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 Select Resonant Quantum Search with Monitor Qubits Frank Wilczek, Hong-Ye Hu, Biao Wu Chin. Phys. Lett.    2020, 37 (5): 050304 .   DOI: 10.1088/0256-307X/37/5/050304 Abstract   HTML   PDF (582KB) We present an algorithm for the generalized search problem (searching $k$ marked items among $N$ items) based on a continuous Hamiltonian and exploiting resonance. This resonant algorithm has the same time complexity $O(\sqrt{N/k})$ as the Grover algorithm. A natural extension of the algorithm, incorporating auxiliary "monitor" qubits, can determine $k$ precisely, if it is unknown. The time complexity of our counting algorithm is $O(\sqrt{N})$, similar to the best quantum approximate counting algorithm, or better, given appropriate physical resources.
 Select Pressure-Dependent Point-Contact Spectroscopy of Superconducting PbTaSe$_2$ Single Crystals Hai Zi, Ling-Xiao Zhao, Xing-Yuan Hou, Lei Shan, Zhian Ren, Gen-Fu Chen, and Cong Ren Chin. Phys. Lett.    2020, 37 (9): 097403 .   DOI: 10.1088/0256-307X/37/9/097403 Abstract   HTML   PDF (5795KB) We develop an experimental tool to investigate the order parameter of superconductors by combining point-contact spectroscopy measurement with high-pressure technique. It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe$_2$ single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar. Under such a high pressure, the normal-state contact resistance is sensitive to the applied pressure, reflecting the underlying variation of contact transparency upon pressures. In a superconducting state, the pressure dependence of the energy gap $\varDelta_0$ and the critical temperature $T_{\rm c}$ for gap opening/closing are extracted based on a generalized Blond–Tinkham–Klapwijk model. The gap ratio $2\varDelta_0/k_{_{\rm B}}T_{\rm c}$ indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe$_2$. Our experimental results show the accessibility and validity of high-pressure point-contact spectroscopy, offering rich information about high-pressure superconductivity.
 Select Electro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical Etching Jun-xia Zhou, Ren-hong Gao, Jintian Lin, Min Wang, Wei Chu, Wen-bo Li, Di-feng Yin, Li Deng, Zhi-wei Fang, Jian-hao Zhang, Rong-bo Wuand Ya Cheng Chin. Phys. Lett.    2020, 37 (8): 084201 .   DOI: 10.1088/0256-307X/37/8/084201 Abstract   HTML   PDF (1221KB) Optical true delay lines (OTDLs) of low propagation losses, small footprints and high tuning speeds and efficiencies are of critical importance for various photonic applications. Here, we report fabrication of electro-optically switchable OTDLs on lithium niobate on insulator using photolithography assisted chemo-mechanical etching. Our device consists of several low-loss optical waveguides of different lengths which are consecutively connected by electro-optical switches to generate different amounts of time delay. The fabricated OTLDs show an ultra-low propagation loss of $\sim 0.03$ dB/cm for waveguide lengths well above 100 cm.
 Select Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn$_2$Bi$_2$Te$_5$ Jinlong Zhang, Dinghui Wang, Minji Shi, Tongshuai Zhu, Haijun Zhang, Jing Wang Chin. Phys. Lett.    2020, 37 (7): 077304 .   DOI: 10.1088/0256-307X/37/7/077304 Abstract   HTML   PDF (2348KB) The dynamical axion field is a new state of quantum matter where the magnetoelectric response couples strongly to its low-energy magnetic fluctuations. It is fundamentally different from an axion insulator with a static quantized magnetoelectric response. The dynamical axion field exhibits many exotic phenomena such as axionic polariton and axion instability. However, these effects have not been experimentally confirmed due to the lack of proper topological magnetic materials. Combining analytic models and first-principles calculations, here we predict a series of van der Waals layered Mn$_2$Bi$_2$Te$_5$-related topological antiferromagnetic materials that could host the long-sought dynamical axion field with a topological origin. We also show that a large dynamical axion field can be achieved in antiferromagnetic insulating states close to the topological phase transition. We further propose the optical and transport experiments to detect such a dynamical axion field. Our results could directly aid and facilitate the search for topological-origin large dynamical axion field in realistic materials.
