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 Select 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.
 Select 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.
 Select 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.
 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   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.
 Select 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.
 Select 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.
 Select Magnetic Sensing inside a Diamond Anvil Cell via Nitrogen-Vacancy Center Spins Yan-Xing Shang, Fang Hong, Jian-Hong Dai, Hui-Yu, Ya-Nan Lu, En-Ke Liu, Xiao-Hui Yu, Gang-Qin Liu, Xin-Yu Pan Chin. Phys. Lett.    2019, 36 (8): 086201 .   DOI: 10.1088/0256-307X/36/8/086201 Abstract   HTML   PDF (1001KB) The diamond anvil cell-based high-pressure technique is a unique tool for creating new states of matter and for understanding the physics underlying some exotic phenomena. In situ sensing of spin and charge properties under high pressure is crucially important but remains technically challenging. While the nitrogen-vacancy (NV) center in diamond is a promising quantum sensor under extreme conditions, its spin dynamics and the quantum control of its spin states under high pressure remain elusive. In this study, we demonstrate coherent control, spin relaxation, and spin dephasing measurements for ensemble NV centers up to 32.8 GPa. With this in situ quantum sensor, we investigate the pressure-induced magnetic phase transition of a micron-size permanent magnet Nd$_{2}$Fe$_{14}$B sample in a diamond anvil cell, with a spatial resolution of $\sim$2 μm, and sensitivity of $\sim$20 $\mu$T/Hz$^{1/2}$. This scheme could be generalized to measure other parameters such as temperature, pressure and their gradients under extreme conditions. This will be beneficial for frontier research of condensed matter physics and geophysics.
 Select Superconducting Single-Layer T-Graphene and Novel Synthesis Routes Qinyan Gu, Dingyu Xing, Jian Sun Chin. Phys. Lett.    2019, 36 (9): 097401 .   DOI: 10.1088/0256-307X/36/9/097401 Abstract   HTML   PDF (15832KB) Single-layer superconductors are ideal materials for fabricating superconducting nano devices. However, up to date, very few single-layer elemental superconductors have been predicted and especially no one has been successfully synthesized yet. Here, using crystal structure search techniques and ab initio calculations, we predict that a single-layer planar carbon sheet with 4- and 8-membered rings called T-graphene is a new intrinsic elemental superconductor with superconducting critical temperature ($T_{\rm c}$) up to around 20.8 K. More importantly, we propose a synthesis route to obtain such a single-layer T-graphene, that is, a T-graphene potassium intercalation compound (C$_4$K with $P4/mmm$ symmetry) is firstly synthesized at high pressure ($>$11.5 GPa) and then quenched to ambient condition; and finally, the single-layer T-graphene can be either exfoliated using the electrochemical method from the bulk C$_4$K, or peeled off from bulk T-graphite C$_4$, where C$_4$ can be obtained from C$_4$K by evaporating the K atoms. Interestingly, we find that the calculated $T_{\rm c}$ of C$_4$K is about 30.4 K at 0 GPa, which sets a new record for layered carbon-based superconductors. The present findings add a new class of carbon-based superconductors. In particular, once the single-layer T-graphene is synthesized, it can pave the way for fabricating superconducting devices together with other 2D materials using the layer-by-layer growth techniques.
 Select Pressure-Induced Metallization Accompanied by Elongated S–S Dimer in Charge Transfer Insulator NiS$_{2}$ Hao Wu, Yong-Hui Zhou, Yi-Fang Yuan, Chun-Hua Chen, Ying Zhou, Bo-Wen Zhang, Xu-Liang Chen, Chuan-Chuan Gu, Chao An, Shu-Yang Wang, Meng-Yao Qi, Ran-Ran Zhang, Li-Li Zhang, Xin-Jian Li, Zhao-Rong Yang Chin. Phys. Lett.    2019, 36 (10): 107101 .   DOI: 10.1088/0256-307X/36/10/107101 Abstract   HTML   PDF (1071KB) The insulator-metal transition triggered by pressure in charge transfer insulator NiS$_{2}$ is investigated by combining high-pressure electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements up to 40–50 GPa. Upon compression, we show that the metallization firstly appears in the low temperature region at $\sim$3.2 GPa and then extends to room temperature at $\sim$8.0 GPa. During the insulator-metal transition, the bond length of S–S dimer extracted from the synchrotron x-ray diffraction increases with pressure, which is supported by the observation of abnormal red-shift of the Raman modes between 3.2 and 7.1 GPa. Considering the decreasing bonding-antibonding splitting due to the expansion of S–S dimer, the charge gap between the S-$pp\pi^*$ band and the upper Hubbard band of Ni-3$d$ $e_{\rm g}$ state is remarkably decreased. These results consistently indicate that the elongated S–S dimer plays a predominant role in the insulator-metal transition under high pressure, even though the $p$-$d$ hybridization is enhanced simultaneously, in accordance with a scenario of charge-gap-controlled type.
