic Bilayers
Weihao Cao, Matisse Wei-Yuan Tu, Jiang Xiao, and Wang Yao
Chin. Phys. Lett.    2020, 37 (10): 107201 .   DOI: 10.1088/0256-307X/37/10/107201
Abstract   HTML   PDF (1297KB)
In cavity quantum electrodynamics, the multiple reflections of a photon between two mirrors defining a cavity is exploited to enhance the light-coupling of an intra-cavity atom. We show that this paradigm for enhancing the interaction of a flying particle with a localized object can be generalized to spintronics based on van der Waals 2D magnets. Upon tunneling through a magnetic bilayer, we find that the spin transfer torques per electron incidence can become orders of magnitude larger than $\hbar /2$, made possible by electron's multi-reflection path through the ferromagnetic monolayers as an intermediate of their angular momentum transfer. Over a broad energy range around the tunneling resonances, the damping-like spin transfer torque per electron tunneling features a universal value of $(\hbar/2)\tan (\theta /2)$, depending only on the angle $\theta$ between the magnetizations. These findings expand the scope of magnetization manipulations for high-performance and high-density storage based on van der Waals magnets.
Fermionic Analogue of High Temperature Hawking Radiation in Black Phosphorus
Hang Liu, Jia-Tao Sun, Chenchen Song, Huaqing Huang, Feng Liu, Sheng Meng
Chin. Phys. Lett.    2020, 37 (6): 067101 .   DOI: 10.1088/0256-307X/37/6/067101
Abstract   HTML   PDF (1956KB)
Time-periodic laser driving can create nonequilibrium states not accessible in equilibrium, opening new regimes in materials engineering and topological phase transitions. We report that black phosphorus (BP) exhibits spatially nonuniform topological Floquet–Dirac states under laser illumination, mimicking the "gravity" felt by fermionic quasiparticles in the same way as that for a Schwarzschild black hole (SBH). Quantum tunneling of electrons from a type-II Dirac cone (inside BH) to a type-I Dirac cone (outside BH) emits an SBH-like Planck radiation spectrum. The Hawking temperature $T_{\rm H}$ obtained for a fermionic analog of BH in the bilayer BP is approximately 3 K, which is several orders of magnitude higher than that in previous works. Our work sheds light on increasing $T_{\rm H}$ from the perspective of engineering 2D materials by time-periodic light illumination. The predicted SBH-like Hawking radiation, accessible in BP thin films, provides clues to probe analogous astrophysical phenomena in solids.
Superconductivity of Lanthanum Superhydride Investigated Using the Standard Four-Probe Configuration under High Pressures
Fang Hong, Liuxiang Yang, Pengfei Shan, Pengtao Yang, Ziyi Liu, Jianping Sun, Yunyu Yin, Xiaohui Yu, Jinguang Cheng, and Zhongxian Zhao
Chin. Phys. Lett.    2020, 37 (10): 107401 .   DOI: 10.1088/0256-307X/37/10/107401
Abstract   HTML   PDF (688KB)
Recently, the theoretically predicted lanthanum superhydride, LaH$_{10 \pm \delta}$, with a clathrate-like structure was successfully synthesized and found to exhibit a record high superconducting transition temperature $T_{\rm c} \approx 250$ K at $\sim $170 GPa, opening a new route for room-temperature superconductivity. However, since in situ experiments at megabar pressures are very challenging, few groups have reported the $\sim $250 K superconducting transition in LaH$_{10 \pm \delta}$. Here, we establish a simpler sample-loading procedure that allows a relatively large sample size for synthesis and a standard four-probe configuration for resistance measurements. Following this procedure, we successfully synthesized LaH$_{10 \pm \delta}$ with dimensions up to $10 \times 20$ μm$^{2}$ by laser heating a thin La flake and ammonia borane at $\sim $1700 K in a symmetric diamond anvil cell under the pressure of 165 GPa. The superconducting transition at $T_{\rm c} \approx 250$ K was confirmed through resistance measurements under various magnetic fields. Our method will facilitate explorations of near-room-temperature superconductors among metal superhydrides.