 Select Pressure-Induced Topological and Structural Phase Transitions in an Antiferromagnetic Topological Insulator Cuiying Pei, Yunyouyou Xia, Jiazhen Wu, Yi Zhao, Lingling Gao, Tianping Ying, Bo Gao, Nana Li, Wenge Yang, Dongzhou Zhang, Huiyang Gou, Yulin Chen, Hideo Hosono, Gang Li, Yanpeng Qi Chin. Phys. Lett.    2020, 37 (6): 066401 .   DOI: 10.1088/0256-307X/37/6/066401 Abstract   HTML   PDF (5392KB) Recently, natural van der Waals heterostructures of (MnBi$_{2}$Te$_{4}$)$_{m}$(Bi$_{2}$Te$_{3}$)$_{n}$ have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states. We systematically investigate both the structural and electronic responses of MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ to external pressure. In addition to the suppression of antiferromagnetic order, MnBi$_{2}$Te$_{4}$ is found to undergo a metal–semiconductor–metal transition upon compression. The resistivity of MnBi$_{4}$Te$_{7}$ changes dramatically under high pressure and a non-monotonic evolution of $\rho (T)$ is observed. The nontrivial topology is proved to persist before the structural phase transition observed in the high-pressure regime. We find that the bulk and surface states respond differently to pressure, which is consistent with the non-monotonic change of the resistivity. Interestingly, a pressure-induced amorphous state is observed in MnBi$_{2}$Te$_{4}$, while two high-pressure phase transitions are revealed in MnBi$_{4}$Te$_{7}$. Our combined theoretical and experimental research establishes MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$ as highly tunable magnetic topological insulators, in which phase transitions and new ground states emerge upon compression.
 Select Imaginary Time Crystal of Thermal Quantum Matter Zi Cai, Yizhen Huang, W. Vincent Liu Chin. Phys. Lett.    2020, 37 (5): 050503 .   DOI: 10.1088/0256-307X/37/5/050503 Abstract   HTML   PDF (2767KB) Temperature is a fundamental thermodynamic variable for matter. Physical observables are often found to either increase or decrease with it, or show a non-monotonic dependence with peaks signaling underlying phase transitions or anomalies. Statistical field theory has established connection between temperature and time: a quantum ensemble with inverse temperature $\beta$ is formally equivalent to a dynamic system evolving along an imaginary time from 0 to $i\beta$ in the space one dimension higher. Here we report that a gas of hard-core bosons interacting with a thermal bath manifests an unexpected temperature-periodic oscillation of its macroscopic observables, arising from the microscopic origin of space-time locked translational symmetry breaking and crystalline ordering. Such a temperature crystal, supported by quantum Monte Carlo simulation, generalizes the concept of purely spatial density-wave order to the imaginary time axis for Euclidean action.
 Select A New Path to Improve High $\beta_{\rm p}$ Plasma Performance on EAST for Steady-State Tokamak Fusion Reactor Baonian Wan and the EAST team Chin. Phys. Lett.    2020, 37 (4): 045202 .   DOI: 10.1088/0256-307X/37/4/045202 Abstract   HTML   PDF (573KB) High $\beta_{\rm p}$ scenario is foreseen to be a promising candidate operational mode for steady-state tokamak fusion reactors. Dedicated experiments on EAST and data analysis find that density gradient $\nabla n$ is a control knob to improve energy confinement in high $\beta_{\rm p}$ plasmas at low toroidal rotation as projected for a fusion reactor. Different from previously known turbulent stabilization mechanisms such as ${\boldsymbol E} \times {\boldsymbol B}$ shear and Shafranov shift, high density gradient can enhance the Shafranov shift stabilizing effect significantly in high $\beta_{\rm p}$ regime, giving that a higher density gradient is readily accessible in future fusion reactors with lower collisionality. This new finding is of great importance for the next-step fusion development because it may open a new path towards even higher energy confinement in the high $\beta_{\rm p}$ scenario. It has been demonstrated in the recent EAST experiments, i.e., a fully non-inductive high $\beta_{\rm p}$ ($\sim$2) H-mode plasma ($H_{98y2}\ge 1.3$) has been obtained for a duration over 100 current diffusion times, which sets another new world record of long-pulse high-performance tokamak plasma operation with the normalized performance approaching the ITER and CFETR regimes.
