Frustrated structural instability in superconducting quasi-one-dimensional K$_2$Cr$_3$As$_3$

TL;DR

This study uncovers a frustrated structural instability in the superconducting quasi-one-dimensional K$_2$Cr$_3$As$_3$, revealing complex lattice distortions and interactions that influence its superconducting properties.

Contribution

It provides the first combined neutron scattering and density functional theory evidence of a frustrated orthorhombic distortion in K$_2$Cr$_3$As$_3$, highlighting complex structural phenomena.

Findings

Identification of a significant phonon instability

Observation of temperature-independent atomic displacements

Detection of localized orthorhombic distortions

Abstract

We present neutron total scattering and density functional theory studies on quasi-one-dimensional superconducting K$_2$Cr$_3$As$_3$ revealing a frustrated structural instability. Our first principles calculations find a significant phonon instability which, under energy minimization, corresponds to a frustrated orthorhombic distortion. In diffraction studies we find large and temperature independent atomic displacement parameters which pair distribution analyses confirms and shows as resulting from highly localized orthorhombic distortions of the CrAs sublattice and coupled K displacements. These results suggest a far more complex phase diagram than previously assumed for this unusual superconductor with the likelihood of subtle interplays of structure, electron-phonon and magnetic interactions.