Enhanced Ferromagnetism of CrI$_{3}$ Bilayer by Self-Intercalation
Yu Guo , Nanshu Liu , Yanyan Zhao , Xue Jiang , Si Zhou, and Jijun Zhao 
Chin. Phys. Lett.    2020, 37 (10): 107506 .   DOI: 10.1088/0256-307X/37/10/107506
Abstract   HTML   PDF (2562KB)
Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI$_{3}$ is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI$_{3}$. We show that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI$_{3}$ bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI$_{3}$ layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.
Exact Solutions of the Dirac Equation for an Electron in a Magnetic Field with Shape Invariant Method
M. R. Setare, O. Hatami
Chin. Phys. Lett.    2008, 25 (11): 3848-3851 .  
Abstract   PDF (169KB)
Based on the shape invariance property we obtain exact solutions of the Dirac equation for an electron moving in the presence of a certain varying magnetic field, then we also show its non-relativistic limit.
Semi-physical Simulation Platform of a Parafoil Nonlinear Dynamic System
GAO Hai-Tao, YANG Sheng-Bo, ZHU Er-Lin, SUN Qing-Lin, CHEN Zeng-Qiang, KANG Xiao-Feng
Chin. Phys. Lett.    2013, 30 (11): 110503 .   DOI: 10.1088/0256-307X/30/11/110503
Abstract   PDF (1888KB)
Focusing on the problems in the process of simulation and experiment on a parafoil nonlinear dynamic system, such as limited methods, high cost and low efficiency, we present a semi-physical simulation platform. It is designed by connecting parts of physical objects to a computer, and remedies the defect that a computer simulation is divorced from a real environment absolutely. The main components of the platform and its functions, as well as simulation flows, are introduced. The feasibility and validity are verified through a simulation experiment. The experimental results show that the platform has significance for improving the quality of the parafoil fixed-point airdrop system, shortening the development cycle and saving cost.
Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe$_{2}$
Ya-Ting Jia, Chun-Sheng Gong, Yi-Xuan Liu, Jian-Fa Zhao, Cheng Dong, Guang-Yang Dai, Xiao-Dong Li, He-Chang Lei, Run-Ze Yu, Guang-Ming Zhang, and Chang-Qing Jin
Chin. Phys. Lett.    2020, 37 (9): 097404 .   DOI: 10.1088/0256-307X/37/9/097404
Abstract   HTML   PDF (1103KB)
The Mott transition is one of the fundamental issues in condensed matter physics, especially in the system with antiferromagnetic long-range order. However, such a transition is rare in quantum spin liquid (QSL) systems without long-range order. Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe$_{2}$ with a triangular lattice of 4$f$ Yb$^{3+}$ ions. Detail analysis of transport properties in metallic state shows an evolution from non-Fermi liquid to Fermi liquid behavior when approaching the vicinity of superconductivity. An irreversible structure phase transition occurs around 11 GPa, which is revealed by the x-ray diffraction. These results shed light on the Mott transition in the QSL systems.
First-Principles Study of Electronic Structure and Optical Properties of Cubic Perovskite CsCaF3
K. Ephraim Babu, A. Veeraiah, D. Tirupati Swamy, V. Veeraiah
Chin. Phys. Lett.    2012, 29 (11): 117102 .   DOI: 10.1088/0256-307X/29/11/117102
Abstract   PDF (895KB)
Electronic, structural and optical properties of the cubic perovskite CsCaF3 are calculated by using the full potential linearized augmented plane wave (FP-LAPW) plus local orbitals method with generalized gradient approximation (GGA) in the framework of the density functional theory. The calculated lattice constant is in good agreement with the experimental result. The electronic band structure shows that the fundamental band gap is wide and indirect at (ΓR) point. The contribution of the different bands is analyzed from the total and partial density of states curves. The charge density plots show strong ionic bonding in Cs-F, and ionic and weak covalent bonding between Ca and F. Calculations of the optical spectra, viz., the dielectric function, optical reflectivity, absorption coefficient, real part of optical conductivity, refractive index, extinction coefficient and electron energy loss, are performed for the energy range 0–30 eV.
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   HTML   PDF (774KB)
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.