 Select Giant Spin Transfer Torque in Atomically Thin Magnetic Bilayers Weihao Cao, Matisse Wei-Yuan Tu, Jiang Xiao, and Wang Yao Chin. Phys. Lett.    2020, 37 (10): 107201 .   DOI: 10.1088/0256-307X/37/10/107201 Abstract   HTML   PDF (1297KB) In cavity quantum electrodynamics, the multiple reflections of a photon between two mirrors defining a cavity is exploited to enhance the light-coupling of an intra-cavity atom. We show that this paradigm for enhancing the interaction of a flying particle with a localized object can be generalized to spintronics based on van der Waals 2D magnets. Upon tunneling through a magnetic bilayer, we find that the spin transfer torques per electron incidence can become orders of magnitude larger than $\hbar /2$, made possible by electron's multi-reflection path through the ferromagnetic monolayers as an intermediate of their angular momentum transfer. Over a broad energy range around the tunneling resonances, the damping-like spin transfer torque per electron tunneling features a universal value of $(\hbar/2)\tan (\theta /2)$, depending only on the angle $\theta$ between the magnetizations. These findings expand the scope of magnetization manipulations for high-performance and high-density storage based on van der Waals magnets.
 Select A Ubiquitous Thermal Conductivity Formula for Liquids, Polymer Glass, and Amorphous Solids Qing Xi, Jinxin Zhong, Jixiong He, Xiangfan Xu, Tsuneyoshi Nakayama, Yuanyuan Wang, Jun Liu, Jun Zhou, and Baowen Li Chin. Phys. Lett.    2020, 37 (10): 104401 .   DOI: 10.1088/0256-307X/37/10/104401 Abstract   HTML   PDF (1069KB) The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time. There have been several approaches to explain the thermal conductivities in these systems, for example, Bridgman's formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers. Here, we present a ubiquitous formula to calculate the thermal conductivities of liquids and amorphous solids in a unified way, and compare it with previous ones. The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data. Our formula not only provides a detailed microscopic mechanism of heat transfer in these systems, but also resolves the discrepancies between existing formulae and experimental data.
 Select Symmetry-Assisted Protection and Compensation of Hidden Spin Polarization in Centrosymmetric Systems Yingjie Zhang, Pengfei Liu, Hongyi Sun, Shixuan Zhao, Hu Xu, and Qihang Liu Chin. Phys. Lett.    2020, 37 (8): 087105 .   DOI: 10.1088/0256-307X/37/8/087105 Abstract   HTML   PDF (3117KB) It was recently noted that in certain nonmagnetic centrosymmetric compounds, spin–orbit interactions couple each local sector that lacks inversion symmetry, leading to visible spin polarization effects in the real space, dubbed “hidden spin polarization (HSP)”. However, observable spin polarization of a given local sector suffers interference from its inversion partner, impeding material realization and potential applications of HSP. Starting from a single-orbital tight-binding model, we propose a nontrivial way to obtain strong sector-projected spin texture through the vanishing hybridization between inversion partners protected by nonsymmorphic symmetry. The HSP effect is generally compensated by inversion partners near the ${\varGamma}$ point but immune from the hopping effect around the boundary of the Brillouin zone. We further summarize 17 layer groups that support such symmetry-assisted HSP and identify hundreds of quasi-2D materials from the existing databases by first-principle calculations, among which a group of rare-earth compounds LnIO (Ln = Pr, Nd, Ho, Tm, and Lu) serves as great candidates showing strong Rashba- and Dresselhaus-type HSP. Our findings expand the material pool for potential spintronic applications and shed light on controlling HSP properties for emergent quantum phenomena.
 Select Predicting the Potential Performance in P-Type SnS Crystals via Utilizing the Weighted Mobility and Quality Factor Wenke He , Bingchao Qin , and Li-Dong Zhao Chin. Phys. Lett.    2020, 37 (8): 087104 .   DOI: 10.1088/0256-307X/37/8/087104 Abstract   HTML   PDF (774KB) The figure of merit $ZT$ is the direct embodiment of thermoelectric performance for a given material. However, as an indicator of performance improvement, the only $ZT$ value is not good enough to identify its outstanding inherent properties, which are highly sought in thermoelectric community. Here, we utilize one powerful parameter to reveal the outstanding properties of a given material. The weighted mobility is used to estimate the carrier transports of p-type SnS crystals, including the differences in doping level, carrier scattering and electronic band structure. We analyze the difference in carrier scattering mechanism for different crystal forms with the same doping level, then evaluate and confirm the temperature-dependent evolution of electronic band structures in SnS. Finally, we calculate the quality factor $B$ based on the weighted mobility, and establish the relationship between $ZT$ and $B$ to further predict the potential performance in p-type SnS crystals with low cost and earth abundance, which can be realized through taking advantage of the inherent material property, thus improving $B$ factor to achieve optimal thermoelectric level.
