A kagome lattice of 3d transition metals hosts flat bands, Dirac fermions and saddle points. It provides a versatile platform for achieving topological superconductivity, anomalous Hall effect, unconventional density wave order and quantum spin liquid when the strong correlation, spin-orbit coupling or magnetic order are involved in such a lattice. Here, using laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations, we investigate the electronic structure of the newly discovered kagome superconductor CsTi$_3$Bi$_5$, which is isostructural to the AV$_3$Sb$_5$ (A=K, Rb or Cs) kagome superconductors and possesses a perfect two-dimensional kagome network of Titanium. We directly observed a strikingly flat band derived from the local destructive interferences of Bloch wave functions within the kagome lattices. We also identify the type-II Dirac nodal loops around the Brillouin zone center, the type-III Dirac nodal loops around the zone corners and type-III Dirac nodal lines along the k$_z$ direction. In addition, around the Brillouin zone center, Z2 nontrivial topological surface states are also observed which is formed from the band inversion due to strong spin orbital coupling. The simultaneous existence of such multi-sets of nontrivial band structures in one kagome superconductor not only provides good opportunities to study related physics in the kagome lattice but also makes CsTi$_3$Bi$_5$ an ideal system to realize noval quantum phenomena by manipulating its chemical potential with chemical doping or pressure.
The kagomé metals RbTi$_3$Bi$_5$ and CsTi$_3$Bi$_5$ were synthesized both as polycrystalline powders by heating the elements an argon atmosphere and as single crystals grown using a self-flux method. The compounds crystallize in the hexagonal crystal system isotypically to KV$_3$Sb$_5$ (P6/mmm, Z = 1, CsTi3Bi5: a = 5.7873(1) Å, c = 9.2062(1) Å; RbTi3Bi5: a = 5.773(1) Å, c = 9.065(1) Å). Titanium atoms form a kagomé net with bismuth atoms in the hexagons as well as above and below the triangles. The alkali metal atoms are coordinated by 12 bismuth atoms and form AlB$_2$-like slabs between the k…
Since the discovery of a new family of vanadium-based kagome superconductor AV3Sb5 (A=K, Rb, and Cs) with topological band structures, extensive effort has been devoted to exploring the origin of superconducting states and the intertwined orders. Meanwhile, searching for new types of superconductors with novel physical properties and higher superconducting transition temperatures has always been a major thread in the history of superconductor research. Here we report a successful fabrication and the topological states of a Titanium-based kagome metal CsTi3Bi5 (CT3B5) crystal. The as-grown CT3B…
Topological superconductors (TSC) become a focus of research due to the accompanying Majorana fermions. However, the experimentally reported TSC are extremely rare. The recent experiments reported the kagome TSC AV$_{3}$Sb$_{5}$ (A=K, Rb, Cs), which exhibit unique superconductivity, topological surface states (TSS), and Majorana bound states. More recently, the first titanium-based kagome superconductor CsTi$ _{3} $Bi$ _{5} $ with nontrivial topology was successfully synthesized as a perspective TSC. Given that Cs contributes little to the electronic structures of CsTi$ _{3} $Bi$ _{5} $ and bi…
Recently superconductivity was discovered in the Kagome metal AV3Sb5 (A = K, Rb, and Cs), which has an ideal Kagome lattice of vanadium. These V-based superconductors also host charge density wave (CDW) and topological nontrivial band structure. Here we report the ultralow-temperature thermal conductivity and high pressure resistance measurements on CsV3Sb5 with Tc = 2.5 K, the highest among AV3Sb5. A finite residual linear term of thermal conductivity at zero magnetic field and its rapid increase in fields suggest nodal superconductivity. By applying pressure, the Tc of CsV3Sb5 increases firs…
Kagome lattice composed of transition-metal ions provides a great opportunity to explore the intertwining between geometry, electronic orders and band topology. The discovery of multiple competing orders that connect intimately with the underlying topological band structure in nonmagnetic kagome metals $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) further pushes this topic to the quantum frontier. Here we report the discovery and characterization of a new class of vanadium-based compounds with kagome bilayers, namely $A$V$_6$Sb$_6$ ($A$ = K, Rb, Cs) and V$_6$Sb$_4$, which, together with $A$V$_3$Sb$_5$, com…
Recently, a family of nonmagnetic kagome metals \textit{A}$\mathrm{V_3Sb_5}$ (\textit{A}=K, Rb, and Cs) has attracted significant attention for realizing the intertwining of quantum order and nontrivial topology. However, these compounds have been identified to host complex band structures. Therefore, it is desirable to design and synthesize novel kagome materials with a simple band topology and good transport properties. In this study, using first-principles calculations, we present the electronic properties and the intrinsic spin Hall effect of V$_6$Sb$_4$, the latest experimentally synthesi…
Unconventional superconductors often feature competing orders, small superfluid density, and nodal electronic pairing. While unusual superconductivity has been proposed in the kagome metals AV3Sb5, key spectroscopic evidence has remained elusive. Here we utilize pressure-tuned (up to 1.85 GPa) and ultra-low temperature (down to 18 mK) muon spin spectroscopy to uncover the unconventional nature of superconductivity in RbV3Sb5. At ambient pressure, we detect an enhancement of the width of the internal magnetic field distribution sensed by the muon ensemble, indicative of time-reversal symmetry b…
Correlated charge order intertwined with time-reversal symmetry-breaking nodal superconductivity in the dual flat band kagome superconductor CeRu$_{3}$Si$_{2}$