 Select 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.
 Select Superconductivity and Fermi Surface Anisotropy in Transition Metal Dichalcogenide NbTe$_{2}$ Xi Zhang, Tianchuang Luo, Xiyao Hu, Jing Guo, Gongchang Lin, Yuehui Li, Yanzhao Liu, Xiaokang Li, Jun Ge, Ying Xing, Zengwei Zhu, Peng Gao, Liling Sun, Jian Wang Chin. Phys. Lett.    DOI: 10.1088/0256-307X/36/5/057402 Abstract   HTML   PDF (1541KB) Transition metal dichalcogenides, featuring layered structures, have aroused enormous interest as a platform for novel physical phenomena and a wide range of potential applications. Among them, special interest has been placed upon WTe$_{2}$ and MoTe$_{2}$, which exhibit non-trivial topology both in single layer and bulk as well as pressure induced or enhanced superconductivity. We study another distorted 1T material NbTe$_{2}$ through systematic electrical transport measurements. Intrinsic superconductivity with onset transition temperature ($T_{\rm c}^{\rm onset}$) up to 0.72 K is detected where the upper critical field ($H_{\rm c}$) shows unconventional quasi-linear behavior, indicating spin-orbit coupling induced p-wave paring. Furthermore, a general model is proposed to fit the angle-dependent magnetoresistance, which reveals the Fermi surface anisotropy of NbTe$_{2}$. Finally, non-saturating linear magnetoresistance up to 50 T is observed and attributed to the quantum limit transport.
 Select Distinct Superconducting Gap on Two Bilayer-Split Fermi Surface Sheets in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Superconductor Ping Ai, Qiang Gao, Jing Liu, Yuxiao Zhang, Cong Li, Jianwei Huang, Chunyao Song, Hongtao Yan, Lin Zhao, Guo-Dong Liu, Gen-Da Gu, Feng-Feng Zhang, Feng Yang, Qin-Jun Peng, Zu-Yan Xu, Xing-Jiang Zhou Chin. Phys. Lett.    2019, 36 (6): 067402 .   DOI: 10.1088/0256-307X/36/6/067402 Abstract   HTML   PDF (2816KB) High resolution laser-based angle-resolved photoemission measurements are carried out on an overdoped superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ with a $T_{\rm c}$ of 75 K. Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels: the bonding sheet corresponds to a doping level of 0.14, which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped, giving an overall doping level of 0.20 for the sample. Different superconducting gap sizes on the two Fermi surface sheets are revealed. The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet. The maximum gap difference between the two Fermi surface sheets near the antinodal region is $\sim$2 meV. These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity, and the layer-dependent superconductivity in high temperature cuprate superconductors.
 Select Machine Learning to Instruct Single Crystal Growth by Flux Method Tang-Shi Yao, Cen-Yao Tang, Meng Yang, Ke-Jia Zhu, Da-Yu Yan, Chang-Jiang Yi, Zi-Li Feng, He-Chang Lei, Cheng-He Li, Le Wang, Lei Wang, You-Guo Shi, Yu-Jie Sun, Hong Ding Chin. Phys. Lett.    2019, 36 (6): 068101 .   DOI: 10.1088/0256-307X/36/6/068101 Abstract   HTML   PDF (691KB) Growth of high-quality single crystals is of great significance for research of condensed matter physics. The exploration of suitable growing conditions for single crystals is expensive and time-consuming, especially for ternary compounds because of the lack of ternary phase diagram. Here we use machine learning (ML) trained on our experimental data to predict and instruct the growth. Four kinds of ML methods, including support vector machine (SVM), decision tree, random forest and gradient boosting decision tree, are adopted. The SVM method is relatively stable and works well, with an accuracy of 81% in predicting experimental results. By comparison, the accuracy of laboratory reaches 36%. The decision tree model is also used to reveal which features will take critical roles in growing processes.
 Select 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.
 Select 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.