 Select Fermionic Analogue of High Temperature Hawking Radiation in Black Phosphorus Hang Liu, Jia-Tao Sun, Chenchen Song, Huaqing Huang, Feng Liu, Sheng Meng Chin. Phys. Lett.    2020, 37 (6): 067101 .   DOI: 10.1088/0256-307X/37/6/067101 Abstract   HTML   PDF (1956KB) Time-periodic laser driving can create nonequilibrium states not accessible in equilibrium, opening new regimes in materials engineering and topological phase transitions. We report that black phosphorus (BP) exhibits spatially nonuniform topological Floquet–Dirac states under laser illumination, mimicking the "gravity" felt by fermionic quasiparticles in the same way as that for a Schwarzschild black hole (SBH). Quantum tunneling of electrons from a type-II Dirac cone (inside BH) to a type-I Dirac cone (outside BH) emits an SBH-like Planck radiation spectrum. The Hawking temperature $T_{\rm H}$ obtained for a fermionic analog of BH in the bilayer BP is approximately 3 K, which is several orders of magnitude higher than that in previous works. Our work sheds light on increasing $T_{\rm H}$ from the perspective of engineering 2D materials by time-periodic light illumination. The predicted SBH-like Hawking radiation, accessible in BP thin films, provides clues to probe analogous astrophysical phenomena in solids.
 Select Enhanced Ferromagnetism of CrI$_{3}$ Bilayer by Self-Intercalation Yu Guo , Nanshu Liu , Yanyan Zhao , Xue Jiang , Si Zhou, and Jijun Zhao  Chin. Phys. Lett.    2020, 37 (10): 107506 .   DOI: 10.1088/0256-307X/37/10/107506 Abstract   HTML   PDF (2562KB) Two-dimensional (2D) ferromagnets with high Curie temperature have long been the pursuit for electronic and spintronic applications. CrI$_{3}$ is a rising star of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling. Here we propose a general strategy of self-intercalation to achieve enhanced ferromagnetism in bilayer CrI$_{3}$. We show that filling either Cr or I atoms into the van der Waals gap of stacked and twisted CrI$_{3}$ bilayers can induce the double exchange effect and significantly strengthen the interlayer ferromagnetic coupling. According to our first-principles calculations, the intercalated native atoms act as covalent bridge between two CrI$_{3}$ layers and lead to discrepant oxidation states for the Cr atoms. These theoretical results offer a facile route to achieve high-Curie-temperature 2D magnets for device implementation.
 Select Metal to Orthogonal Metal Transition Chuang Chen, Xiao Yan Xu, Yang Qi, Zi Yang Meng Chin. Phys. Lett.    2020, 37 (4): 047103 .   DOI: 10.1088/0256-307X/37/4/047103 Abstract   HTML   PDF (1184KB) Orthogonal metal is a new quantum metallic state that conducts electricity but acquires no Fermi surface (FS) or quasiparticles, and hence orthogonal to the established paradigm of Landau's Fermi-liquid (FL). Such a state may hold the key of understanding the perplexing experimental observations of quantum metals that are beyond FL, i.e., dubbed non-Fermi-liquid (nFL), ranging from the Cu- and Fe-based oxides, heavy fermion compounds to the recently discovered twisted graphene heterostructures. However, to fully understand such an exotic state of matter, at least theoretically, one would like to construct a lattice model and to solve it with unbiased quantum many-body machinery. Here we achieve this goal by designing a 2D lattice model comprised of fermionic and bosonic matter fields coupled with dynamic $\mathbb{Z}_2$ gauge fields, and obtain its exact properties with sign-free quantum Monte Carlo simulations. We find that as the bosonic matter fields become disordered, with the help of deconfinement of the $\mathbb{Z}_2$ gauge fields, the system reacts with changing its nature from the conventional normal metal with an FS to an orthogonal metal of nFL without FS and quasiparticles and yet still responds to magnetic probe like an FL. Such a quantum phase transition from a normal metal to an orthogonal metal, with its electronic and magnetic spectral properties revealed, is calling for the establishment of new paradigm of quantum metals and their transition with conventional ones.
