Time-reversal symmetry-breaking charge order in a kagome superconductor

TL;DR

This study provides direct evidence of time-reversal symmetry-breaking charge order in a kagome superconductor, revealing its intertwining with unconventional superconductivity through muon spin relaxation measurements.

Contribution

The paper demonstrates the presence of time-reversal symmetry-breaking charge order in KV_3Sb_5 and uncovers its connection with unconventional superconductivity, using muon spin relaxation techniques.

Findings

Enhanced internal magnetic fields below charge order temperature

Time-reversal symmetry-breaking charge order observed

Multigap unconventional superconductivity identified

Abstract

The kagome lattice, the most prominent structural motif in quantum physics, benefits from inherent nontrivial geometry to host diverse quantum phases, ranging from spin-liquid phases, topological matter to intertwined orders, and most rarely unconventional superconductivity. Recently, charge sensitive probes have suggested that the kagome superconductors AV_3Sb_5 (A = K, Rb, Cs) (A = K, Rb, Cs) exhibit unconventional chiral charge order, which is analogous to the long-sought-after quantum order in the Haldane model or Varma model. However, direct evidence for the time-reversal symmetry-breaking of the charge order remains elusive. Here we utilize muon spin relaxation to probe the kagome charge order and superconductivity in KV_3Sb_5. We observe a striking enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Remarkably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV_3Sb_5 and that the T_c/lambda_{ab}^{-2} ratio is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.