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

Abstract

The kagome lattice1, 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 orders2, 3,4,5,6,7,8 and, most rarely, to unconventional su-perconductivity6,9. Recently, charge sensitive probes have indicated that the kagome superconductors AV3Sb5 (A = K, Rb, Cs)9,10,11 exhibit unconventional chiral charge order12, 13,14,15,16,17,18,19, which is analogous to the long-sought-after quantum order in the Haldane model20 or Varma model21. 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 ordering temperature and persists into the superconducting state. Notably, 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 Tc/−2ab ratio (where Tc is the superconducting transition temperature and 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.

Tunable nodal kagome superconductivity in charge ordered RbV3Sb5

“…The increase in the exponential relaxation of RbV 3 Sb 5 between T * 1 and 2 K is about 0.05 µs −1 , which can be interpreted as a characteristic field strength Γ 12 /γ µ 0.6 G. While these ZF-µSR results are consistent with the onset of time-reversal symmetry-breaking at T co , highfield µSR experiments, illustrated in the inset of Fig. 1g, are essential to confirm this effect, as we discussed previously [24]. Fig.…”

“…(1) Here, ∆/γ µ is the width of the local field distribution due to the nuclear moments and γ µ = 0.085 µs −1 G −1 is the muon gyromagnetic ratio. As we discussed previously in our work that reported time-reversal symmetrybreaking in KV 3 Sb 5 [24], the exponential relaxation rate Γ is mostly sensitive to the temperature dependence of the electronic contribution to the muon spin relaxation. In Fig.…”

“…The ZF-µSR spectra (see Fig. 1d) were fitted using the Gaussian Kubo-Toyabe (GKT) depolarization function [41] multiplied with an exponential decay function [24]:…”

“…Similarly to charge order, superconductivity, with transition temperature T c ∼ 1 K at ambient pressure, was also reported to display intriguing features, such as multiple gaps in (K,Cs)V 3 Sb 5 [31][32][33], diminished superfluid density in KV 3 Sb 5 [24], and double-dome structures in the pressure phase diagrams of all three compounds [34][35][36]. However, no consensus on the superconducting gap structure has been reached yet [24,[31][32][33][37][38][39][40], partly due to the challenges of performing spectroscopic studies under extreme conditions including ultra low temperatures and large pressures. Moreover, the role of the unconventional charge order in the emergence of these unusual superconducting features remains unclear, since the former onsets at a much higher temperature than the latter.…”

“…The novel family of kagome metals AV 3 Sb 5 (A = K, Rb, Cs) [7][8][9][10][11] exhibit an array of interesting effects such as giant anomalous Hall conductivity [12,13], charge order [11,[14][15][16][17][18][19][20][21][22], orbital order [23], and possible unconventional superconductivity [7,8,10]. An important feature of the charge order, which onsets at temperatures T co ∼ 100 K at ambient pressure, is the breaking of time-reversal symmetry, as reported by scanning tunneling microscopy (STM) measurements [11,14,15,17] in (K,Rb,Cs)V 3 Sb 5 , by muon spin relaxation (µSR) in KV 3 Sb 5 [24] and CsV 3 Sb 5 [25], and by Kerr effect measurements in CsV 3 Sb 5 [26]. This implies that the chargeordered state displays not only bond distortions, but also orbital current loops (see Fig.…”