 Select Superconductivity of Lanthanum Superhydride Investigated Using the Standard Four-Probe Configuration under High Pressures Fang Hong, Liuxiang Yang, Pengfei Shan, Pengtao Yang, Ziyi Liu, Jianping Sun, Yunyu Yin, Xiaohui Yu, Jinguang Cheng, and Zhongxian Zhao Chin. Phys. Lett.    2020, 37 (10): 107401 .   DOI: 10.1088/0256-307X/37/10/107401 Abstract   HTML   PDF (688KB) Recently, the theoretically predicted lanthanum superhydride, LaH$_{10 \pm \delta}$, with a clathrate-like structure was successfully synthesized and found to exhibit a record high superconducting transition temperature $T_{\rm c} \approx 250$ K at $\sim$170 GPa, opening a new route for room-temperature superconductivity. However, since in situ experiments at megabar pressures are very challenging, few groups have reported the $\sim$250 K superconducting transition in LaH$_{10 \pm \delta}$. Here, we establish a simpler sample-loading procedure that allows a relatively large sample size for synthesis and a standard four-probe configuration for resistance measurements. Following this procedure, we successfully synthesized LaH$_{10 \pm \delta}$ with dimensions up to $10 \times 20$ μm$^{2}$ by laser heating a thin La flake and ammonia borane at $\sim$1700 K in a symmetric diamond anvil cell under the pressure of 165 GPa. The superconducting transition at $T_{\rm c} \approx 250$ K was confirmed through resistance measurements under various magnetic fields. Our method will facilitate explorations of near-room-temperature superconductors among metal superhydrides.
 Select Ferromagnetic MnSn Monolayer Epitaxially Grown on Silicon Substrate Qian-Qian Yuan, Zhaopeng Guo, Zhi-Qiang Shi, Hui Zhao, Zhen-Yu Jia, Qianjin Wang, Jian Sun, Di Wu, and Shao-Chun Li Chin. Phys. Lett.    2020, 37 (7): 077502 .   DOI: 10.1088/0256-307X/37/7/077502 Abstract   HTML   PDF (2498KB) Two-dimensional (2D) ferromagnetic materials have been exhibiting promising potential in applications, such as spintronics devices. To grow epitaxial magnetic films on silicon substrate, in the single-layer limit, is practically important but challenging. In this study, we realized the epitaxial growth of MnSn monolayer on Si(111) substrate, with an atomically thin Sn/Si(111)-$2\sqrt 3 \times 2\sqrt 3$-buffer layer, and controlled the MnSn thickness with atomic-layer precision. We discovered the ferromagnetism in MnSn monolayer with the Curie temperature ($T_{\rm c}$) of ${\sim} 54$ K. As the MnSn film is grown to 4 monolayers, $T_{\rm c}$ increases accordingly to ${\sim} 235$ K. The lattice of the epitaxial MnSn monolayer as well as the Sn/Si(111)-$2\sqrt 3 \times 2\sqrt 3$ is perfectly compatible with silicon, and thus an sharp interface is formed between MnSn, Sn and Si. This system provides a new platform for exploring the 2D ferromagnetism, integrating magnetic monolayers into silicon-based technology, and engineering the spintronics heterostructures.
 Select Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe$_{2}$ Ya-Ting Jia, Chun-Sheng Gong, Yi-Xuan Liu, Jian-Fa Zhao, Cheng Dong, Guang-Yang Dai, Xiao-Dong Li, He-Chang Lei, Run-Ze Yu, Guang-Ming Zhang, and Chang-Qing Jin Chin. Phys. Lett.    2020, 37 (9): 097404 .   DOI: 10.1088/0256-307X/37/9/097404 Abstract   HTML   PDF (1103KB) The Mott transition is one of the fundamental issues in condensed matter physics, especially in the system with antiferromagnetic long-range order. However, such a transition is rare in quantum spin liquid (QSL) systems without long-range order. Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe$_{2}$ with a triangular lattice of 4$f$ Yb$^{3+}$ ions. Detail analysis of transport properties in metallic state shows an evolution from non-Fermi liquid to Fermi liquid behavior when approaching the vicinity of superconductivity. An irreversible structure phase transition occurs around 11 GPa, which is revealed by the x-ray diffraction. These results shed light on the Mott transition in the QSL systems.
 Select Soliton Molecules and Some Hybrid Solutions for the Nonlinear Schrödinger Equation Bao Wang, Zhao Zhang, Biao Li Chin. Phys. Lett.    2020, 37 (3): 030501 .   DOI: 10.1088/0256-307X/37/3/030501 Abstract   HTML   PDF (719KB) Based on velocity resonance and Darboux transformation, soliton molecules and hybrid solutions consisting of soliton molecules and smooth positons are derived. Two new interesting results are obtained: the first is that the relationship between soliton molecules and smooth positons is clearly pointed out, and the second is that we find two different interactions between smooth positons called strong interaction and weak interaction, respectively. The strong interaction will only disappear when $t \to \infty$. This strong interaction can also excite some periodic phenomena.
