Time-reversal symmetry breaking charge order in a kagome superconductor

The kagome lattice, the most prominent structural motif in quantum physics, benefits from inherent nontrivial geometry to host diverse quantum phases [1,2,3], 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 AV₃Sb₅ (A = K, Rb, Cs) [4] exhibit unconventional chiral charge order [5]. However, direct evidence for the time-reversal symmetry-breaking of the charge order remained elusive. We utilized muon spin relaxation to probe the kagome charge order and superconductivity in KV₃Sb₅ [6]. 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 KV₃Sb₅ and that the T_c/λ⁻²_ab 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. While low-temperature time-reversal symmetry-breaking superconductivity has been discussed for many systems, high-temperature time-reversal symmetry-breaking charge order is extremely rare, and finds a direct comparison with the fundamental Haldane and Varma models.

[1] J.-X. Yin et. al., Nature 583, 533-536 (2020).

[2] Z. Guguchia et. al., Nature Communications 11, 559 (2020).

[3] C. Mielke III et al., and Z. Guguchia, Phys. Rev. Mat. 5, 034803 (2021).

[4] Ortiz, B. et al. Phys. Rev. Lett. 125, 247002 (2020).

[5] Y.-X. Jiang et. al., Nature Materials 20, 1353 (2021).

[6] C. Mielke III et. al., and Z. Guguchia, Nature (2022).