#### Announcement

 Select Universal Theory and Basic Rules of Strain-Dependent Doping Behaviors in Semiconductors Xiaolan Yan, Pei Li, Su-Huai Wei, and Bing Huang Chin. Phys. Lett.    2021, 38 (8): 087103 .   DOI: 10.1088/0256-307X/38/8/087103 Abstract   HTML   PDF (1503KB) Enhancing the dopability of semiconductors via strain engineering is critical to improving their functionalities, which is, however, largely hindered by the lack of basic rules. In this study, for the first time, we develop a universal theory to understand the total energy changes of point defects (or dopants) with different charge states under strains, which can exhibit either parabolic or superlinear behaviors, determined by the size of defect-induced local volume change ($\Delta V$). In general, $\Delta V$ increases (decreases) when an electron is added (removed) to (from) the defect site. Consequently, in terms of this universal theory, three basic rules can be obtained to further understand or predict the diverse strain-dependent doping behaviors, i.e., defect formation energies, charge-state transition levels, and Fermi pinning levels, in semiconductors. These three basic rules could be generally applied to improve the doping performance or overcome the doping bottlenecks in various semiconductors.
 Select S-Wave Superconductivity in Kagome Metal CsV$_{3}$Sb$_{5}$ Revealed by $^{121/123}$Sb NQR and $^{51}$V NMR Measurements Chao Mu, Qiangwei Yin, Zhijun Tu, Chunsheng Gong, Hechang Lei, Zheng Li, and Jianlin Luo Chin. Phys. Lett.    2021, 38 (7): 077402 .   DOI: 10.1088/0256-307X/38/7/077402 Abstract   HTML   PDF (1234KB) We report $^{121/123}$Sb nuclear quadrupole resonance (NQR) and $^{51}$V nuclear magnetic resonance (NMR) measurements on kagome metal CsV$_3$Sb$_5$ with $T_{\rm c}=2.5$ K. Both $^{51}$V NMR spectra and $^{121/123}$Sb NQR spectra split after a charge density wave (CDW) transition, which demonstrates a commensurate CDW state. The coexistence of the high temperature phase and the CDW phase between $91$ K and $94$ K manifests that it is a first-order phase transition. At low temperature, electric-field-gradient fluctuations diminish and magnetic fluctuations become dominant. Superconductivity emerges in the charge order state. Knight shift decreases and $1/T_{1}T$ shows a Hebel–Slichter coherence peak just below $T_{\rm c}$, indicating that CsV$_3$Sb$_5$ is an s-wave superconductor.
 Select Strong Coupled Magnetic and Electric Ordering in Monolayer of Metal Thio(seleno)phosphates Chenqiang Hua, Hua Bai, Yi Zheng, Zhu-An Xu, Shengyuan A. Yang, Yunhao Lu, and Su-Huai Wei Chin. Phys. Lett.    2021, 38 (7): 077501 .   DOI: 10.1088/0256-307X/38/7/077501 Abstract   HTML   PDF (2464KB) The coupling between electric ordering and magnetic ordering in two-dimensional (2D) materials is important for both fundamental research of 2D multiferroics and future development of magnetism-based information storage and operation. Here, we introduce a scheme for realizing a magnetic phase transition through the transition of electric ordering. We take CuMoP$_{2}$S$_{6}$ monolayer as an example, which is a member of the large 2D transition-metal chalcogen-phosphates family. Based on first-principles calculations, we find that it is a multiferroic with unprecedented characters, namely, it exhibits two different phases: an antiferroelectric-antiferromagnetic phase and a ferroelectric-ferromagnetic phase, in which the electric and magnetic orderings are strongly coupled. Importantly, the electric polarization is out-of-plane, so the magnetism can be readily switched by using the gate electric field. Our finding reveals a series of 2D multiferroics with special magnetoelectric coupling, which hold great promise for experimental realization and practical applications.
 Select Rabi Spectroscopy and Sensitivity of a Floquet Engineered Optical Lattice Clock Mo-Juan Yin, Tao Wang, Xiao-Tong Lu, Ting Li, Ye-Bing Wang, Xue-Feng Zhang, Wei-Dong Li, Augusto Smerzi, and Hong Chang Chin. Phys. Lett.    2021, 38 (7): 073201 .   DOI: 10.1088/0256-307X/38/7/073201 Abstract   HTML   PDF (1464KB) We periodically modulate the lattice trapping potential of a $^{87}$Sr optical clock to Floquet engineer the clock transition. In the context of atomic gases in lattices, Floquet engineering has been used to shape the dispersion and topology of Bloch quasi-energy bands. Differently from these previous works manipulating the external (spatial) quasi-energies, we target the internal atomic degrees of freedom. We shape Floquet spin quasi-energies and measure their resonance profiles with Rabi spectroscopy. We provide the spectroscopic sensitivity of each band by measuring the Fisher information and show that this is not depleted by the Floquet dynamical modulation. The demonstration that the internal degrees of freedom can be selectively engineered by manipulating the external degrees of freedom inaugurates a novel device with potential applications in metrology, sensing and quantum simulations.
 Select Resolving the Bethe–Salpeter Kernel Si-Xue Qin and Craig D. Roberts Chin. Phys. Lett.    2021, 38 (7): 071201 .   DOI: 10.1088/0256-307X/38/7/071201 Abstract   HTML   PDF (787KB) A novel method for constructing a kernel for the meson bound-state problem is described. It produces a closed form that is symmetry-consistent (discrete and continuous) with the gap equation defined by any admissible gluon-quark vertex, $\varGamma$. Applicable even when the diagrammatic content of $\varGamma$ is unknown, the scheme can foster new synergies between continuum and lattice approaches to strong interactions. The framework is illustrated by showing that the presence of a dressed-quark anomalous magnetic moment in $\varGamma$, an emergent feature of strong interactions, can remedy many defects of widely used meson bound-state kernels, including the mass splittings between vector and axial-vector mesons and the level ordering of pseudoscalar and vector meson radial excitations.