 Select Negative Thermal Transport in Conduction and Advection Liujun Xu and Jiping Huang Chin. Phys. Lett.    2020, 37 (8): 080502 .   DOI: 10.1088/0256-307X/37/8/080502 Abstract   HTML   PDF (1064KB) Negative refractive index has drawn a great deal of attention due to its unique properties and practical applications in wave systems. To promote the related physics in thermotics, here we manage to coin a complex thermal conductivity whose imaginary part corresponds to the real part of complex refractive index. Therefore, the thermal counterpart of negative refractive index is just negative imaginary thermal conductivity, which is featured by the opposite directions of energy flow and wave vector in thermal conduction and advection, thus called negative thermal transport herein. To avoid violating causality, we design an open system with energy exchange and explore three different cases to reveal negative thermal transport. We further provide experimental suggestions with a solid ring structure. All finite-element simulations agree with theoretical analyses, indicating that negative thermal transport is physically feasible. These results have potential applications such as designing the inverse Doppler effect in thermal conduction and advection.
 Select A Two-Dimensional Architecture for Fast Large-Scale Trapped-Ion Quantum Computing Y.-K. Wu  and L.-M. Duan Chin. Phys. Lett.    2020, 37 (7): 070302 .   DOI: 10.1088/0256-307X/37/7/070302 Abstract   HTML   PDF (1064KB) Building blocks of quantum computers have been demonstrated in small to intermediate-scale systems. As one of the leading platforms, the trapped ion system has attracted wide attention. A significant challenge in this system is to combine fast high-fidelity gates with scalability and convenience in ion trap fabrication. Here we propose an architecture for large-scale quantum computing with a two-dimensional array of atomic ions trapped at such large distance which is convenient for ion-trap fabrication but usually believed to be unsuitable for quantum computing as the conventional gates would be too slow. Using gate operations far outside of the Lamb–Dicke region, we show that fast and robust entangling gates can be realized in any large ion arrays. The gate operations are intrinsically parallel and robust to thermal noise, which, together with their high speed and scalability of the proposed architecture, makes this approach an attractive one for large-scale quantum computing.
 Select High-Pressure Ultrafast Dynamics in Sr$_{2}$IrO$_{4}$: Pressure-Induced Phonon Bottleneck Effect Yanling Wu, Xia Yin, Jiazila Hasaien, Yang Ding, Jimin Zhao Chin. Phys. Lett.    2020, 37 (4): 047801 .   DOI: 10.1088/0256-307X/37/4/047801 Abstract   HTML   PDF (4486KB) By integrating pump-probe ultrafast spectroscopy with diamond anvil cell (DAC) technique, we demonstrate a time-resolved ultrafast dynamics study on non-equilibrium quasiparticle (QP) states in Sr$_{2}$IrO$_{4}$ under high pressure. On-site in situ condition is realized, where both the sample and DAC have fixed position during the experiment. The QP dynamics exhibits a salient pressure-induced phonon bottleneck feature at 20 GPa, which corresponds to a gap shrinkage in the electronic structure. A structural transition is also observed at 32 GPa. In addition, the slowest relaxation component reveals possible heat diffusion or pressure-controlled local spin fluctuation associated with the gap shrinkage. Our work enables precise pressure dependence investigations of ultrafast dynamics, paving the way for reliable studies of high-pressure excited state physics.
 Select High-Fidelity Manipulation of the Quantized Motion of a Single Atom via Stern–Gerlach Splitting Kun-Peng Wang, Jun Zhuang, Xiao-Dong He, Rui-Jun Guo, Cheng Sheng, Peng Xu, Min Liu, Jin Wang, Ming-Sheng Zhan Chin. Phys. Lett.    2020, 37 (4): 044209 .   DOI: 10.1088/0256-307X/37/4/044209 Abstract   HTML   PDF (3267KB) We demonstrate high-fidelity manipulation of the quantized motion of a single $^{87}$Rb atom in an optical tweezer via microwave couplings induced by Stern–Gerlach splitting. The Stern–Gerlach splitting is mediated by polarization gradient of a strongly focused tweezer beam that functions as fictitious magnetic field gradient. The spatial splitting removes the orthogonality of the atomic spatial wavefunctions, thus enables the microwave couplings between the motional states. We obtain coherent Rabi oscillations for up to third-order sideband transitions, in which a high fidelity of larger than $0.99$ is obtained for the spin-flip transition on the first order sideband after subtraction of the state preparation and detection error. The Stern–Gerlach splitting is measured at a precision of better than $0.05$ nm. This work paves the way for quantum engineering of motional states of single atoms, and may have wide applications in few body physics and ultracold chemistry.
