Observation of ubiquitous charge correlations and hidden quantum critical point in hole-doped kagome superconductors

TL;DR

This study reveals pervasive charge correlations and a hidden quantum critical point in hole-doped kagome superconductors, showing how disorder and doping influence charge density wave phases and their relation to superconductivity.

Contribution

It uncovers the presence of incipient and fragmented charge-density wave phases and identifies a hidden quantum critical point in hole-doped kagome superconductors using nuclear quadrupole resonance.

Findings

Static CDW puddles exist above transition temperature.

The ISD-$ ext{pi}$ CDW order vanishes near doping level 0.12.

Carrier doping fragments the CDW order while disorder preserves patches.

Abstract

The interplay between superconductivity and charge-density wave (CDW) order, and its evolution with carrier density, is central to the physics of many quantum materials, notably high-$T_c$ cuprates and kagome metals. Hole-doped kagome compounds exhibit puzzling double-dome superconductivity and, as chemical substitution inevitably introduces quenched disorder, their properties remain poorly understood. Here, by leveraging the sensitivity of nuclear quadrupole resonance to local and static orderings, we uncover new features, primarily the incipient and fragmented CDW phases, in the charge landscape of CsV$_3$Sb$_{5-x}$Sn$_x$. Static CDW puddles are observed well above the transition temperature, a hallmark of pinning by defects. Their doping and temperature evolution indicate that, in the absence of disorder, the inverse Star-of-David $\pi$-shifted (ISD-$\pi$) CDW order would vanish near $x=0.12$, between the two superconducting domes. This critical doping represents a hidden quantum critical point. Nevertheless, the ISD-$\pi$ pattern persists well beyond previous reports, although its volume fraction is progressively reduced up to the critical doping at which it saturates. We establish that carrier doping promotes fragmentation of the ISD-$\pi$ order, whereas randomness preserves the ISD-$\pi$ patches.