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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
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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.
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): .   DOI: 10.1088/0256-307X/36/10/107101
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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.
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
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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.
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
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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.
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
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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$2 meV. These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity, and the layer-dependent superconductivity in high temperature cuprate superconductors.
Low-Frequency Noise in Amorphous Indium Zinc Oxide Thin Film Transistors with Aluminum Oxide Gate Insulator
Ya-Yi Chen, Yuan Liu, Zhao-Hui Wu, Li Wang, Bin Li, Yun-Fei En, Yi-Qiang Chen
Chin. Phys. Lett.    2018, 35 (4): .   DOI: 10.1088/0256-307X/35/4/048502
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Low-frequency noise (LFN) in all operation regions of amorphous indium zinc oxide (a-IZO) thin film transistors (TFTs) with an aluminum oxide gate insulator is investigated. Based on the LFN measured results, we extract the distribution of localized states in the band gap and the spatial distribution of border traps in the gate dielectric, and study the dependence of measured noise on the characteristic temperature of localized states for a-IZO TFTs with Al$_2$O$_3$ gate dielectric. Further study on the LFN measured results shows that the gate voltage dependent noise data closely obey the mobility fluctuation model, and the average Hooge's parameter is about $1.18\times10^{-3}$. Considering the relationship between the free carrier number and the field effect mobility, we simulate the LFN using the $\Delta N$–$\Delta\mu$ model, and the total trap density near the IZO/oxide interface is about $1.23\times 10^{18}$ cm$^{-3}$eV$^{-1}$.
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
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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.
Soliton, Breather and Rogue Wave Solutions for the Nonlinear Schrödinger Equation Coupled to a Multiple Self-Induced Transparency System
Xin Wang, Lei Wang
Chin. Phys. Lett.    2018, 35 (3): .   DOI: 10.1088/0256-307X/35/3/030201
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We derive an $N$-fold Darboux transformation for the nonlinear Schrödinger equation coupled to a multiple self-induced transparency system, which is applicable to optical fiber communications in the erbium-doped medium. The $N$-soliton, $N$-breather and $N$th-order rogue wave solutions in the compact determinant representations are derived using the Darboux transformation and limit technique. Dynamics of such solutions from the first- to second-order ones are shown.
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
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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.
The Search for the Quantum Spin Liquid in Kagome Antiferromagnets
J.-J. Wen, Y. S. Lee
Chin. Phys. Lett.    2019, 36 (5): .   DOI: 10.1088/0256-307X/36/5/050101
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Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH$_{3}$NH$_{3}$PbI$_{3}$$^*$
Yue-Yu Zhang, Shiyou Chen, Peng Xu, Hongjun Xiang, Xin-Gao Gong, Aron Walsh, Su-Huai Wei
Chin. Phys. Lett.    2018, 35 (3): 036104.   DOI: 10.1088/0256-307X/35/3/036104
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The organic-inorganic hybrid perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH$_{3}$NH$_{3}$PbI$_{3}$ solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH$_{3}$NH$_{3}$PbI$_{3}$ is thermodynamically unstable with respect to the phase separation into CH$_{3}$NH$_{3}$I + PbI$_{2}$, i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH$_{3}$NH$_{3}$PbI$_{3}$ is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH$_{3}$NH$_{3}$ by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH$_{3}$NH$_{3}$PbI$_{3}$ and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.
Growth of $\beta$-Ga$_{2}$O$_{3}$ Films on Sapphire by Hydride Vapor Phase Epitaxy
Ze-Ning XIONG, Xiang-Qian XIU, Yue-Wen LI, Xue-Mei HUA, Zi-Li XIE, Peng CHEN, Bin LIU, Ping HAN, Rong ZHANG, You-Dou ZHENG
Chin. Phys. Lett.    2018, 35 (5): .   DOI: 10.1088/0256-307X/35/5/058101
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Two-inch Ga$_{2}$O$_{3}$ films with ($\bar{2}$01)-orientation are grown on $c$-sapphire at 850–1050$^{\circ}\!$C by hydride vapor phase epitaxy. High-resolution x-ray diffraction shows that pure $\beta$-Ga$_{2}$O$_{3}$ with a smooth surface has a higher crystal quality, and the Raman spectra reveal a very small residual strain in $\beta$-Ga$_{2}$O$_{3}$ grown by hydride vapor phase epitaxy compared with bulk single crystal. The optical transmittance is higher than 80% in the visible and near-UV regions, and the optical bandgap energy is calculated to be 4.9 eV.
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
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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.
