Ferroelectric Soft Phonons, Charge Density Wave Instability, and Strong Electron-Phonon Coupling in BiS$_2$-Layered Superconductors

Very recently a new family of layered materials, containing BiS$_2$ planes was discovered to be superconducting at temperatures up to T$_c$=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 [1]. The parent compound LaOBiS$_2$ 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 BiFeO$_3$. 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 BiS$_2$ 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 T$_c$ in this new class of materials.\\[4pt] [1] T. Yildirim, arXiv:1210.2418 (2012).