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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$2 meV. These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity, and the layer-dependent superconductivity in high temperature cuprate superconductors.
Machine Learning to Instruct Single Crystal Growth by Flux Method
Tang-Shi Yao, Cen-Yao Tang, Meng Yang, Ke-Jia Zhu, Da-Yu Yan, Chang-Jiang Yi, Zi-Li Feng, He-Chang Lei, Cheng-He Li, Le Wang, Lei Wang, You-Guo Shi, Yu-Jie Sun, Hong Ding
Chin. Phys. Lett.    2019, 36 (6): 068101.   DOI: 10.1088/0256-307X/36/6/068101
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Growth of high-quality single crystals is of great significance for research of condensed matter physics. The exploration of suitable growing conditions for single crystals is expensive and time-consuming, especially for ternary compounds because of the lack of ternary phase diagram. Here we use machine learning (ML) trained on our experimental data to predict and instruct the growth. Four kinds of ML methods, including support vector machine (SVM), decision tree, random forest and gradient boosting decision tree, are adopted. The SVM method is relatively stable and works well, with an accuracy of 81% in predicting experimental results. By comparison, the accuracy of laboratory reaches 36%. The decision tree model is also used to reveal which features will take critical roles in growing processes.
Superconductivity and Fermi Surface Anisotropy in Transition Metal Dichalcogenide NbTe$_{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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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.
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.
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.
Ionic-Liquid-Gating Induced Protonation and Superconductivity in FeSe, FeSe$_{0.93}$S$_{0.07}$, ZrNCl, 1$T$-TaS$_2$ and Bi$_2$Se$_3$
Yi Cui, Ze Hu, Jin-Shan Zhang, Wen-Long Ma, Ming-Wei Ma, Zhen Ma, Cong Wang, Jia-Qiang Yan, Jian-Ping Sun, Jin-Guang Cheng, Shuang Jia, Yuan Li, Jin-Sheng Wen, He-Chang Lei, Pu Yu, Wei Ji, Wei-Qiang Yu
Chin. Phys. Lett.    2019, 36 (7): 077401.   DOI: 10.1088/0256-307X/36/7/077401
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We report protonation in several compounds by an ionic-liquid-gating method, under optimized gating conditions. This leads to single superconducting phases for several compounds. Non-volatility of protons allows post-gating magnetization and transport measurements. The superconducting transition temperature $T_{\rm c}$ is enhanced to 43.5 K for FeSe$_{0.93}$S$_{0.07}$, and 41 K for FeSe after protonation. Superconducting transitions with $T_{\rm c} \sim 15$ K for ZrNCl, $\sim$7.2 K for 1$T$-TaS$_2$, and $\sim$3.8 K for Bi$_2$Se$_3$ are induced after protonation. Electric transport in protonated FeSe$_{0.93}$S$_{0.07}$ confirms high-temperature superconductivity. Our $^{1}$H nuclear magnetic resonance (NMR) measurements on protonated FeSe$_{1-x}$S$_{x}$ reveal enhanced spin-lattice relaxation rate $1/^{1}T_1$ with increasing $x$, which is consistent with the LDA calculations that H$^{+}$ is located in the interstitial sites close to the anions.
Formation of Two-Dimensional AgTe Monolayer Atomic Crystal on Ag(111) Substrate
Li Dong, Aiwei Wang, En Li, Qin Wang, Geng Li, Qing Huan, Hong-Jun Gao
Chin. Phys. Lett.    2019, 36 (2): 028102.   DOI: 10.1088/0256-307X/36/2/028102
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We report on the formation of two-dimensional monolayer AgTe crystal on Ag(111) substrates. The samples are prepared in ultrahigh vacuum by deposition of Te on Ag(111) followed by annealing. Using a scanning tunneling microscope (STM) and low electron energy diffraction (LEED), we investigate the atomic structure of the samples. The STM images and the LEED pattern show that monolayer AgTe crystal is formed on Ag(111). Four kinds of atomic structures of AgTe and Ag(111) are observed: (i) flat honeycomb structure, (ii) bulked honeycomb, (iii) stripe structure, (iv) hexagonal structure. The structural analysis indicates that the formation of the different atomic structures is due to the lattice mismatch and relief of the intrinsic strain in the AgTe layer. Our results provide a simple and convenient method to produce monolayer AgTe atomic crystal on Ag(111) and a template for study of novel physical properties and for future quantum devices.
