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

The kagome lattice(1), which is the most prominent structural motif in quantum physics, benefits from inherent non-trivial geometry so that it can host diverse quantum phases, ranging from spin-liquid phases, to topological matter, to intertwined orders(2-8) and, most rarely, to unconventional superconductivity(6,9). Recently, charge sensitive probes have indicated that the kagome superconductors AV(3)Sb(5) (A = K, Rb, Cs)(9-11) exhibit unconventional chiral charge order(12-19), which is analogousto the long-sought-after quantum order in the Haldane model(20) or Varma model(21). However, direct evidence for the time-reversal symmetry breaking of the charge order remains elusive. Here we use muon spin relaxation to probe the kagome charge order and superconductivity in KV3Sb5. We observe a noticeable enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge orderingtemperature and persists into the superconducting state. Notably, the muon spin relaxation rate below the charge orderingtemperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV3Sb5 and that the T-c/lambda(-2)(ab) ratio (where T-c is the superconducting transition temperature and lambda(ab) is the magnetic penetration depth in the kagome plane) 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.

Automated summary

The kagome lattice is a prominent structural motif in quantum physics that can host diverse quantum phases. Recent studies have shown that kagome superconductors exhibit unconventional chiral charge order, but direct evidence for time-reversal symmetry breaking is still lacking. Using muon spin relaxation, we observed an enhancement of the internal field width below the charge ordering temperature in KV3Sb5, indicating the presence of time-reversal symmetry-breaking charge order intertwining with unconventional superconductivity.