PSI Center for Neutron and Muon Sciences（保罗谢尔研究所中子与μ子科学中心）；Physik-Institut, Universität Zürich（苏黎世大学物理学院）；Department of Quantum Matter Physics, University of Geneva（日内瓦大学量子物质物理系）；TU Dortmund University, Department of Physics（多特蒙德工业大学物理系）；Research Center Future Energy Materials and Systems (RC FEMS)（未来能源材料与系统研究中心）
$T_{3}$Pb$_{2}Ch_{2}$ ($T$=Pd,Pt and $Ch$=S,Se) with transition metal kagome net: Dynamical properties, phonon nodal line, phonon surface states, and chiral phonons
Recently the kagome compounds have inspired enormous interest and made some great progress such as in the field of superconductivity and topology. Here we predict a different kagome magnesium triboride (MgB3) superconductor with a calculated Tc ~12.2 K and Tc ~15.4 K by external stress, the potentially highest among the reported diverse kagome-type superconductors. We reveal its various exotic physical properties including the van Hove singularity, flat-band, multiple Dirac points, and nontrivial topology. The system can be described by a two-band model with highly anisotropic superconducting …
Strain-Tuned Nodal Superconductivity in the Charge-Ordered Kagome Metal CsV$_3$Sb$_5$
Okayama University（冈山大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；Beijing National Laboratory for Condensed Matter Physics（北京凝聚态物理国家实验室）；Fujian Provincial Collaborative Innovation Center for Advanced High-Field Superconducting Materials and Engineering（福建省先进高场超导材料与工程协同创新中心）；College of Physics and Energy, Fujian Normal University（福建师范大学物理与能源学院）；Guang’an Institute of Technology
The superconducting gap structures in the transition-metal-based kagome metal AV$_3$Sb$_5$ (A=K,Rb,Cs), the first family of quasi-two-dimensional kagome superconductors, remain elusive as there is strong experimental evidence for both nodal and nodaless gap structures. Here we show that the dichotomy can be resolved because of the coexistence of time-reversal symmetry breaking with a conventional fully gapped superconductivity. The symmetry protects the edge states which arise on the domains of the lattice symmetry breaking order to remain gapless in proximity to a conventional pairing. We dem…
Pseudo-spin-polarized topological superconductivity in kagome RbV$_3$Sb$_5$
Hong Kong University of Science and Technology（香港科技大学）；International Quantum Academy, and Shenzhen Branch, Hefei National Laboratory（国际量子学院合肥国家实验室深圳分院）
Recently, intensive studies have revealed fascinating physics, such as charge density wave and superconducting states, in the newly synthesized kagome-lattice materials $A$V$_3$Sb$_5$ ($A$=K, Rb, Cs). Despite the rapid progress, fundamental aspects like the magnetic properties and electronic correlations in these materials have not been clearly understood yet. Here, based on the density functional theory plus the single-site dynamical mean-field theory calculations, we investigate the correlated electronic structure and the magnetic properties of the KV$_3$Sb$_5$ family materials in the normal…
Effect of superconductivity by Nb and V substitution in kagome CaPd5
Henan Normal University（河南师范大学）；Suzhou University of Science and Technology（苏州科技大学）；East China Normal University（华东师范大学）；Jinan University（暨南大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）
The recently discovered kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) possess a unique band structure with van Hove singularities and Dirac dispersions, in which unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking have been reported. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries, including exotic states with chiral and topological properties accompanied by a sign-changing superconducting ga…
Motivated by recent experimental evidences signalling the chiral charge order in the vanadium-based kagome superconductors, we theoretically investigate the impact of the chiral flux charge order over the experimental outcomes for the normal and the SC properties. It is revealed that the spectral weight on the Fermi surface (FS) is partially gaped by the chiral flux charge order with the reservation of the spectral weight on the $M$ points and the midpoint between the two adjacent $M$ points, resulting in the momentum-dependent energy gap being consistent with the recent experimental observati…
Resolving unconventional gap structure in kagome superconductors with hybrid microwave circuits
Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体和化学物理研究所）；Institute of Solid State and Material Physics, Technische Universität Dresden（德累斯顿工业大学固态与材料物理研究所）；Institute of Applied Physics, Technische Universität Dresden（德累斯顿工业大学应用物理研究所）；Leibniz Institute for Solid State and Materials Research Dresden（德累斯顿莱布尼茨固体与材料研究所）；Department of Physics, University of Naples Federico II（那不勒斯费德里科二世大学物理系）；Department of Electrical Engineering and Information Technology, University of Naples Federico II（那不勒斯费德里科二世大学电气与信息工程系）
The discovery of superconductivity in layered vanadium-based kagome metals $A$V$_3$Sb$_5$ ($A$: K, Rb, Cs) has added a new family of materials to the growing class of possible unconventional superconductors. However, the nature of the superconducting pairing in these materials remains elusive. We present a microscopic theoretical study of the leading superconducting instabilities on the kagome lattice based on spin- and charge-fluctuation mediated Cooper pairing. The applied methodology includes effects of both on-site and nearest-neighbor repulsive Coulomb interactions. Near the upper van Hov…
Signatures of spin-polarized p-wave superconductivity in the kagome material RbV$_3$Sb$_5$
International Quantum Academy（国际量子学院）；Hong Kong University of Science and Technology（香港科技大学）；Southern University of Science and Technology（南方科技大学）；Nankai University（南开大学）；Peking University（北京大学）
Binary kagome superconducting candidates hosting topological electronic states
Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所）；Peng Huanwu Collaborative Center for Research and Education, Beihang University（北京航空航天大学彭恒武理论研究中心）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences（中国科学院大学科维理理论科学研究所）