Finite-Key Analysis for a Practical High-Dimensional Quantum Key Distribution System Based on Time-Phase States
Ya-Hui Gan, Yang Wang, Wan-Su Bao, Ru-Shi He, Chun Zhou, Mu-Sheng Jiang
Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/040301
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High-dimensional quantum states key distribution (HD-QKD) can enable more than one bit per photon and tolerate more noise. Recently, a practical HD-QKD system based on time-phase states has provided a secret key at Mbps over metropolitan distances. For the purposes of further improving the secret key rate of a practical HD-QKD system, we make two main contributions in this work. Firstly, we present an improved parameter estimation for this system in the finite-key scenario based on the Chernoff bound and the improved Chernoff bound. Secondly, we analyze how the dimension $d$ affects the performance of the practical HD-QKD system. We present numerical simulations about the secret key rate of the practical HD-QKD system based on different parameter estimation methods. It is found that using the improved Chernoff bound can improve the secret key rate and maximum channel loss of the practical HD-QKD system. In addition, a mixture of the 4-level and 8-level practical HD-QKD system can provide better performance in terms of the key generation rate over metropolitan distances.
An Atomic Magnetometer with Spin-Projection Noise Proportional to $\sqrt{{T_2}}$
Hai-Feng Dong, Xiao-Fei Wang, Ji-Min Li, Jing-Ling Chen, Yuan Ren
Chin. Phys. Lett.    2019, 36 (2): .   DOI: 10.1088/0256-307X/36/2/020701
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There is a common sense view for atomic magnetometers that their spin-projection-noises (SPNs) are inversely proportional to $\sqrt{{T_2}}$, where $T_2$ is the transverse relaxation time. We analyze the current atomic magnetometer types and give a counter-example of this common sense, which is the all-optical spin precession modulated three-axis atomic magnetometer proposed by our group in 2015. Unlike the other atomic magnetometers, the SPN of this kind of atomic magnetometers is proportional to $\sqrt{{T_2}}$ due to the fact that the scale factor between $P_x$ and $B$ can be unrelated to the transverse relaxation time $T_2$. We demonstrate this irrelevance experimentally and analyze the SPN theoretically. Using short-pulse ultra-high power laser to fully polarize the atoms, the phenomenon that SPN decreases with $T_2$ may also be demonstrated experimentally and a new tool for researching SPN in atomic magnetometers may be realized.
Coherent Coupling between Microwave and Optical Fields via Cold Atoms
Zhen-Tao Liang, Qing-Xian Lv, Shan-Chao Zhang, Wei-Tao Wu, Yan-Xiong Du, Hui Yan, Shi-Liang Zhu
Chin. Phys. Lett.    2019, 36 (8): .   DOI: 10.1088/0256-307X/36/8/080301
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We demonstrate a long-coherent-time coupling between microwave and optical fields through cold atomic ensembles. The phase information of the microwave field is stored in a coherent superposition state of a cold atomic ensemble and is then read out by two optical fields after 12 ms. A similar operation of mapping the phase of optical fields into a cold atomic ensemble and then retrieving by microwave is also demonstrated. These studies demonstrate that long-coherent-time cold atomic ensembles could resonantly couple with microwave and optical fields simultaneously, which paves the way for realizing high-efficiency, high-bandwidth, and noiseless atomic quantum converters.
Construction of Complete Orthogonal Genuine Multipartite Entanglement State
Feng-Lin Wu, Si-Yuan Liu, Wen-Li Yang, Heng Fan
Chin. Phys. Lett.    2019, 36 (6): .   DOI: 10.1088/0256-307X/36/6/060301
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With the development of quantum information processing, multipartite entanglement measures are needed in many cases. However, there are still no complete orthogonal genuine multipartite entanglement (GME) bases available as Bell states to bipartite systems. To achieve this goal, we find a method to construct complete orthogonal GME states, and we exclude many equivalent states by leveraging the group theory. We also provide the case of a $3$-order $3$-dimensional Hilbert space as an example and study the application of general results in the dense coding scheme as an application. Moreover, we discuss some open questions and believe that this work will enlighten extensive studies in this field.
