Kagome materials $A$V$_3$Sb$_5$ ($A$=K,Rb,Cs): pairing symmetry and pressure-tuning studies

TL;DR

This review discusses the unconventional superconductivity and charge density wave phenomena in kagome metals $A$V$_3$Sb$_5$, highlighting how pressure and thickness tuning reveal their complex interplay and evolution.

Contribution

It provides a comprehensive synthesis of experimental findings on pairing symmetry and pressure effects in $A$V$_3$Sb$_5$, emphasizing the competition and coexistence of SC and CDW.

Findings

CsV$_3$Sb$_5$ exhibits nodeless superconductivity.

Pressure suppresses CDW and enhances superconductivity.

Emergence of a new CDW state under pressure leads to two superconducting domes.

Abstract

The vanadium-based kagome metals $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) host a superconducting ground state that coexists with an unconventional charge density wave (CDW). The CDW state exhibits experimental signatures of chirality, electronic nematicity, and time-reversal-symmetry-breaking, raising the questions whether the superconductivity (SC) in $A$V$_3$Sb$_5$ may also be unconventional, how SC interplays with CDW, and how the two orders evolve upon tuning. This article reviews studies of the superconducting pairing symmetry, and the tuning of SC and CDW in the $A$V$_3$Sb$_5$ compounds. Various experimental techniques consistently find that CsV$_3$Sb$_5$ exhibits nodeless SC, which remains robust regardless whether the CDW is present. Under hydrostatic pressure, SC in $A$V$_3$Sb$_5$ becomes enhanced as the CDW is gradually suppressed, revealing a competition between the two orders. In CsV$_3$Sb$_5$, a new CDW state emerges under pressure that competes more strongly with SC relative to the CDW at ambient pressure, and results in two superconducting domes that coexist with CDW. After the CDW in $A$V$_3$Sb$_5$ is fully suppressed with hydrostatic pressure, a further increase in pressure leads to a nonmonotonic evolution of the superconducting transition temperature driven by lattice modulations. Thickness is shown to be a powerful tuning parameter in $A$V$_3$Sb$_5$ thin flakes, revealing the evolution of CDW and SC upon dimensional reduction, and can be combined with hydrostatic pressure to shed light on the interplay between SC and CDW. Based on results reviewed in this article, we discuss outstanding issues to be addressed in the $A$V$_3$Sb$_5$ systems.