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    SPECIAL TOPIC — Superconductivity in vanadium-based kagome materials

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    Tri-hexagonal charge order in kagome metal CsV3Sb5 revealed by 121Sb nuclear quadrupole resonance
    Chao Mu(牟超), Qiangwei Yin(殷蔷薇), Zhijun Tu(涂志俊), Chunsheng Gong(龚春生), Ping Zheng(郑萍), Hechang Lei(雷和畅), Zheng Li(李政), and Jianlin Luo(雒建林)
    Chin. Phys. B, 2022, 31 (1): 017105.   DOI: 10.1088/1674-1056/ac422c
    Abstract148)   HTML5)    PDF (3092KB)(83)      
    We report 121Sb nuclear quadrupole resonance (NQR) measurements on kagome superconductor CsV3Sb5 with Tc=2.5 K. 121Sb NQR spectra split after a charge density wave (CDW) transition at 94 K, 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. The CDW order exhibits tri-hexagonal deformation with a lateral shift between the adjacent kagome layers, which is consistent with 2×2×2 superlattice modulation. The superconducting state coexists with CDW order and shows a conventional s-wave behavior in the bulk state.
    A density-wave-like transition in the polycrystalline V3Sb2 sample with bilayer kagome lattice
    Ningning Wang(王宁宁), Yuhao Gu(顾雨豪), M. A. McGuire, Jiaqiang Yan, Lifen Shi(石利粉), Qi Cui(崔琦), Keyu Chen(陈科宇), Yuxin Wang(王郁欣), Hua Zhang(张华), Huaixin Yang(杨槐馨), Xiaoli Dong(董晓莉), Kun Jiang(蒋坤), Jiangping Hu(胡江平), Bosen Wang(王铂森), Jianping Sun(孙建平), and Jinguang Cheng(程金光)
    Chin. Phys. B, 2022, 31 (1): 017106.   DOI: 10.1088/1674-1056/ac4227
    Abstract198)   HTML4)    PDF (2380KB)(171)      
    Recently, transition-metal-based kagome metals have aroused much research interest as a novel platform to explore exotic topological quantum phenomena. Here we report on the synthesis, structure, and physical properties of a bilayer kagome lattice compound V3Sb2. The polycrystalline V3Sb2 samples were synthesized by conventional solid-state-reaction method in a sealed quartz tube at temperatures below 850 ℃. Measurements of magnetic susceptibility and resistivity revealed consistently a density-wave-like transition at Tdw ≈ 160 K with a large thermal hysteresis, even though some sample-dependent behaviors were observed presumably due to the different preparation conditions. Upon cooling through Tdw, no strong anomaly in lattice parameters and no indication of symmetry lowering were detected in powder x-ray diffraction measurements. This transition can be suppressed completely by applying hydrostatic pressures of about 1.8 GPa, around which no sign of superconductivity was observed down to 1.5 K. Specific-heat measurements revealed a relatively large Sommerfeld coefficient γ = 18.5 mJ·mol-1·K-2, confirming the metallic ground state with moderate electronic correlations. Density functional theory calculations indicate that V3Sb2 shows a non-trivial topological crystalline property. Thus, our study makes V3Sb2 a new candidate of metallic kagome compound to study the interplay between density-wave-order, nontrivial band topology, and possible superconductivity.
    Evolution of superconductivity and charge order in pressurized RbV3Sb5
    Feng Du(杜锋), Shuaishuai Luo(罗帅帅), Rui Li(李蕊), Brenden R. Ortiz, Ye Chen(陈晔), Stephen D. Wilson, Yu Song(宋宇), and Huiqiu Yuan(袁辉球)
    Chin. Phys. B, 2022, 31 (1): 017404.   DOI: 10.1088/1674-1056/ac4232
    Abstract126)   HTML2)    PDF (3024KB)(174)      
    The kagome metals AV3Sb5 (A= K, Rb, Cs) under ambient pressure exhibit an unusual charge order, from which superconductivity emerges. In this work, by applying hydrostatic pressure using a liquid pressure medium and carrying out electrical resistance measurements for RbV3Sb5, we find that the charge order becomes suppressed under a modest pressure pc (1.4 GPa<pc<1.6 GPa), while the superconducting transition temperature Tc is maximized. Tc is then gradually weakened with further increase of pressure and reaches a minimum around 14.3 GPa, before exhibiting another {maximum} around 22.8 GPa, signifying the presence of a second superconducting dome. Distinct normal state resistance anomalies are found to be associated with the second superconducting dome, similar to KV3Sb5. Our findings point to qualitatively similar temperature-pressure phase diagrams in KV3Sb5 and RbV3Sb5, {and suggest a close link} between the second superconducting dome and the high-pressure resistance anomalies.