 Select The Search for the Quantum Spin Liquid in Kagome Antiferromagnets J.-J. Wen, Y. S. Lee Chin. Phys. Lett.    2019, 36 (5): 050101 .   DOI: 10.1088/0256-307X/36/5/050101 Abstract   HTML   PDF (307KB)
 Select Quantum Approach to Fast Protein-Folding Time Li-Hua Lu, You-Quan Li Chin. Phys. Lett.    2019, 36 (8): 080305 .   DOI: 10.1088/0256-307X/36/8/080305 Abstract   HTML   PDF (1996KB) In the traditional random-conformational-search model, various hypotheses with a series of meta-stable intermediate states were proposed to resolve the Levinthal paradox in protein-folding time. Here we introduce a quantum strategy to formulate protein folding as a quantum walk on a definite graph, which provides us a general framework without making hypotheses. Evaluating it by the mean of first passage time, we find that the folding time via our quantum approach is much shorter than the one obtained via classical random walks. This idea is expected to evoke more insights for future studies.
 Select Experimental Hamiltonian Learning of an 11-Qubit Solid-State Quantum Spin Register P.-Y. Hou, L. He, F. Wang, X.-Z. Huang, W.-G. Zhang, X.-L. Ouyang, X. Wang, W.-Q. Lian, X.-Y. Chang, L.-M. Duan Chin. Phys. Lett.    2019, 36 (10): 100303 .   DOI: 10.1088/0256-307X/36/10/100303 Abstract   HTML   PDF (1448KB) Learning the Hamiltonian of a quantum system is indispensable for prediction of the system dynamics and realization of high fidelity quantum gates. However, it is a significant challenge to efficiently characterize the Hamiltonian which has a Hilbert space dimension exponentially growing with the system size. Here, we develop and implement an adaptive method to learn the effective Hamiltonian of an 11-qubit quantum system consisting of one electron spin and ten nuclear spins associated with a single nitrogen-vacancy center in a diamond. We validate the estimated Hamiltonian by designing universal quantum gates based on the learnt Hamiltonian and implementing these gates in the experiment. Our experimental result demonstrates a well-characterized 11-qubit quantum spin register with the ability to test quantum algorithms, and shows our Hamiltonian learning method as a useful tool for characterizing the Hamiltonian of the nodes in a quantum network with solid-state spin qubits.
 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   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.
 Select 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.
 Select Quantized Conductance of Majorana Zero Mode in the Vortex of the Topological Superconductor (Li$_{0.84}$Fe$_{0.16}$)OHFeSe C. Chen, Q. Liu, T. Z. Zhang, D. Li, P. P. Shen, X. L. Dong, Z.-X. Zhao, T. Zhang, D. L. Feng Chin. Phys. Lett.    DOI: 10.1088/0256-307X/36/5/057403 Abstract   HTML   PDF (1786KB) The Majorana zero mode (MZM), which manifests as an exotic neutral excitation in superconductors, is the building block of topological quantum computing. It has recently been found in the vortices of several iron-based superconductors as a zero-bias conductance peak in tunneling spectroscopy. In particular, a clean and robust MZM has been observed in the cores of free vortices in (Li$_{0.84}$Fe$_{0.16}$)OHFeSe. Here using scanning tunneling spectroscopy, we demonstrate that Majorana-induced resonant Andreev reflection occurs between the STM tip and this zero-bias bound state, and consequently, the conductance at zero bias is quantized as $2e^{2}/h$. Our results present a hallmark signature of the MZM in the vortex of an intrinsic topological superconductor, together with its intriguing behavior.
 Select Towards the Same Line of Liquid–Liquid Phase Transition of Dense Hydrogen from Various Theoretical Predictions Binbin Lu, Dongdong Kang, Dan Wang, Tianyu Gao, Jiayu Dai Chin. Phys. Lett.    2019, 36 (10): 103102 .   DOI: 10.1088/0256-307X/36/10/103102 Abstract   HTML   PDF (1985KB) For a long time, there have been huge discrepancies between different models and experiments concerning the liquid–liquid phase transition (LLPT) in dense hydrogen. We present the results of extensive calculations of the LLPT in dense hydrogen using the most expensive first-principle path-integral molecular dynamics simulations available. The nonlocal density functional rVV10 and the hybrid functional PBE0 are used to improve the description of the electronic structure of hydrogen. Of all the density functional theory calculations available, we report the most consistent results through quantum Monte Carlo simulations and coupled electron-ion Monte Carlo simulations of the LLPT in dense hydrogen. The critical point of the first-order LLPT is estimated to be above 2000 K according to the equation of state. Moreover, the metallization pressure obtained from the jump of dc electrical conductivity almost coincides with the plateau of equation of state.
