Evidence of a hidden flux phase in the topological kagome metal CsV$_3$Sb$_5$

TL;DR

This study provides experimental evidence of a hidden flux phase with time reversal symmetry breaking in the kagome superconductor CsV$_3$Sb$_5$, revealing a novel TRSB state emerging from a charge density wave phase.

Contribution

It identifies a chiral flux phase as the candidate for TRSB in CsV$_3$Sb$_5$ using muon spin relaxation and optical SHG, advancing understanding of symmetry-breaking in kagome metals.

Findings

Strong TRSB signals below 70 K detected by ZF-$$SR.

Inversion symmetry remains intact in the studied temperature range.

The chiral flux phase is the most consistent candidate for the TRSB state.

Abstract

Phase transitions governed by spontaneous time reversal symmetry breaking (TRSB) have long been sought in many quantum systems, including materials with anomalous Hall effect (AHE), cuprate high temperature superconductors, Iridates and so on. However, experimentally identifying such a phase transition is extremely challenging because the transition is hidden from many experimental probes. Here, using zero-field muon spin relaxation (ZF-$\mu$SR) technique, we observe strong TRSB signals below 70 K in the newly discovered kagome superconductor CsV$_3$Sb$_5$. The TRSB state emerges from the 2 x 2 charge density wave (CDW) phase present below ~ 95 K. By carrying out optical second-harmonic generation (SHG) experiments, we also find that inversion symmetry is maintained in the temperature range of interest. Combining all the experimental results and symmetry constraints, we conclude that the interlayer coupled chiral flux phase (CFP) is the most promising candidate for the TRSB state among all theoretical proposals of orbital current orders. Thus, this prototypical kagome metal CsV3Sb5 can be a platform to establish a TRSB current-ordered state and explore its relationship with CDW, giant AHE, and superconductivity.