Ferroelectric soft phonons, charge density wave instability, and strong electron-phonon coupling in BiS2 layered superconductors: A first-principles study

Very recently a new family of layered materials, containing BiS2 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. The parent compound LaOBiS2 possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of approximate to 10 mu C/cm(2). Upon electron doping, new unstable phonon branches appear along 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 directions to increase Tc in this new class of materials.