Title: Understanding and Modulating a New Unconventional Kagome Superconductor

Speaker: Haitao Yang, Professor, Institute of Physics, Chinese Academy of Sciences

Time: November Third 9:30am

Venue: Ladder conference room on the third floor of Research Building

Host: Prof. Xiaohui Qiu

Abstrct: AV₃Sb₅ (A = K, Rb, Cs) is a newly discovered Kagome family that attracts great interest due to its topological nontrivial band structure and the coexistence of superconductivity (SC) and charge-density-wave (CDW) with exotic properties including a giant anomalous Hall effect. All these properties make AV₃Sb₅ an ideal system to study the interplay among the superconductivity, topological nontrivial band structure, and other rich phenomena within the Kagome lattice. Here, we report the preparation of the high-quality CsV₃Sb₅ single crystal with a SC transition temperature ～ 3.5 K and Ti-doping CsV_3-xTi_xSb₅ single crystals with the modulating SC and CDW. Based on the electronical transport, magnetization, ARPES and μSR characterization, we find that a two-fold anisotropy of in-plane angular-dependent magnetoresistance in the mixed state and the CDW gap is strongly momentum dependent. We also observe strong TRSB signals below 70 K in CsV₃Sb₅ and conclude that the interlayer coupled chiral flux phase (CFP) is the most promising candidate for the TRSB state. Moreover, we observe a dome-shape SC diagram separated by the two distinct SC phase in the Ti-doping CsV_3-xTi_xSb₅ single crystals. These finding provide new insights into the mechanism of unconventional SC and multiple orders in Kagome systems.

Title: Roton pair density wave in a strong-coupling kagome superconductor

Speaker: Hui Chen, Associate Professor, Institute of Physics, Chinese Academy of Sciences

Time: November Third 9:30am

Venue: Ladder conference room on the third floor of Research Building

Host: Prof. Xiaohui Qiu

Abstrct: The transition-metal kagome lattice materials host frustrated, correlated, and topological quantum states of matter. Recently, a new family of vanadium-based kagome metals AV₃Sb₅ (A=K, Rb, and Cs) with topological band structures has been discovered. These layered compounds are nonmagnetic and undergo charge density wave transitions before developing superconductivity at low temperatures. Here we report the observation of unconventional superconductivity and pair density wave (PDW) in CsV₃Sb₅ using scanning tunneling microscope/spectroscopy (STM/STS) and Josephson STS. We find that CsV₃Sb₅ exhibits a V-shaped pairing gap Δ~0.5 meV and is a strong-coupling superconductor (2Δ/k_BT_c~5) that coexists with 4a₀ unidirectional and 2a₀×2a₀ charge order. Remarkably, we discover a 3Q PDW accompanied by bidirectional 4a₀/3 spatial modulations of the superconducting gap, coherence peak and gap-depth in the tunneling conductance. We term this novel quantum state a roton-PDW associated with an underlying vortex-antivortex lattice that can account for the observed conductance modulations. Probing the electronic states in the vortex halo in an applied magnetic field, in strong-field that suppresses superconductivity, and in zero-field above Tc reveals that the PDW is a primary state responsible for an emergent pseudogap and intertwined electronic order. Our findings show striking analogies and distinctions to the phenomenology of high-T_c cuprate superconductors, and provide groundwork for understanding the microscopic origin of correlated electronic states and superconductivity in vanadium-based kagome metals.