Ferroelectric Soft Phonons, Charge Density Wave Instability and Strong Electron-Phonon Coupling in BiS2-Layered Superconductors

TL;DR

This study uses advanced first principles calculations to explore ferroelectricity, charge density wave instability, and electron-phonon interactions in BiS2-layered superconductors, revealing mechanisms behind their superconductivity.

Contribution

It provides the first detailed theoretical analysis of phonon behavior, charge ordering, and electron-phonon coupling in BiS2 superconductors, highlighting the interplay of ferroelectric and CDW phases.

Findings

Presence of anharmonic ferroelectric soft phonons in LaOBiS2.

Electron doping induces charge density wave instabilities.

BiS2 is a strong electron-phonon coupled superconductor near competing phases.

Abstract

Very recently a new family of layered materials, containing BiS2 planes was discovered to be superconducting at temperatures up to Tc=10 K, raising questions about the mechanism of superconductivity in these systems. Here, we present state-of-the-art first principles calculations that directly address this question and reveal several surprising findings. The parent compound LaOBiS2 possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of $\approx 10 \mu C/cm^2$, which is comparable to the well-known ferroelectric BiFeO3. Upon electron doping, new unstable phonon branches appear along the entire line Q=(q,q,0), causing Bi/S atoms to order in a one-dimensional charge density wave (CDW). We find that BiS2 is a strong electron-phonon coupled superconductor in the vicinity of competing ferroelectric and CDW phases. Our results suggest new directions to tune the balance between these phases and increase Tc in this new class of materials.