Orbital ordering and fluctuations in a kagome superconductor CsV3Sb5

Recently, competing electronic instabilities, including superconductivity and density-wave-like order, have been discovered in vanadium-based kagome metals A V 3 Sb 5 ( A = K, Rb, Cs) with a nontrivial band topology. This finding stimulates considerable interest to study the interplay of these competing electronic orders and possible exotic excitations in the superconducting state. Here, we performed 51 V and 133 Cs nuclear magnetic resonance (NMR) measurements on a CsV 3 Sb 5 single crystal to clarify the nature of density-wave-like transition in these kagome superconductors. A first-order structural transition is unambiguously revealed below T s ∼ 94 K by observing the sudden splitting of Knight shift in 51 V NMR spectrum. Moreover, combined with 133 Cs NMR spectrum, the present result confirms a three-dimensional structural modulation. By further analyzing the anisotropy of Knight shift and 1/ T 1 T at 51 V nuclei, we proposed that the orbital order is the primary electronic order induced by the first-order structural transition, which is supported by further analysis on electric field gradient at 51 V nuclei. In addition, the evidence for possible orbital fluctuations is also revealed above T s . The present work sheds light on a rich orbital physics in kagome superconductors A V 3 Sb 5 .