 Select Asymptotical Locking Tomography of High-Dimensional Entanglement Ling-Jun Kong, Rui Liu, Wen-Rong Qi, Zhou-Xiang Wang, Shuang-Yin Huang, Chenghou Tu, Yongnan Li, Hui-Tian Wang Chin. Phys. Lett.    2020, 37 (3): 034204 .   DOI: 10.1088/0256-307X/37/3/034204 Abstract   HTML   PDF (7396KB) High-dimensional (HD) entanglement provides a very promising way of transcending the limitations of the two-dimensional entanglement between qubits for increasing channel capacity in many quantum protocols. In the pursuit of capitalizing on the HD entangled states, one of the central issues is to unambiguously and comprehensively quantify and reconstruct them. The full quantum state tomography is a unique solution, but it is undesirable and even impractical because the measurements increase rapidly in $d^4$ for a bipartite $d$-dimensional quantum state. Here we present a very efficient and practical tomography method—asymptotical locking tomography (ALT), which can harvest full information of bipartite $d$-dimensional entangled states by very few measurements less than $2d^2$ only. To showcase the validity and reasonableness of our ALT, we carry out the test with the two-photon spin-orbital angular momentum hyperentangled states in a four-dimensional subspace. Besides high-efficiency and practicality, our ALT is also universal and can be generalized into multipartite HD entanglement and other quantum systems.
 Select Lax Pairs of Integrable Systems in Bidifferential Graded Algebras Danda Zhang, Da-Jun Zhang, Sen-Yue Lou Chin. Phys. Lett.    2020, 37 (4): 040201 .   DOI: 10.1088/0256-307X/37/4/040201 Abstract   HTML   PDF (416KB) Lax pairs regarded as foundations of the inverse scattering methods play an important role in integrable systems. In the framework of bidifferential graded algebras, we propose a straightforward approach to constructing the Lax pairs of integrable systems in functional environment. Some continuous equations and discrete equations are presented.
 Select Anomalous Hall Effect in Layered Ferrimagnet MnSb$_{2}$Te$_{4}$ Gang Shi, Mingjie Zhang, Dayu Yan, Honglei Feng, Meng Yang, Youguo Shi, Yongqing Li Chin. Phys. Lett.    2020, 37 (4): 047301 .   DOI: 10.1088/0256-307X/37/4/047301 Abstract   HTML   PDF (682KB) We report on low-temperature electron transport properties of MnSb$_{2}$Te$_{4}$, a candidate of ferrimagnetic Weyl semimetal. Long-range magnetic order is manifested as a nearly square-shaped hysteresis loop in the anomalous Hall resistance, as well as sharp jumps in the magnetoresistance. At temperatures below 4 K, a ${\rm ln}T$-type upturn appears in the temperature dependence of longitudinal resistance, which can be attributed to the electron-electron interaction (EEI), since the weak localization can be excluded by the temperature dependence of magnetoresistance. Although the anomalous Hall resistance exhibits a similar ${\rm ln}T$-type upturn in the same temperature range, such correction is absent in the anomalous Hall conductivity. Our work demonstrates that MnSb$_{2}$Te$_{4}$ microflakes provide an ideal system to test the theory of EEI correction to the anomalous Hall effect.
 Select Controlling the Coffee Ring Effect on Graphene and Polymer by Cations Haijun Yang, Yizhou Yang, Shiqi Sheng, Binghai Wen, Nan Sheng, Xing Liu, Rongzheng Wan, Long Yan, Zhengchi Hou, Xiaoling Lei, Guosheng Shi, Haiping Fang Chin. Phys. Lett.    2020, 37 (2): 028103 .   DOI: 10.1088/0256-307X/37/2/028103 Abstract   HTML   PDF (3598KB) Recently, there are great efforts that have been taken to suppressing/controlling the coffee ring effect, but it is of challenge to achieve inexpensive and efficient control with less disturbance, suitable for scalable production and highly enhancing the printing/dyeing color fastness. By only adding trace amounts of salt into the suspensions, here we experimentally achieve the facile and highly efficient control of the coffee ring effect of suspended matter on substrates of graphene, natural graphite, and polyethylene terephthalate surfaces. Notably, friction force measurements show that ion-controlled uniform patterns also greatly enhance color fastness. Molecular dynamics simulations reveal that, due to strong hydrated cation-$\pi$ interactions between hydrated cations and aromatic rings in the substrate surface, the suspended matters are adsorbed on the surfaces mediated by cations so that the suspended matters are uniformly distributed. These findings will open new avenues for fabricating functional patterns on graphene substrates and will benefit practical applications including printing, coating, and dyeing.