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
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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.
Resistivity and Radio-Frequency Properties of Two-Generation Trap-Rich Silicon-on-Insulator Substrates
Lei Zhu, Yong-Wei Chang, Nan Gao, Xin Su, YeMin Dong, Lu Fei, Xing Wei, Xi Wang
Chin. Phys. Lett.    2018, 35 (4): .   DOI: 10.1088/0256-307X/35/4/047302
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Crystal morphologies and resistivity of polysilicon trap-rich layers of two-generation trap-rich silicon-on-insulator (TR-SOI) substrates are studied. It is found that the resistivity of the trap-rich layer of generation 2 (TR-G2) is higher than that of generation 1 (TR-G1), although the crystal morphologies of the trap rich layers are the same. In addition, the rf performance of two-generation TR-SOI substrates is investigated by coplanar waveguide lines and inductors. The results show that both the rf loss and the second harmonic distortion of TR-G2 are smaller than those of TR-G1. These results can be attributed to the higher resistivity values of both the trap-rich layer and the high-resistivity silicon (HR-Si) substrate of TR-G2. Moreover, the rf performance of the TR-SOI substrate with thicker buried oxide is slightly better. The second harmonics of various TR-SOI substrates are simulated and evaluated with the harmonic quality factor model as well. It can be predicted that the TR-SOI substrate will see further improvement in rf performance if the resistivities of both the trap-rich layer and HR-Si substrate increase.
Theoretical Prediction of Diamond Betavoltaic Batteries Performance Using $^{63}$Ni
Yu-Min Liu, Jing-Bin Lu, Xiao-Yi Li, Xu Xu, Rui He, Ren-Zhou Zheng, Guo-Dong Wei
Chin. Phys. Lett.    2018, 35 (7): .   DOI: 10.1088/0256-307X/35/7/072301
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A diamond p-n junction is used to convert the decay energy of $^{63}$Ni source into electrical energy. The self-absorption effect of the $^{63}$Ni source, the backscatter process and the transport process of beta particles in diamond materials are studied. Then the theoretical maximum of electrical properties and the energy conversion efficiencies of diamond-$^{63}$Ni p-n junction batteries are achieved. Finally, a feasible design of $p^{+}p^{-}n^{+}$ junction battery, which has the maximum output power density of 0.42 $\mu$W/cm$^{2}$ and the optimal device conversion efficiency of 26.8%, is proposed.
From Claringbullite to a New Spin Liquid Candidate Cu$_3$Zn(OH)$_6$FCl
Zili Feng, Wei Yi, Kejia Zhu, Yuan Wei, Shanshan Miao, Jie Ma, Jianlin Luo, Shiliang Li, Zi Yang Meng, Youguo Shi
Chin. Phys. Lett.    2019, 36 (1): 017502.   DOI: 10.1088/0256-307X/36/1/017502
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The search for quantum spin liquid (QSL) materials has attracted significant attention in the field of condensed matter physics in recent years, however so far only a handful of them are considered as candidates hosting QSL ground state. Owning to their geometrically frustrated structures, Kagome materials are ideal systems to realize QSL. We synthesize the kagome structured material claringbullite (Cu$_4$(OH)$_6$FCl) and then replace inter-layer Cu with Zn to form Cu$_3$Zn(OH)$_6$FCl. Comprehensive measurements reveal that doping Zn$^{2+}$ ions transforms magnetically ordered Cu$_4$(OH)$_6$FCl into a non-magnetic QSL candidate Cu$_3$Zn(OH)$_6$FCl. Therefore, the successful syntheses of Cu$_4$(OH)$_6$FCl and Cu$_3$Zn(OH)$_6$FCl provide not only a new platform for the study of QSL but also a novel pathway of investigating the transition between QSL and magnetically ordered systems.
Realization of Quantum Maxwell's Demon with Solid-State Spins
W.-B. Wang, X.-Y. Chang, F. Wang, P.-Y. Hou, Y.-Y. Huang, W.-G. Zhang, X.-L. Ouyang, X.-Z. Huang, Z.-Y. Zhang, H.-Y. Wang, L. He, L.-M. Duan
Chin. Phys. Lett.    2018, 35 (4): .   DOI: 10.1088/0256-307X/35/4/040301
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We report experimental realization of a quantum version of Maxwell's demon using solid state spins where the information acquiring and feedback operations by the demon are achieved through conditional quantum gates. A unique feature of this implementation is that the demon can start in a quantum superposition state or in an entangled state with an ancilla observer. Through quantum state tomography, we measure the entropy in the system, demon, and the ancilla, showing the influence of coherence and entanglement on the result. A quantum implementation of Maxwell's demon adds more controllability to this paradoxical thermal machine and may find applications in quantum thermodynamics involving microscopic systems.
