Symmetry-broken ground state and phonon mediated superconductivity in Kagome CsV$_3$Sb$_5$

TL;DR

This paper uses first-principles calculations to reveal the atomistic structure of the charge density wave in CsV$_3$Sb$_5$, showing a symmetry-broken ground state that supports phonon-mediated superconductivity, thus clarifying its complex quantum phases.

Contribution

It provides the first detailed atomistic phase diagram of CsV$_3$Sb$_5$, identifying the reconstructed Kagome layers and their stacking orders, and demonstrates phonon-mediated superconductivity in this material.

Findings

The CDW ground state involves reconstructed Kagome layers with degenerate stacking orders.

The symmetry-broken state explains experimental observations of isotropic transport.

Superconductivity in CsV$_3$Sb$_5$ is phonon-mediated with a critical temperature matching experiments.

Abstract

The newly discovered family of non-magnetic Kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) provides a unique platform for exploring the interplay between charge density wave (CDW) order, superconductivity, non-trivial topology, and spontaneous time-reversal symmetry breaking. Although characterizing the CDW phase is essential for understanding and modeling these exotic phenomena, its nature remains unresolved. In this work, we employ first-principles free-energy calculations, accounting for both ionic kinetic energy and anharmonic effects, to resolve the atomistic phase diagram of CsV$_3$Sb$_5$ and its charge ordering structure. Our results uncover that the CDW ground state is formed by reconstructed vanadium Kagome layers in a triangular hexagonal pattern, featuring energetically degenerate different stacking orders. This accounts for the various out-of-plane modulations observed experimentally and supports the coexistence of multiple domains. The discovered symmetry-broken ground state is consistent with the absence of any electronic anisotropy in transport experiments. By combining anharmonic phonons with the calculation of electron-phonon matrix elements, we predict a superconducting critical temperature for the CDW phase in agreement with experiments, showing that superconductivity is phonon mediated. These findings not only resolve a long-standing structural puzzle, but also clarify the impact of the CDW in superconductivity, highlighting its fundamental importance in shaping the low-temperature quantum phase diagram of Kagome metals.