Structural evolution of the kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) through charge density wave order

TL;DR

This study investigates the structural evolution of kagome superconductors AV3Sb5 (A=K, Rb, Cs) through charge density wave transitions, revealing complex superstructures and the effects of magnetic fields using advanced crystallography and machine learning.

Contribution

It provides detailed structural insights into AV3Sb5 compounds, especially the complex evolution in CsV3Sb5, using synchrotron X-ray crystallography and machine learning techniques.

Findings

KV3Sb5 and RbV3Sb5 have 2x2x2 superstructures with tri-hexagonal deformation.

CsV3Sb5 exhibits staged structural evolution from 2x2x1 to 2x2x4 supercells.

The CDW state in CsV3Sb5 is insensitive to magnetic fields up to 28 T.

Abstract

The kagome superconductors KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$ are known to display charge density wave (CDW) order which impacts the topological characteristics of their electronic structure. Details of their structural ground states and how they evolve with temperature are revealed here using single crystal X-ray crystallographic refinements as a function of temperature, carried out with synchrotron radiation. The compounds KV$_3$Sb$_5$ and RbV$_3$Sb$_5$ present 2$\times$2$\times$2 superstructures in the $Fmmm$ space group with a staggered tri-hexagonal deformation of vanadium layers. CsV$_3$Sb$_5$ displays more complex structural evolution, whose details have been unravelled by applying machine learning methods to the scattering data. Upon cooling through the CDW transition, CsV$_3$Sb$_5$ displays a staged progression of ordering from a 2$\times$2$\times$1 supercell and a 2$\times$2$\times$2 supercell into a final 2$\times$2$\times$4 supercell that persists to $T$ = 11 K and exhibits an average structure where vanadium layers display both tri-hexagonal and Star of David patterns of deformations. Diffraction from CsV$_3$Sb$_5$ under pulsed magnetic fields up to $\mu_0H$ = 28 T suggest the real component of the CDW state is insensitive to external magnetic fields.