Nonvolatile Resistive Switching and Physical Mechanism in LaCrO$_{3}$ Thin Films
Wan-Jing Hu, Ling Hu, Ren-Huai Wei, Xian-Wu Tang, Wen-Hai Song, Jian-Ming Dai, Xue-Bin Zhu, Yu-Ping Sun
Chin. Phys. Lett.    2018, 35 (4): .   DOI: 10.1088/0256-307X/35/4/047301
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Polycrystalline LaCrO$_{3}$ (LCO) thin films are deposited on Pt/Ti/SiO$_{2}$/Si substrates by pulsed laser deposition and used as the switching material to construct resistive random access memory devices. The unipolar resistive switching (RS) behavior in the Au/LCO/Pt devices exhibits a high resistance ratio of $\sim 10^{4}$ between the high resistance state (HRS) and low resistance state (LRS) and exhibits excellent endurance/retention characteristics. The conduction mechanism of the HRS in the high voltage range is dominated by the Schottky emission, while the Ohmic conduction dictates the LRS and the low voltage range of HRS. The RS behavior in the Au/LCO/Pt devices can be understood by the formation and rupture of conducting filaments consisting of oxygen vacancies, which is validated by the temperature dependence of resistance and x-ray photoelectron spectroscopy results. Further analysis shows that the reset current $I_{\rm R}$ and reset power $P_{\rm R}$ in the reset processes exhibit a scaling law with the resistance in LRS ($R_{0}$), which indicates that the Joule heating effect plays an essential role in the RS behavior of the Au/LCO/Pt devices.
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
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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.
Charge Density Wave States in 2H-MoTe$_{2}$ Revealed by Scanning Tunneling Microscopy
Lu Dong, Guan-Yong Wang, Zhen Zhu, Chen-Xiao Zhao, Xin-Yi Yang, Ai-Min Li, Jing-Lei Chen, Dan-Dan Guan, Yao-Yi Li, Hao Zheng, Mao-Hai Xie, Jin-Feng Jia
Chin. Phys. Lett.    2018, 35 (6): .   DOI: 10.1088/0256-307X/35/6/066801
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2H- and 1T$'$-phase monolayer MoTe$_{2}$ films on highly oriented pyrolytic graphite are studied using scanning tunneling microscopy and spectroscopy (STM/STS). The phase transition of MoTe$_{2}$ can be controlled by a post-growth annealing process, and the intermediate state during the phase transition is directly observed by STM. For 2H-MoTe$_{2}$, inversion domain boundaries are presented as bright lines at high sample bias, but as dark lines at lower sample bias. The $dI/dV$ mappings reveal the distinct distributions of electronic states between domain boundaries and interiors of domains. It should be noted that a $2\times2$ periodic structure is clearly discernable inside the domains, where the STS measurement shows a small dip of size $\sim$150 meV at the vicinity of the Fermi level, indicating that the $2\times2$ periodic structure may be an incommensurate charge density wave. Moreover, a $4\times4$ periodic structure appears in 2H-MoTe$_{2}$ grown at a higher substrate temperature.
Beam Steering Analysis in Optically Phased Vertical Cavity Surface Emitting Laser Array
Meng Xun, Yun Sun, Chen Xu, Yi-Yang Xie, Zhi Jin, Jing-Tao Zhou, Xin-Yu Liu, De-Xin Wu
Chin. Phys. Lett.    2018, 35 (3): .   DOI: 10.1088/0256-307X/35/3/034202
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Beam steering in implant defined coherently coupled vertical cavity surface emitting laser (VCSEL) arrays is simulated using the FDTD solution software. Angular deflection dependent on relative phase differences among elements, inter-element spacing, element size and emitted wavelength is analyzed detailedly and systematically. We design and fabricate 1$\times$2 implant defined VCSEL arrays for optimum beam steering performance. Electronically controlled beam steering with a maximum deflection angle of 1.6$^{\circ}$ is successfully achieved in the 1$\times$2 VCSEL arrays. The percentage of the power in the central lobe is above 39% when steering. The results show that the steering is controllable. Compared with other beam steering methods, the fabrication process is simple and of low cost.
