Kagome superconductors

Abstract

The newly discovered Kagome superconductors AV3Sb5(A = K, Rb, Cs) provide a platform to investigate the interplay of the topological property, superconductivity and geometrical frustration. Since their discovery, many research groups, especially many groups in China, have made tremendous progress in this field, including time-reversal-symmetry-breaking (TRSB), charge density wave (CDW), electronic nematicity, superconductivity properties and pair density wave (PDW). In this paper, we introduce the AV3Sb5 properties, discuss the recent research progress and highlight the future focus of this Kagome superconductor. The paper is organized as follows. We start from the exotic normal states of AV3Sb5, where a CDW emerges at the temperature around 70-100 K depending on A. This CDW enlarges the unit cell size to 2×2 with additional c-direction modulation as observed by scanning tunneling microscope (STM) and X-ray scattering experiments. Interestingly, this CDW behaves differently under opposite magnetic fields. Namely, this CDW may break the time reversal symmetry. To confirm this property, the zero field muon spin relaxation (ZFμSR) experiment is performed with increasing relaxation rates after the CDW transition. Additionally, the intrinsic anomalous Hall effect is also observed, which may relate to this time reversal symmetry breaking (TRSB). Since there are no long-range magnetic orders observed in the elastic neutron scattering experiment and μSR, the TRSB is not related to the electron spin degree of freedom. To explain the TRSB, the chiral flux phase (CFP) with orbital magnetism is theoretically proposed. Moreover, the electronic nematicity is also observed at about 30-50 K below the CDW transition temperature. This phase breaks the C6 rotation symmetry of the Kagome lattice as confirmed by STM and nuclear magnetic resonance (NMR). What is the microscopic origin of this nematicity is still under investigation. Then, we move to the superconducting properties of AV3Sb5. Combining the inversion symmetry property found in optical measurement and decreasing of the spin susceptibility found in NMR, the AV3Sb5 superconductor is proven to be a spin-singlet superconductor. Experiments in NMR, angle-resolved photoemission, superfluid density and specific heat further confirm the superconductivity in Kagome superconductors is a conventional s-wave superconductor. Although this superconductor is conventional, AV3Sb5 also contains the unconventional property. Importantly, a PDW is observed in CsV3Sb5 by high-resolution STM. What is the PDW origin or microscopic mechanism is still an open question. These new progress reveal the intriguing physical properties behind the Kagome superconductors and also bring many unsolved questions, which calls for further investigations.