Quantum Critical Point Concealed by Disorder in Hole-doped Kagome Superconductors

Superconductivity (SC) and charge-density wave (CDW) order can have a complex relationship that influences the behavior of many quantum materials. In the hole-doped kagome compounds CsV₃Sb₅₋ₓSnₓ, the long-range CDW order is suppressed at the doping level that corresponds to the first SC dome of the double-dome structure. Doping is prone to introducing disorder, which may complicate the relative roles of CDW and disorder in sustaining SC. Here, we use nuclear quadrupole resonance (NQR) to explore the different charge orderings in CsV₃Sb₅₋ₓSnₓ. Firstly, we detect charge correlations well above the CDW transition temperature whose effective extent grows with doping—suggesting that defects stabilize the CDW order. Secondly, their behavior with temperature and doping reveals that the CDW order should vanish at a doping level near the minimum of the two superconducting domes. Thirdly, we observe local CDW persisting even up to the maximum of the second SC dome. The last two findings point to the critical role of disorder in sustaining the short-range CDW. Therefore, the doping level at which the charge correlations point to a suppression of the CDW is a quantum critical point in absence of disorder.