Electric Field Induced Permanent Superconductivity in Layered Metal Nitride Chlorides HfNCl and ZrNCl
Shuai Zhang, Mo-Ran Gao, Huan-Yan Fu, Xin-Min Wang, Zhi-An Ren, Gen-Fu Chen
Chin. Phys. Lett.    2018, 35 (9): 097401.   DOI: 10.1088/0256-307X/35/9/097401
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Devices of electric double-layer transistors (EDLTs) with ionic liquid have been employed as an effective way to dope carriers over a wide range. However, the induced electronic states can hardly survive in the materials after releasing the gate voltage $V_{\rm G}$ at temperatures higher than the melting point of the selected ionic liquid. Here we show that a permanent superconductivity with transition temperature $T_{\rm c}$ of 24 and 15 K is realized in single crystals and polycrystalline samples of HfNCl and ZrNCl upon applying proper $V_{\rm G}$'s at different temperatures. Reversible change between insulating and superconducting states can be obtained by applying positive and negative $V_{\rm G}$ at low temperature such as 220 K, whereas $V_{\rm G}$'s applied at 250 K induce the irreversible superconducting transition. The upper critical field $H_{\rm c2}$ of the superconducting states obtained at different gating temperatures shows similar temperature dependence. We propose a reasonable scenario that partial vacancy of Cl ions could be caused by applying proper $V_{\rm G}$'s at slightly higher processing temperatures, which consequently results in a permanent electron doping in the system. Such a technique shows great potential to systematically tune the bulk electronic state in the similar two-dimensional systems.
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
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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.
Electronic Phase Separation in Iron Selenide (Li,Fe)OHFeSe Superconductor System
Yiyuan Mao, Jun Li, Yulong Huan, Jie Yuan, Zi-an Li, Ke Chai, Mingwei Ma, Shunli Ni, Jinpeng Tian, Shaobo Liu, Huaxue Zhou, Fang Zhou, Jianqi Li, Guangming Zhang, Kui Jin, Xiaoli Dong, Zhongxian Zhao
Chin. Phys. Lett.    2018, 35 (5): 57402.   DOI: 10.1088/0256-307X/35/5/057402
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The phenomenon of phase separation into antiferromagnetic (AFM) and superconducting (SC) or normal-state regions has great implication for the origin of high-temperature (high-$T_{\rm c}$) superconductivity. However, the occurrence of an intrinsic antiferromagnetism above the $T_{\rm c}$ of (Li,Fe)OHFeSe superconductor is questioned. Here we report a systematic study on a series of (Li,Fe)OHFeSe single crystal samples with $T_{\rm c}$ up to $\sim$41 K. We observe an evident drop in the static magnetization at $T_{\rm afm} \sim 125$ K, in some of the SC ($T_{\rm c} \lesssim 38$ K, cell parameter $c \lesssim 9.27$ Å) and non-SC samples. We verify that this AFM signal is intrinsic to (Li,Fe)OHFeSe. Thus, our observations indicate mesoscopic-to-macroscopic coexistence of an AFM state with the normal (below $T_{\rm afm}$) or SC (below $T_{\rm c}$) state in (Li,Fe)OHFeSe. We explain such coexistence by electronic phase separation, similar to that in high-$T_{\rm c}$ cuprates and iron arsenides. However, such an AFM signal can be absent in some other samples of (Li,Fe)OHFeSe, particularly it is never observed in the SC samples of $T_{\rm c} \gtrsim 38$ K, owing to a spatial scale of the phase separation too small for the macroscopic magnetic probe. For this case, we propose a microscopic electronic phase separation. The occurrence of two-dimensional AFM spin fluctuations below nearly the same temperature as $T_{\rm afm}$, reported previously for a (Li,Fe)OHFeSe ($T_{\rm c} \sim 42$ K) single crystal, suggests that the microscopic static phase separation reaches vanishing point in high-$T_{\rm c}$ (Li,Fe)OHFeSe. A complete phase diagram is thus established. Our study provides key information of the underlying physics for high-$T_{\rm c}$ superconductivity.
