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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.
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.
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.
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.
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.
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$2 meV. These observations provide important information for studying the relationship between the Fermi surface topology and superconductivity, and the layer-dependent superconductivity in high temperature cuprate superconductors.
Superconductivity and Fermi Surface Anisotropy in Transition Metal Dichalcogenide NbTe$_{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.
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.
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.
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.
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.
Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd$_{3}$As$_{2}$
Fei Sun, M. Yang, M. W. Yang, Q. Wu, H. Zhao, X. Ye, Youguo Shi, Jimin Zhao
Chin. Phys. Lett.    2018, 35 (11): .   DOI: 10.1088/0256-307X/35/11/116301
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Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd$_{3}$As$_{2}$ is carried out. An acoustic phonon (AP) mode with central frequency $f=0.037$ THz (i.e., 1.23 cm$^{-1}$ or 0.153 meV) is unambiguously generated and detected, which we attribute to laser-induced thermal strain. An AP chirping (i.e., variation of the phonon frequency) is clearly detected, which is ascribed to heat capacity variation with time. By comparing our experimental results and the theoretical model, we obtain a chirping time constant, which is 31.2 ps at 6 K and 19.8 ps at 300 K, respectively. Significantly, we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping, instead of a Fano resonance. Moreover, we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence, as well as temperature, which endows Cd$_{3}$As$_{2}$ application potentials in thermoelectric devices.
Solutions to Nonlocal Integrable Discrete Nonlinear Schrödinger Equations via Reduction
Ya-Hong Hu, Jun-Chao Chen
Chin. Phys. Lett.    2018, 35 (11): .   DOI: 10.1088/0256-307X/35/11/110201
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Solutions to local and nonlocal integrable discrete nonlinear Schrödinger (IDNLS) equations are studied via reduction on the bilinear form. It is shown that these solutions to IDNLS equations can be expressed in terms of the single Casorati determinant under different constraint conditions.
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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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.
Endoscopic Optical Doppler Tomography Based on Two-Axis Scanning MEMS Mirror
Dong-Lin Wang, Yun-Qi Hao, Feng-Xiao Zhai, Kun Yang, Hong-Qiong Liu, Qiao Chen, Huikai Xie
Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/120701
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We study the feasibility of endoscopic optical Doppler tomography with a micro-electro-mechanical system (MEMS) mirror based probe. The additional phase shifts introduced by the probe are tracked and formulated. The suppression method of the probe phase shifts is proposed and validated by fluid flow detection experiments. In vivo blood flow detection is also implemented on a hairless mouse. The velocities of the blood flow in two directions are obtained to be $-$8.1 mm/s and 6.6 mm/s, respectively.
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.
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.
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.
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.
Magnetotransport Properties of a Nodal Line Semimetal TiSi
Moran Gao, Junbao He, Wenliang Zhu, Shuai Zhang, Xinmin Wang, Jing Li, Chaoyang Ma, Hui Liang, Zhian Ren, Genfu Chen
Chin. Phys. Lett.    2018, 35 (11): .   DOI: 10.1088/0256-307X/35/11/117101
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We report the magnetoresistance (MR), de Haas-van Alphen (dHvA) effect and Hall effect measurements on a single crystal of TiSi, which is predicted to be a nodal line semimetal. With application of a magnetic field, a metal-to-insulator-like transition in $\rho(T)$ and a nonsaturating MR are observed at low temperatures. The dHvA oscillations reveal a small Fermi-surface pocket with a nontrivial Berry phase. The analysis of the nonlinear Hall resistivity shows that TiSi is a multiband system with low carrier densities and high mobilities. All these results unambiguously prove the existence of Dirac fermions in TiSi.
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.
Properties of One-Dimensional Highly Polarized Fermi Gases
Ya-Dong Song, Xiao-Ming Cai
Chin. Phys. Lett.    2018, 35 (11): .   DOI: 10.1088/0256-307X/35/11/110301
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Using both the exact Bethe ansatz method and the variational method, we study properties of the one-dimensional Fermi polaron. We focus on the binding energy, effective mass, momentum distributions, Tan contact and correlation functions. As the attraction increases, the impurity is more tightly bound and correlated with the surrounding particles, and the size of formed polaron decreases. In addition, compared with the Bethe ansatz method, the variational method is totally qualified to study the one-dimensional Fermi polaron. The intrinsic reason is that the number of particle-hole excitations in a Fermi sea, caused by a single impurity, is always rather small. The variational method can be well extended to other impurity systems.