Ultrafast Carrier Dynamics and Terahertz Photoconductivity of Mixed-Cation and Lead Mixed-Halide Hybrid Perovskites
Wan-Ying Zhao, Zhi-Liang Ku, Li-Ping Lv, Xian Lin, Yong Peng, Zuan-Ming Jin, Guo-Hong Ma, Jian-Quan Yao
Chin. Phys. Lett.    2019, 36 (2): .   DOI: 10.1088/0256-307X/36/2/028401
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Using time-dependent terahertz spectroscopy, we investigate the role of mixed-cation and mixed-halide on the ultrafast photoconductivity dynamics of two different methylammonium (MA) lead-iodide perovskite thin films. It is found that the dynamics of conductivity after photoexcitation reveals significant correlation on the microscopy crystalline features of the samples. Our results show that mixed-cation and lead mixed-halide affect the charge carrier dynamics of the lead-iodide perovskites. In the (5-AVA)$_{0.05}$(MA)$_{0.95}$PbI$_{2.95}$Cl$_{0.05}$/spiro thin film, we observe a much weaker saturation trend of the initial photoconductivity with high excitation fluence, which is attributed to the combined effect of sequential charge carrier generation, transfer, cooling and polaron formation.
Bound-State Soliton Solutions of the Nonlinear Schrödinger Equation and Their Asymmetric Decompositions
Yong-Shuai Zhang, Jing-Song He
Chin. Phys. Lett.    2019, 36 (3): .   DOI: 10.1088/0256-307X/36/3/030201
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We study the asymmetric decompositions of bound-state (BS) soliton solutions to the nonlinear Schrödinger equation. Assuming that the BS solitons are split into multiple solitons with different displacements, we obtain more accurate decompositions compared to the symmetric decompositions. Through graphical techniques, the asymmetric decompositions are shown to overlap very well with the real trajectories of the BS soliton solutions.
Theoretical Proposal for a Planar Single-Layer Carbon That Shows a Potential in Superconductivity
Yan-Ming Ma
Chin. Phys. Lett.    2019, 36 (9): .   DOI: 10.1088/0256-307X/36/9/090101
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Nanosecond Pulses Generation with Samarium Oxide Film Saturable Absorber
N. F. Zulkipli, M. Batumalay, F. S. M. Samsamnun, M. B. H. Mahyuddin, E. Hanafi, T. F. T. M. N. Izam, M. I. M. A. Khudus, S. W. Harun
Chin. Phys. Lett.    2019, 36 (7): .   DOI: 10.1088/0256-307X/36/7/074203
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Nanosecond pulse generation is demonstrated in a mode-locked erbium-doped fiber laser (EDFL) utilizing a samarium oxide (Sm$_{2}$O$_{3}$) film. The Sm$_{2}$O$_{3}$ film exhibits a modulation depth of 33%, which is suitable for mode-locking operation. The passively pulsed EDFL operates stably at 1569.8 nm within a pumping power from 109 to 146 mW. The train of generated output pulses has a pulse width of 356 nm repeated at a fundamental frequency of 0.97 MHz. The average output power of 3.91 mW is obtained at a pump power of 146 mW, corresponding to 4.0 nJ pulse energy. The experimental result indicates that the proposed Sm$_{2}$O$_{3}$ saturable absorber is viable for the construction of a flexible and reliably stable mode-locked pulsed fiber laser operating in the 1.5 μm region.
Observation of Two-Level Critical State in the Superconducting FeTe Thin Films$^*$
Hao Ru, Yi-Shi Lin, Yin-Cong Chen, Yang Feng, Yi-Hua Wang
Chin. Phys. Lett.    2019, 36 (7): 077402.   DOI: 10.1088/0256-307X/36/7/077402
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FeTe, a non-superconducting parent compound in the iron-chalcogenide family, becomes superconducting after annealing in oxygen. Under the presence of magnetism, spin-orbit coupling, inhomogeneity and lattice distortion, the nature of its superconductivity is not well understood. Here we combine the mutual inductance technique with magneto transport to study the magnetization and superconductivity of FeTe thin films. It is found that the films with the highest $T_{\rm C}$ show non-saturating superfluid density and a strong magnetic hysteresis distinct from that in a homogeneous superconductor. Such a hysteresis can be well explained by a two-level critical state model and suggests the importance of granularity to superconductivity in this compound.