 Select Measurement of Spin Singlet-Triplet Qubit in Quantum Dots Using Superconducting Resonator Xing-Yu Zhu, Tao Tu, Ao-Lin Guo, Zong-Quan Zhou, Guang-Can Guo Chin. Phys. Lett.    2020, 37 (2): 020302 .   DOI: 10.1088/0256-307X/37/2/020302 Abstract   HTML   PDF (636KB) The spin qubit in quantum dots is one of the leading platforms for quantum computation. A crucial requirement for scalable quantum information processing is the high efficient measurement. Here we analyze the measurement process of a quantum-dot spin qubit coupled to a superconducting transmission line resonator. Especially, the phase shift of the resonator is sensitive to the spin states and the gate operations. The response of the resonator can be used to measure the spin qubit efficiently, which can be extend to read out the multiple spin qubits in a scalable solid-state quantum processor.
 Select Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in (Li$_{0.84}$Fe$_{0.16}$)OHFe$_{0.98}$Se Qiong Wu, Huaxue Zhou, Yanling Wu, Lili Hu, Shunli Ni, Yichao Tian, Fei Sun, Fang Zhou, Xiaoli Dong, Zhongxian Zhao, and Jimin Zhao Chin. Phys. Lett.    2020, 37 (9): 097802 .   DOI: 10.1088/0256-307X/37/9/097802 Abstract   HTML   PDF (3320KB) Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors. We investigate the ultrafast quasiparticle (QP) dynamics of an intercalated superconductor (Li$_{1-x}$Fe$_{x}$)OHFe$_{1-y}$Se, which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO$_{3}$. We obtain the electron-phonon coupling (EPC) constant $\lambda_{{A}_{\rm 1g}}$ ($0.22 \pm 0.04$), which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO$_{3}$. Significantly, we find that such a positive correlation between $\lambda_{{A}_{\rm 1g}}$ and superconducting $T_{\rm c}$ holds among all known FeSe-based superconductors, even in line with reported FeAs-based superconductors. Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors, which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors.
 Select Unexpectedly Strong Diamagnetism of Self-Assembled Aromatic Peptides Haijun Yang, Zixin Wang, Liuhua Mu, Yongshun Song, Jun Hu, Feng Zhang, and Haiping Fang Chin. Phys. Lett.    2020, 37 (8): 087504 .   DOI: 10.1088/0256-307X/37/8/087504 Abstract   HTML   PDF (471KB) There is a considerable amount of work that shows the biomagnetism of organic components without ferromagnetic components at the molecular level, but it is of great challenge to cover the giant gap of biomagnetism between their experimental and theoretical results. Here we show that the diamagnetism of aromatic peptides is greatly enhanced for about 11 times by self-assembling, reaching two orders of magnitude higher than the mass susceptibility of pure water. The self-assembly of aromatic rings in the peptide molecules plays the key role in such a strong diamagnetism.
 Select Production of $^{87}$Rb Bose–Einstein Condensate with a Simple Evaporative Cooling Method Rehman Fazal, Jia-Zhen Li, Zhi-Wen Chen, Yuan Qin, Ya-Yi Lin, Zuan-Xian Zhang, Shan-Chao Zhang, Wei Huang, Hui Yan, Shi-Liang Zhu Chin. Phys. Lett.    2020, 37 (3): 036701 .   DOI: 10.1088/0256-307X/37/3/036701 Abstract   HTML   PDF (712KB) A Bose–Einstein condensate with a large atom number is an important experimental platform for quantum simulation and quantum information research. An optical dipole trap is the a conventional way to hold the ultracold atoms, where an atomic cloud is evaporatively cooled down before reaching the Bose–Einstein condensate. A carefully designed trap depth controlling curve is typically required to realize the optimal evaporation cooling. We present and demonstrate a simple way to optimize the evaporation cooling in a crossed optical dipole trap. A polyline shape optical power control profile is easily obtained with our method, by which a pure Bose–Einstein condensate with atom number $1.73\times10^5$ is produced. Theoretically, we numerically simulate the optimal evaporation cooling using the parameters of our apparatus based on a kinetic theory. Compared to the simulation results, our evaporation cooling shows a good performance. We believe that our simple method can be used to quickly realize evaporation cooling in optical dipole traps.