Strain Tunable Berry Curvature Dipole, Orbital Magnetization and Nonlinear Hall Effect in WSe$_{2}$ Monolayer
Mao-Sen Qin , Peng-Fei Zhu , Xing-Guo Ye , Wen-Zheng Xu , Zhen-Hao Song , Jing Liang , Kaihui Liu , and Zhi-Min Liao
Chin. Phys. Lett.    2021, 38 (1): 017301 .   DOI: 10.1088/0256-307X/38/1/017301
Abstract   HTML   PDF (2896KB)
The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent $K$ and $K'$ valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization of asymmetrical distribution of Berry curvature in a single valley in monolayer WSe$_2$ via applying uniaxial strain to break $C_{3v}$ symmetry. As a result, although the Berry curvature itself is still opposite in $K$ and $K'$ valleys, the two valleys would contribute equally to nonzero Berry curvature dipole. Upon applying electric field ${\boldsymbol E}$, the emergent Berry curvature dipole ${\boldsymbol D}$ would lead to an out-of-plane orbital magnetization $M \propto {\boldsymbol D} \cdot {\boldsymbol E}$, which further induces an anomalous Hall effect with a linear response to $E^2$, known as nonlinear Hall effect. We show the strain modulated transport properties of nonlinear Hall effect in monolayer WSe$_2$ with moderate hole-doping by gating. The second-harmonic Hall signals show quadratic dependence on electric field, and the corresponding orbital magnetization per current density $M/J$ can reach as large as 60. In contrast to the conventional Rashba–Edelstein effect with in-plane spin polarization, such current-induced orbital magnetization is along the out-of-plane direction, thus promising for high-efficient electrical switching of perpendicular magnetization.
A Polymer-Rich Re-deposition Technique for Non-volatile Etching By-products in Reactive Ion Etching Systems
A. Limcharoen, C. Pakpum, P. Limsuwan
Chin. Phys. Lett.    2013, 30 (7): 075202 .   DOI: 10.1088/0256-307X/30/7/075202
Abstract   PDF (666KB)
Re-deposition is a non-volatile etching by-product in reactive ion etching systems that is well known to cause dirt on etching work. In this study, we propose a novel etching method called the polymer-rich re-deposition technique, used particularly for improving the etched sidewall where the re-deposition is able to accumulate. This technique works by allowing the accumulated re-deposition on the etched sidewall to have a higher polymer species than the new compounds in the non-volatile etching by-product. The polymer-rich re-deposition is easy to remove along with the photo-resist mask residual at the photo-resist strip step using an isopropyl alcohol-based solution. The traditional, additional cleaning process step used to remove the re-deposition material is not required anymore, so this reduces the overall processing time. The technique is demonstrated on an Al2O3-TiC substrate by C4F8 plasma, and the EDX spectrum confirms that the polymer re-deposition has C and F atoms as the dominant atoms, suggesting that it is a C–F polymer re-deposition.
Room-Temperature Inductively Coupled Plasma Etching of InP Using Cl2N2 and Cl2/CH4/H2
LEE Chee-Wei, CHIN Mee-Koy
Chin. Phys. Lett.    2006, 23 (4): 903-906 .  
Abstract   PDF (638KB)
We optimize the room-temperature etching of InP using Cl2N2 and Cl2/CH4/H2 inductively coupled plasma reactive ions. A design of experiment is used in the optimization. The results, in terms of etch rate, surface roughness and etched profile, are presented. These Cl2-based recipes do not require substrate heating and thus can be more cost effectively and widely applied. The Cl2/CH4/H2 process is able to give a higher etch rate (about 850nm/min) and cleaner surface with less polymer formation compared to the conventional CH4/H2 process. The Cl2/N2 process produces even higher etch rate (as high as 2μm/min), but rougher surface with slight sidewall undercut. The Cl2/N2 process also has no polymer formation due to the absence of methane gas. Both the processes give very good selectivity to the silicon dioxide (SiO2) etch mask. The selectivity of InP to the oxide mask (up to 55:1) for the Cl2/N2 process is one of the highest reported so far. The etched structures possess reasonably good sidewall verticality and surface quality comparable to that obtained under elevated temperature condition (>200°C).