Gapless excitations inside the fully gapped kagome superconductors AV$_3$Sb$_5$

TL;DR

This paper reveals that kagome superconductors AV$_3$Sb$_5$ can host gapless excitations within a fully gapped state due to symmetry-protected edge states arising from coexistence of time-reversal symmetry breaking and conventional superconductivity.

Contribution

It introduces a theoretical model explaining the coexistence of gapless edge states with fully gapped bulk in AV$_3$Sb$_5$ superconductors, resolving experimental contradictions.

Findings

Edge states remain gapless near symmetry-breaking domains.

The model captures key features of the Fermi surface.

Coexistence of TRSB and conventional pairing explains observations.

Abstract

The superconducting gap structures in the transition-metal-based kagome metal AV$_3$Sb$_5$ (A=K,Rb,Cs), the first family of quasi-two-dimensional kagome superconductors, remain elusive as there is strong experimental evidence for both nodal and nodaless gap structures. Here we show that the dichotomy can be resolved because of the coexistence of time-reversal symmetry breaking with a conventional fully gapped superconductivity. The symmetry protects the edge states which arise on the domains of the lattice symmetry breaking order to remain gapless in proximity to a conventional pairing. We demonstrate this result in a four-band tight-binding model using the V $d_{X^2-Y^2}$-like and the in-plane Sb $p_z$-like Wannier functions that can faithfully capture the main feature of the materials near the Fermi level.