Nodeless electron pairing in CsV$_3$Sb$_5$-derived kagome superconductors

TL;DR

This study reveals a nodeless, isotropic superconducting gap in CsV3Sb5-derived kagome superconductors using ARPES, showing robustness against charge order variations and evidence of time-reversal symmetry breaking, advancing understanding of their unconventional pairing.

Contribution

First direct momentum-resolved observation of a nodeless, isotropic, and orbital-independent superconducting gap in CsV3Sb5-derived kagome superconductors, with evidence of TRS breaking.

Findings

Superconducting gap is nodeless and isotropic in momentum space.

The gap structure is robust against charge order variations.

Signatures of time-reversal symmetry breaking in the superconducting state.

Abstract

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.