The Unconventional Influence of a Nearby Molecule onto Transport of Single C$_{60}$ Molecule Transistor
Xiao Guo, Wen-jie Liang
Chin. Phys. Lett.    2019, 36 (12): 127301.   DOI: 10.1088/0256-307X/36/12/127301
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We study the transport property of single C$_{60}$ molecular transistors with special focus on the situation that other molecules are in vicinity. The devices are prepared using electromigration and thermal deposition techniques. Pure single C$_{60}$ molecule transistors show typical coulomb blockade behavior at low temperature. When we increase the coverage of molecules slightly by extending the deposition time, the transport spectrum of devices displays a switching behavior in the general coulomb blockade pattern. We attribute this unconventional phenomenon to the influence from a nearby C$_{60}$ molecule. By analyzing this transport behavior quantitatively based on the parallel-double-quantum-dot model, the interaction from the nearby molecule is proved to be of capacity and tunneling coupling. Thermal stimulation is also applied to the device to investigate the effect of local charging environment variation on intermolecular interaction.
Multi-Path Ghost Imaging by Means of an Additional Time Correlation
Rui Liu, Ling-Jun Kong, Yu Si, Zhou-Xiang Wang, Wen-Rong Qi, Chenghou Tu, Yongnan Li, Hui-Tian Wang
Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/044205
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Ghost imaging functions achieved by means of the spatial correlations between two photons is a new modality in imaging systems. With a small number of photons, ghost imaging is usually realized based on the position correlation of photon pairs produced from the spontaneous parametric down-conversion process. Here we demonstrate a way to realize multi-path ghost imaging by introducing an additional time correlation. Different delays of paths will induce the shift of the coincidence peak, which carries the information about objects. By choosing the suitable coincidence window, we obtain images of three objects simultaneously, with a visibility of 87.2%. This method provides insights and techniques into multi-parameter ghost imaging. It can be applied to other correlated imaging systems, for example, quantum spiral imaging.
Double Resonance Raman Scattering in Single-Layer MoSe$_{2}$ under Moderate Pressure
Jian-mei Li, Yi-kun Yao, Li-huan Sun, Xin-yan Shan, Cong Wang, Xing-hua Lu
Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/048201
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Pressure-dependent properties in layered transition-dichalcogenides are important for our understanding of their basic structures and applications. We investigate the electronic structure in MoSe$_{2}$ monolayer under external pressure up to 5.73 GPa by Raman spectroscopy and photoluminescence (PL) spectroscopy. The double resonance out-of-plane acoustic mode ($2ZA$) phonon is observed in Raman spectroscopy near 250 cm$^{-1}$, which presents pronounced intensity and pressure dependence. Significant variation in $2ZA$ peak intensity under different pressures reflects the change in electronic band structure as pressure varies, which is consistent with the blue shift in PL spectroscopy. The high sensitivity in both Raman and PL spectroscopy under moderate pressure in such a two-dimensional material may have many advantages for optoelectronic applications.