Rare-Earth Chalcogenides: A Large Family of Triangular Lattice Spin Liquid Candidates
Weiwei Liu, Zheng Zhang, Jianting Ji, Yixuan Liu, Jianshu Li, Xiaoqun Wang, Hechang Lei, Gang Chen, Qingming Zhang
Chin. Phys. Lett.    2018, 35 (11): 117501.   DOI: 10.1088/0256-307X/35/11/117501
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Frustrated quantum magnets are expected to host many exotic quantum spin states like quantum spin liquid (QSL), and have attracted numerous interest in modern condensed matter physics. The discovery of the triangular lattice spin liquid candidate YbMgGaO$_4$ stimulated an increasing attention on the rare-earth-based frustrated magnets with strong spin-orbit coupling. Here we report the synthesis and characterization of a large family of rare-earth chalcogenides AReCh$_2$ (A = alkali or monovalent ions, Re = rare earth, Ch = O, S, Se). The family compounds share the same structure ($R\bar{3}m$) as YbMgGaO$_4$, and antiferromagnetically coupled rare-earth ions form perfect triangular layers that are well separated along the $c$-axis. Specific heat and magnetic susceptibility measurements on NaYbO$_2$, NaYbS$_2$ and NaYbSe$_2$ single crystals and polycrystals, reveal no structural or magnetic transition down to 50 mK. The family, having the simplest structure and chemical formula among the known QSL candidates, removes the issue on possible exchange disorders in YbMgGaO$_4$. More excitingly, the rich diversity of the family members allows tunable charge gaps, variable exchange coupling, and many other advantages. This makes the family an ideal platform for fundamental research of QSLs and its promising applications.
Exact Equivalence between Quantum Adiabatic Algorithm and Quantum Circuit Algorithm
Hongye Yu, Yuliang Huang, Biao Wu
Chin. Phys. Lett.    2018, 35 (11): 110303.   DOI: 10.1088/0256-307X/35/11/110303
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We present a rigorous proof that quantum circuit algorithm can be transformed into quantum adiabatic algorithm with the exact same time complexity. This means that from a quantum circuit algorithm of $L$ gates we can construct a quantum adiabatic algorithm with time complexity of $O(L)$. Additionally, our construction shows that one may exponentially speed up some quantum adiabatic algorithms by properly choosing an evolution path.
Preservation of Quantum Coherence for Gaussian-State Dynamics in a Non-Markovian Process
Jun Wen, Guan-Qiang Li
Chin. Phys. Lett.    2018, 35 (6): .   DOI: 10.1088/0256-307X/35/6/060301
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Coherence is a key resource in quantum information science. Exactly understanding and controlling the variation of coherence are vital for implementation in realistic quantum systems. Using $P$-representation of density matrix, we obtain the analytical solution of the master equation for the classical states in the non-Markovian process and investigate the coherent dynamics of Gaussian states. It is found that quantum coherence can be preserved in such a process if the coupling strength between system and environment exceeds a threshold value. We also discuss the characteristic function of the Gaussian states in the non-Markovian process, which provides an inevitable bridge for the control and operation of quantum coherence.
Multi-Soliton Solutions for the Coupled Fokas–Lenells System via Riemann–Hilbert Approach
Zhou-Zheng Kang, Tie-Cheng Xia, Xi Ma
Chin. Phys. Lett.    2018, 35 (7): .   DOI: 10.1088/0256-307X/35/7/070201
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We aim to construct multi-soliton solutions for the coupled Fokas–Lenells system which arises as a model for describing the nonlinear pulse propagation in optical fibers. Starting from the spectral analysis of the Lax pair, a Riemann–Hilbert problem is presented. Then in the framework of the Riemann–Hilbert problem corresponding to the reflectionless case, $N$-soliton solutions to the coupled Fokas–Lenells system are derived explicitly.
Two-Dimensional Borane with 'Banana' Bonds and Dirac-Like Ring
Hong Wu, Yun-Hui Wang, Zhi-Hong Yang, Feng Li
Chin. Phys. Lett.    2018, 35 (3): .   DOI: 10.1088/0256-307X/35/3/037101
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Designing new two-dimensional (2D) semiconductors with novel topological characters is highly desirable for further material innovation. We propose a theoretical design of a stable 2D inorganic material, namely, borane, which is jointly stabilized by traditional B–B localized and unique B–H–B delocalized chemical bonds. In borane, the bonding natures along different directions are distinguishing, which lead to huge differences in mechanical strengths of 142.73 and 97.47 N/m for $a$ and $b$ directions, respectively. In a unit cell, each hydrogen atom binds to two boron atoms forming a three-center-two-electron (3c-2e) bridge bond B–H–B. This can be considered as an extension of diborane molecules from 0D to 2D. The collaboration of localized and delocalized chemical bonds endows borane with high structural stability, as indicated by its favorable cohesive energy, high mechanical strength, absence of imaginary modes in the phonon spectrum, and moderate melting point. Remarkably, borane has a fascinating electronic property featured with a Dirac-like ring in the electronic band structure. The unique bonding nature and electronic property in borane would attract intensive interests in both theory and experiment.