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

Yu Li, Chennan Wang+18 more|arXiv (Cornell University)|Jul 22, 2021

Topological Materials and Phenomena50 references90 citations

TL;DR

The paper reports time-reversal symmetry breaking in CsV3Sb5 detected by zero-field μSR, emerging from the 2×2 CDW phase, and proposes an interlayer coupled chiral flux phase as the leading current-order candidate compatible with SHG symmetry constraints.

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-$μ$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.

Motivation & Objective

• Motivate search for spontaneous time-reversal symmetry breaking in CsV3Sb5 and related kagome metals.
• Identify the TRSB phase and its relationship to the CDW and superconductivity.
• Characterize the symmetry properties of the TRSB state using multiple experimental probes.
• Propose a concrete current-ordered state consistent with experimental constraints.

Proposed method

• Perform zero-field μSR to detect TRSB signals in CsV3Sb5 and determine the onset temperature (~70 K).
• Use optical second-harmonic generation (SHG) to test inversion symmetry in the relevant temperature range.
• Apply symmetry considerations to constrain possible TRSB order parameters.
• Compare experimental findings with theoretical orbital current order proposals and assess consistency with CDW and AHE phenomenology.

Experimental results

Research questions

• RQ1 Does CsV3Sb5 exhibit spontaneous time-reversal symmetry breaking associated with its CDW phase?
• RQ2 What TRSB order parameter(s) are compatible with the observed μSR signals and SHG symmetry constraints?
• RQ3 Is an interlayer coupled chiral flux phase a viable realization of the TRSB state in CsV3Sb5?
• RQ4 How does the TRSB state relate to the 2×2 CDW, AHE, and superconductivity in this material?

Key findings

• Strong TRSB signals appear below 70 K in CsV3Sb5 as detected by ZF-μSR.
• The TRSB state emerges from the 2×2 CDW phase present below ~95 K.
• SHG experiments show that inversion symmetry is maintained in the temperature range studied.
• Symmetry analysis supports the interlayer coupled chiral flux phase as the most promising TRSB candidate among orbital current orders.
• CsV3Sb5 behaves as a platform to study a TRSB current-ordered state and its relation to CDW, AHE, and superconductivity.