Ferro-lattice-distortions and charge fluctuations in superconducting LaO$_{1-x}$F$_{x}$BiS$_{2}$

TL;DR

This study reveals ferro-lattice distortions and charge fluctuations in LaO$_{1-x}$F$_{x}$BiS$_{2}$ superconductors, showing in-plane sulfur displacements form ferro-distortive patterns affecting charge carrier availability.

Contribution

It uncovers coherent ferro-distortive lattice patterns in LaO$_{1-x}$F$_{x}$BiS$_{2}$, challenging previous symmetry assumptions and linking lattice distortions to charge carrier trapping.

Findings

Superlattice pattern from sulfur displacements observed.

Ferro-distortive patterns persist in superconducting state.

Charge trapping reduces carriers for pairing.

Abstract

Competing ferroelectric and charge density wave phases have been proposed to be present in the electron-phonon coupled LaO$_{1-x}$F$_{x}$BiS$_{2}$ superconductor. The lattice instability arises from unstable phonon modes that can break the crystal symmetry. Upon examination of the crystal structure using single crystal diffraction, we find a superlattice pattern arising from coherent in-plane displacements of the sulfur atoms in the BiS$_{2}$ superconducting planes. The distortions morph into coordinated ferro-distortive patterns, challenging previous symmetry suggestions including the possible presence of unstable antiferro-distortive patterns. The ferro-distortive pattern remains in the superconducting state, but with the displacements diminished in magnitude. Moreover, the sulfur displacements can exist in several polytypes stacked along the c-axis. Charge carriers can get trapped in the lattice deformations reducing the effective number of carriers available for pairing.