Sub-Doppler Laser Cooling of $^{23}$Na in Gray Molasses on the $D_{2}$ Line
Zhenlian Shi, Ziliang Li, Pengjun Wang, Zengming Meng, Lianghui Huang, Jing Zhang
Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/123701
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We report on the efficient gray molasses cooling of sodium atoms using the $D_{2}$ optical transition at 589.1 nm. Thanks to the hyperfine split about 6${\it \Gamma}$ between $|F'=2\rangle$ and $|F'=3\rangle$ in the excited state 3$^{2}P_{3/2}$, this atomic transition is effective for the gray molasses cooling mechanism. Using this cooling technique, the atomic sample in $F=2$ ground manifold is cooled from 700 $\mu$K to 56 $\mu$K in 3.5 ms. We observe that the loading efficiency into magnetic trap is increased due to the lower temperature and high phase space density of atomic cloud after gray molasses. This technique offers a promising route for the fast cooling of the sodium atoms in the $F=2$ state.
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.
Erratum and Note: Measurement of Zeeman Shift of Cesium Atoms Using an Optical Nanofiber [Chin. Phys. Lett. 35(2018)083201]
Chuan-Biao Zhang, Dian-Qiang Su, Zhong-Hua Ji, Yan-Ting Zhao, Lian-Tuan Xiao, Suo-Tang Jia
Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/129901
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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.
Finite Volume Time Domain with the Green Function Method for Electromagnetic Scattering in Schwarzschild Spacetime
Shou-Qing Jia
Chin. Phys. Lett.    2019, 36 (1): .   DOI: 10.1088/0256-307X/36/1/010401
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The finite volume time domain (FVTD) algorithm and Green function algorithm are extended to Schwarzschild spacetime for numerical simulation of electromagnetic scattering. The FVTD method in Schwarzschild spacetime is developed by filling the flat spacetime with an equivalent medium. The Green function in Schwarzschild spacetime is acquired by solving initial value problems. Both the FVTD code and the Green function code are validated by numerical results. Scattering in Schwarzschild spacetime is simulated with these methods.
Relativistic Spherical Plasma Waves in a Collisional and Warm Plasma
Zhong-Kui Kuang, Li-Hong Cheng, Pan-Fei Geng, Rong-An Tang, Ju-Kui Xue
Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/125202
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Under Lagrange coordinates, the relativistic spherical plasma wave in a collisional and warm plasma is discussed theoretically. Within the Lagrange coordinates and using the Maxwell and hydrodynamics equations, a wave equation describing the relativistic spherical wave is derived. The damped oscillating spherical wave solution is obtained analytically using the perturbation theory. Because of the coupled effects of spherical geometry, thermal pressure, and collision effect, the electron damps the periodic oscillation. The oscillation frequency and the damping rate of the wave are related to not only the collision and thermal pressure effect but also the space coordinate. Near the center of the sphere, the thermal pressure significantly reduces the oscillation period and the damping rate of the wave, while the collision effect can strongly influence the damping rate. Far away from the spherical center, only the collision effect can reduce the oscillation period of the wave, while the collision effect and thermal pressure have weak influence on the damping rate.
Shaping the Wavefront of Incident Light with a Strong Robustness Particle Swarm Optimization Algorithm
Bi-Qi Li, Bin Zhang, Qi Feng, Xiao-Ming Cheng, Ying-Chun Ding, Qiang Liu
Chin. Phys. Lett.    2018, 35 (12): .   DOI: 10.1088/0256-307X/35/12/124201
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We demonstrate a modified particle swarm optimization (PSO) algorithm to effectively shape the incident light with strong robustness and short optimization time. The performance of the modified PSO algorithm and genetic algorithm (GA) is numerically simulated. Then, using a high speed digital micromirror device, we carry out light focusing experiments with the modified PSO algorithm and GA. The experimental results show that the modified PSO algorithm has greater robustness and faster convergence speed than GA. This modified PSO algorithm has great application prospects in optical focusing and imaging inside in vivo biological tissue, which possesses a complicated background.