Time-reversal symmetry broken by charge order in CsV_{3}Sb_{5}

Abstract

The recently discovered vanadium-based kagome metals AV_{3}Sb_{5} (A = K, Rb, Cs) exhibit superconductivity at low temperatures and charge density wave (CDW) order at high temperatures. A prominent feature of the charge ordered state in this family is that it breaks time-reversal symmetry (TRSB), which is connected to the underlying topological nature of the band structure. In this work, a powerful combination of zero-field and high-field muon-spin rotation/relaxation is used to study the signatures of TRSB of the charge order in CsV_{3}Sb_{5}, as well as its anisotropic character. By tracking the temperature evolution of the in-plane and out-of-plane components of the muon-spin polarization, an enhancement of the internal field width sensed by the muon-spin ensemble was observed below T_{TRSB}=T_{CDW}≃95 K. Additional increase of the internal field width, accompanied by a change of the local field direction at the muon site from the ab plane to the c axis, was detected below T^{*}≃30 K. Remarkably, this two-step feature becomes well pronounced when a magnetic field of 8 T is applied along the crystallographic caxis, thus indicating a field-induced enhancement of the electronic response at the CDW transition. These results point to a TRSB in CsV_{3}Sb_{5} by charge order with an onset of ≃95 K, followed by an enhanced electronic response below ≃30 K. The observed two-step transition is discussed within the framework of different charge-order instabilities, which, in accordance with density functional theory calculations, are nearly degenerate in energy.