Tripartite Entanglement Measures of Generalized GHZ State in Uniform Acceleration
Qian Dong, M. A. Mercado Sanchez, Guo-Hua Sun, Mohamad Toutounji, Shi-Hai Dong
Chin. Phys. Lett.    2019, 36 (10): .   DOI: 10.1088/0256-307X/36/10/100301
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Using the single-mode approximation, we study entanglement measures including two independent quantities; i.e., negativity and von Neumann entropy for a tripartite generalized Greenberger–Horne–Zeilinger (GHZ) state in noninertial frames. Based on the calculated negativity, we study the whole entanglement measures named as the algebraic average $\pi_{3}$-tangle and geometric average ${\it \Pi}_{3}$-tangle. We find that the difference between them is very small or disappears with the increase of the number of accelerated qubits. The entanglement properties are discussed from one accelerated observer and others remaining stationary to all three accelerated observers. The results show that there will always exist entanglement, even if acceleration $r$ arrives to infinity. The degree of entanglement for all 1–1 tangles are always equal to zero, but 1–2 tangles always decrease with the acceleration parameter $r$. We notice that the von Neumann entropy increases with the number of the accelerated observers and $S_{\kappa_{\rm I}\zeta_{\rm I}}$ ($\kappa, \zeta\in ({\rm A, B, C})$) first increases and then decreases with the acceleration parameter $r$. This implies that the subsystem $\rho_{\kappa_{\rm I}\zeta_{\rm I}}$ is first more disorder and then the disorder will be reduced as the acceleration parameter $r$ increases. Moreover, it is found that the von Neumann entropies $S_{\rm ABCI}$, $S_{\rm ABICI}$ and $S_{\rm AIBICI}$ always decrease with the controllable angle $\theta$, while the entropies of the bipartite subsystems $S_{2-2_{\rm non}}$ (two accelerated qubits), $S_{2-1_{\rm non}}$ (one accelerated qubit) and $S_{2-0_{\rm non}}$ (without accelerated qubit) first increase with the angle $\theta$ and then decrease with it.
Experimental Point-to-Multipoint Plug-and-Play Measurement-Device-Independent Quantum Key Distribution Network
Guang-Zhao Tang, Shi-Hai Sun, Chun-Yan Li
Chin. Phys. Lett.    2019, 36 (7): .   DOI: 10.1088/0256-307X/36/7/070301
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Measurement-device-independent quantum key distribution (MDI-QKD) offers a practical way to realize a star-type quantum network. Previous experiments on MDI-QKD networks can only support the point-to-point communication. We experimentally demonstrate a plug-and-play MDI-QKD network which can support the point-to-multipoint communication among three users. Benefiting from the plug-and-play MDI-QKD architecture, the whole network is automatically stabilized in spectrum, polarization, arrival time, and phase reference. The users only need the encoding devices, which means that the hardware requirements are greatly reduced. Our experiment shows that it is feasible to establish a point-to-multipoint MDI-QKD network.
Temperature-Dependent Dielectric Characterization of Magneto-Optical Tb$_{3}$Sc$_{2}$Al$_{3}$O$_{12}$ Crystal Investigated by Terahertz Time-Domain Spectroscopy
Ju-Geng Li, Sen-Miao Yang, Xin Chen, Nai-Feng Zhuang, Qi-Biao Zhu, An-Hua Wu, Xian Lin, Guo-Hong Ma, Zuan-Ming Jin, Jian-Quan Yao
Chin. Phys. Lett.    2019, 36 (4): .   DOI: 10.1088/0256-307X/36/4/044203
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Terbium scandium aluminum garnet (TSAG) crystals have been widely used in magneto-optical systems. We investigate the complex refractive index of the TSAG crystal in the terahertz frequency range using terahertz (THz) time-domain spectroscopy in the temperature range 100–300 K. It is observed that the refractive index and the absorption coefficient increase with the THz frequency. The refractive index increases with the temperature. We measure the temperature coefficient of the refractive index of the TSAG crystal in the frequency range 0.4–1.4 THz. Furthermore, the loss tangent, i.e., the ratio of experimental values of the imaginary and real part of the dielectric permittivity, is found to be almost independent of frequency. TSAG is very promising for applications in THz optoelectronics because it has a high dielectric constant, low loss, and low thermal coefficient of the dielectric constant.
Eigenstate Distribution Fluctuation of a Quenched Disordered Bose–Hubbard System in Thermal-to-Localized Transitions
Junjun Xu, Yanxing Li
Chin. Phys. Lett.    2019, 36 (2): .   DOI: 10.1088/0256-307X/36/2/027201
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We study the thermalization of a quenched disordered Bose–Hubbard system. By considering the eigenstate distribution fluctuation, we show that the thermal to many-body localized transition is always connected to a minimum of this distribution fluctuation. We also observe a Mott-localized regime, where the system fails to thermalize due to the strong on-site repulsion. Lastly, we show how to detect this eigenstate distribution fluctuation in a cold atom system, which is equivalent to measure the Loschmidt echo of the system. Our work suggests a way to measure the thermal-to-localized transitions in experiments, especially for a large system.