 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   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.
 Select 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.
 Select 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.
 Select Metal to Orthogonal Metal Transition Chuang Chen, Xiao Yan Xu, Yang Qi, Zi Yang Meng Chin. Phys. Lett.    2020, 37 (4): 047103 .   DOI: 10.1088/0256-307X/37/4/047103 Abstract   HTML   PDF (1184KB) Orthogonal metal is a new quantum metallic state that conducts electricity but acquires no Fermi surface (FS) or quasiparticles, and hence orthogonal to the established paradigm of Landau's Fermi-liquid (FL). Such a state may hold the key of understanding the perplexing experimental observations of quantum metals that are beyond FL, i.e., dubbed non-Fermi-liquid (nFL), ranging from the Cu- and Fe-based oxides, heavy fermion compounds to the recently discovered twisted graphene heterostructures. However, to fully understand such an exotic state of matter, at least theoretically, one would like to construct a lattice model and to solve it with unbiased quantum many-body machinery. Here we achieve this goal by designing a 2D lattice model comprised of fermionic and bosonic matter fields coupled with dynamic $\mathbb{Z}_2$ gauge fields, and obtain its exact properties with sign-free quantum Monte Carlo simulations. We find that as the bosonic matter fields become disordered, with the help of deconfinement of the $\mathbb{Z}_2$ gauge fields, the system reacts with changing its nature from the conventional normal metal with an FS to an orthogonal metal of nFL without FS and quasiparticles and yet still responds to magnetic probe like an FL. Such a quantum phase transition from a normal metal to an orthogonal metal, with its electronic and magnetic spectral properties revealed, is calling for the establishment of new paradigm of quantum metals and their transition with conventional ones.
 Select 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.
 Select Ferromagnetic MnSn Monolayer Epitaxially Grown on Silicon Substrate Qian-Qian Yuan, Zhaopeng Guo, Zhi-Qiang Shi, Hui Zhao, Zhen-Yu Jia, Qianjin Wang, Jian Sun, Di Wu, and Shao-Chun Li Chin. Phys. Lett.    2020, 37 (7): 077502 .   DOI: 10.1088/0256-307X/37/7/077502 Abstract   HTML   PDF (2498KB) Two-dimensional (2D) ferromagnetic materials have been exhibiting promising potential in applications, such as spintronics devices. To grow epitaxial magnetic films on silicon substrate, in the single-layer limit, is practically important but challenging. In this study, we realized the epitaxial growth of MnSn monolayer on Si(111) substrate, with an atomically thin Sn/Si(111)-$2\sqrt 3 \times 2\sqrt 3$-buffer layer, and controlled the MnSn thickness with atomic-layer precision. We discovered the ferromagnetism in MnSn monolayer with the Curie temperature ($T_{\rm c}$) of ${\sim} 54$ K. As the MnSn film is grown to 4 monolayers, $T_{\rm c}$ increases accordingly to ${\sim} 235$ K. The lattice of the epitaxial MnSn monolayer as well as the Sn/Si(111)-$2\sqrt 3 \times 2\sqrt 3$ is perfectly compatible with silicon, and thus an sharp interface is formed between MnSn, Sn and Si. This system provides a new platform for exploring the 2D ferromagnetism, integrating magnetic monolayers into silicon-based technology, and engineering the spintronics heterostructures.
 Select 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.
 Select Critical Scaling Behaviors of Entanglement Spectra Qi-Cheng Tang, Wei Zhu Chin. Phys. Lett.    2020, 37 (1): 010301 .   DOI: 10.1088/0256-307X/37/1/010301 Abstract   HTML   PDF (1294KB) We investigate the evolution of entanglement spectra under a global quantum quench from a short-range correlated state to the quantum critical point. Motivated by the conformal mapping, we find that the dynamical entanglement spectra demonstrate distinct finite-size scaling behaviors from the static case. As a prototypical example, we compute real-time dynamics of the entanglement spectra of a one-dimensional transverse-field Ising chain. Numerical simulation confirms that the entanglement spectra scale with the subsystem size $l$ as $\sim$$l^{-1}$ for the dynamical equilibrium state, much faster than $\propto$ $\ln^{-1} l$ for the critical ground state. In particular, as a byproduct, the entanglement spectra at the long time limit faithfully gives universal tower structure of underlying Ising criticality, which shows the emergence of operator-state correspondence in the quantum dynamics.