 Select Unusual Normal and Superconducting State Properties Observed in Hydrothermal Fe$_{1-\delta}$Se Flakes Shaobo Liu, Sheng Ma, Zhaosheng Wang, Wei Hu, Zian Li, Qimei Liang, Hong Wang, Yuhang Zhang, Zouyouwei Lu, Jie Yuan, Kui Jin, Jian-Qi Li, Li Pi, Li Yu, Fang Zhou, Xiaoli Dong, and Zhongxian Zhao Chin. Phys. Lett.    2021, 38 (5): 057401 .   DOI: 10.1088/0256-307X/38/5/057401 Abstract   HTML   PDF (1134KB) The electronic and superconducting properties of Fe$_{1-\delta}$Se single-crystal flakes grown hydrothermally are studied by the transport measurements under zero and high magnetic fields up to 38.5 T. The results contrast sharply with those previously reported for nematically ordered FeSe by chemical-vapor-transport (CVT) growth. No signature of the electronic nematicity, but an evident metal-to-nonmetal crossover with increasing temperature, is detected in the normal state of the present hydrothermal samples. Interestingly, a higher superconducting critical temperature $T_{\rm c}$ of 13.2 K is observed compared to a suppressed $T_{\rm c}$ of 9 K in the presence of the nematicity in the CVT FeSe. Moreover, the upper critical field in the zero-temperature limit is found to be isotropic with respect to the field direction and to reach a higher value of $\sim$42 T, which breaks the Pauli limit by a factor of 1.8.
 Select Anisotropic Superconducting Properties of Kagome Metal CsV$_{3}$Sb$_{5}$ Shunli Ni, Sheng Ma, Yuhang Zhang, Jie Yuan, Haitao Yang, Zouyouwei Lu, Ningning Wang, Jianping Sun, Zhen Zhao, Dong Li, Shaobo Liu, Hua Zhang, Hui Chen, Kui Jin, Jinguang Cheng, Li Yu, Fang Zhou, Xiaoli Dong, Jiangping Hu, Hong-Jun Gao, and Zhongxian Zhao Chin. Phys. Lett.    2021, 38 (5): 057403 .   DOI: 10.1088/0256-307X/38/5/057403 Abstract   HTML   PDF (838KB) We systematically measure the superconducting (SC) and mixed state properties of high-quality CsV$_{3}$Sb$_{5}$ single crystals with $T_{\rm c} \sim 3.5$ K. We find that the upper critical field $H_{\rm c2}(T)$ exhibits a large anisotropic ratio of $H_{\rm c2}^{ab}/H_{\rm c2}^{c} \sim 9$ at zero temperature and fitting its temperature dependence requires a minimum two-band effective model. Moreover, the ratio of the lower critical field, $H_{\rm c1}^{ab}/H_{\rm c1}^{c}$, is also found to be larger than 1, which indicates that the in-plane energy dispersion is strongly renormalized near Fermi energy. Both $H_{\rm c1}(T)$ and SC diamagnetic signal are found to change little initially below $T_{\rm c} \sim 3.5$ K and then to increase abruptly upon cooling to a characteristic temperature of $\sim$2.8 K. Furthermore, we identify a two-fold anisotropy of in-plane angular-dependent magnetoresistance in the mixed state. Interestingly, we find that, below the same characteristic $T \sim 2.8$ K, the orientation of this two-fold anisotropy displays a peculiar twist by an angle of 60$^{\circ}$ characteristic of the Kagome geometry. Our results suggest an intriguing superconducting state emerging in the complex environment of Kagome lattice, which, at least, is partially driven by electron-electron correlation.
 Select Highly Robust Reentrant Superconductivity in CsV$_{3}$Sb$_{5}$ under Pressure Xu Chen, Xinhui Zhan, Xiaojun Wang, Jun Deng, Xiao-Bing Liu, Xin Chen, Jian-Gang Guo, and Xiaolong Chen Chin. Phys. Lett.    2021, 38 (5): 057402 .   DOI: 10.1088/0256-307X/38/5/057402 Abstract   HTML   PDF (3241KB) We present the superconducting (SC) property and high-robustness of structural stability of kagome CsV$_{3}$Sb$_{5}$ under in situ high pressures. For the initial SC-I phase, its $T_{\rm c}$ is quickly enhanced from 3.5 K to 7.6 K and then totally suppressed at $P \sim 10$ GPa. With further increasing pressure, an SC-II phase emerges at $P \sim 15$ GPa and persists up to 100 GPa. The $T_{\rm c}$ rapidly increases to the maximal value of 5.2 K at $P=53.6$ GPa and slowly decreases to 4.7 K at $P=100$ GPa. A two-dome-like variation of $T_{\rm c}$ in CsV$_{3}$Sb$_{5}$ is concluded here. The Raman measurements demonstrate that weakening of $E_{\rm 2g}$ mode and strengthening of $E_{\rm 1g}$ mode occur without phase transition in the SC-II phase, which is supported by the results of phonon spectra calculations. Electronic structure calculations reveal that exertion of pressure may bridge the gap of topological surface nontrivial states near $E_{\rm F}$, i.e., disappearance of $Z_{2}$ invariant. Meanwhile, the Fermi surface enlarges significantly, consistent with the increased carrier density. The findings here suggest that the change of electronic structure and strengthened electron-phonon coupling should be responsible for the pressure-induced reentrant SC.