    Pressure tuning of the anomalous Hall effect in the kagome superconductor CsV3Sb5
    Fang-Hang Yu(喻芳航), Xi-Kai Wen(温茜凯), Zhi-Gang Gui(桂智刚), Tao Wu(吴涛), Zhenyu Wang(王震宇), Zi-Ji Xiang(项子霁), Jianjun Ying(应剑俊), and Xianhui Chen(陈仙辉)
    Chin. Phys. B, 2022, 31 (1): 017405.   DOI: 10.1088/1674-1056/ac3990
    Abstract155)   HTML4)    PDF (2601KB)(160)      
    Controlling the anomalous Hall effect (AHE) inspires potential applications of quantum materials in the next generation of electronics. The recently discovered quasi-2D kagome superconductor CsV3Sb5 exhibits large AHE accompanying with the charge-density-wave (CDW) order which provides us an ideal platform to study the interplay among nontrivial band topology, CDW, and unconventional superconductivity. Here, we systematically investigated the pressure effect of the AHE in CsV3Sb5. Our high-pressure transport measurements confirm the concurrence of AHE and CDW in the compressed CsV3Sb5. Remarkably, distinct from the negative AHE at ambient pressure, a positive anomalous Hall resistivity sets in below 35 K with pressure around 0.75 GPa, which can be attributed to the Fermi surface reconstruction and/or Fermi energy shift in the new CDW phase under pressure. Our work indicates that the anomalous Hall effect in CsV3Sb5 is tunable and highly related to the band structure.
    Surface-induced orbital-selective band reconstruction in kagome superconductor CsV3Sb5
    Linwei Huai(淮琳崴), Yang Luo(罗洋), Samuel M. L. Teicher, Brenden R. Ortiz, Kaize Wang(王铠泽),Shuting Peng(彭舒婷), Zhiyuan Wei(魏志远), Jianchang Shen(沈建昌), Bingqian Wang(王冰倩), Yu Miao(缪宇),Xiupeng Sun(孙秀鹏), Zhipeng Ou(欧志鹏), Stephen D. Wilson, and Junfeng He(何俊峰)
    Chin. Phys. B, 2022, 31 (5): 057403.   DOI: 10.1088/1674-1056/ac4f50
    Abstract125)   HTML2)    PDF (1156KB)(60)      
    The two-dimensional (2D) kagome superconductor CsV3Sb5 has attracted much recent attention due to the coexistence of superconductivity, charge orders, topology and kagome physics, which manifest themselves as distinct electronic structures in both bulk and surface states of the material. An interesting next step is to manipulate the electronic states in this system. Here, we report angle-resolved photoemission spectroscopy (ARPES) evidence for a surface-induced orbital-selective band reconstruction in CsV3Sb5. A significant energy shift of the electron-like band around Γ and a moderate energy shift of the hole-like band around M are observed as a function of time. This evolution is reproduced in a much shorter time scale by in-situ annealing of the CsV3Sb5 sample. Orbital-resolved density functional theory (DFT) calculations reveal that the momentum-dependent band reconstruction is associated with different orbitals for the bands around Γ and M, and the time-dependent evolution points to the change of sample surface that is likely caused by the formation of Cs vacancies on the surface. Our results indicate the possibility of orbital-selective control of the band structure via surface modification, which may open a new avenue for manipulating exotic phenomena in this material system, including superconductivity.
    Robustness of the unidirectional stripe order in the kagome superconductor CsV3Sb5
    Bin Hu(胡彬), Yuhan Ye(耶郁晗), Zihao Huang(黄子豪), Xianghe Han(韩相和), Zhen Zhao(赵振),Haitao Yang(杨海涛), Hui Chen(陈辉), and Hong-Jun Gao(高鸿钧)
    Chin. Phys. B, 2022, 31 (5): 058102.   DOI: 10.1088/1674-1056/ac5888
    Abstract122)   HTML2)    PDF (2401KB)(83)      
    V-based kagome materials AV3Sb5 (A=K, Rb, Cs) have attracted much attention due to their novel properties such as unconventional superconductivity, giant anomalous Hall effect, charge density wave (CDW) and pair density wave. Except for the 2a0×2a0 CDW (charge density wave with in-plane 2×2 superlattice modulation) in AV3Sb5, an additional 1×4 (4a0) unidirectional stripe order has been observed at the Sb surface of RbV3Sb5 and CsV3Sb5. However, the stability and electronic nature of the 4a0 stripe order remain controversial and unclear. Here, by using low-temperature scanning tunneling microscopy/spectroscopy (STM/S), we systematically study the 4a0 stripe order on the Sb-terminated surface of CsV3Sb5. We find that the 4a0 stripe order is visible in a large energy range. The STM images with positive and negative bias show contrast inversion, which is the hallmark for the Peierls-type CDW. In addition, below the critical temperature about 60 K, the 4a0 stripe order keeps unaffected against the topmost Cs atoms, point defects, step edges and magnetic field up to 8 T. Our results provide experimental evidences on the existence of unidirectional CDW in CsV3Sb5.