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 Select $X_0(2900)$ and $X_1(2900)$: Hadronic Molecules or Compact Tetraquarks Hua-Xing Chen, Wei Chen, Rui-Rui Dong, and Niu Su Chin. Phys. Lett.    2020, 37 (10): 101201 .   DOI: 10.1088/0256-307X/37/10/101201 Abstract   HTML   PDF (601KB) Very recently the LHCb collaboration reported their observation of the first two fully open-flavor tetraquark states, the $X_0(2900)$ of $J^P = 0^+$ and the $X_1(2900)$ of $J^P = 1^-$. We study their possible interpretations using the method of QCD sum rules, paying special attention to an interesting feature of this experiment that the higher resonance $X_1(2900)$ has a width significantly larger than the lower one $X_0(2900)$. Our results suggest that the $X_0(2900)$ can be interpreted as the s-wave $D^{*-}K^{*+}$ molecule state of $J^P = 0^+$, and the $X_1(2900)$ can be interpreted as the p-wave $\bar c \bar s u d$ compact tetraquark state of $J^P = 1^-$. Mass predictions of their bottom partners are also given.
 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 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 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 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 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 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 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 Structure-Spin-Transport Anomaly in Quasi-One-Dimensional Ba$_{9}$Fe$_{3}$Te$_{15}$ under High Pressure Jun Zhang, Mei-Ling Jin, Xiang Li, Xian-Cheng Wang, Jian-Fa Zhao, Ying Liu, Lei Duan, Wen-Min Li, Li-Peng Cao, Bi-Juan Chen, Li-Juan Wang, Fei Sun, Yong-Gang Wang, Liu-Xiang Yang, Yu-Ming Xiao, Zheng Deng, Shao-Min Feng, Chang-Qing Jin, and Jin-Long Zhu Chin. Phys. Lett.    2020, 37 (8): 087106 .   DOI: 10.1088/0256-307X/37/8/087106 Abstract   HTML   PDF (2065KB) Recently, a series of novel compounds Ba$_{3}$MX$_{5}$ (M =  Fe, Ti, V; X = Se, Te) with hexagonal crystal structures composed of quasi-1-dimensional (1D) magnetic chains has been synthesized by our research team using high-pressure and high-temperature methods. The initial hexagonal phases persist to the maximum achievable pressure, while spin configurations and magnetic interactions may change dramatically as a result of considerable reductions in interchain separations upon pressurization. These compounds therefore offer unique possibilities for studying the evolution of intrinsic electronic structures in quasi-1D magnetic systems. Here we present a systematic investigation of Ba$_{9}$Fe$_{3}$Te$_{15}$, in which the interchain separations between trimerized 1D chains ($\sim$10.2 Å) can be effectively modulated by external high pressure. The crystal structure especially along the 1D chains exhibits an abnormal expansion at $\sim$5 GPa, which accompanies trimerization entangled anomalous mixed-high-low spin transition. An insulator-metal transition has been observed under high pressure as a result of charge-transfer gap closing. Pressure-induced superconductivity emerges at 26 GPa, where the charge-transfer gap fully closes, 3D electronic configuration forms and local spin fully collapses.
 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.
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