Crystal structure, lattice vibrations, and superconductivity of LaO1-xFxBiS2

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO0.5F0.5BiS2 and its nonsuperconducting parent compound LaOBiS2. The crystal structures and vibrational modes have been examined. Bragg peaks from the superconducting sample exhibit pronounced broadening compared to those from the nonsuperconducting sample. In the inelastic measurements, a large difference in the high-energy phonon modes was observed upon F doping. Alternatively, the low-energy modes remain almost unchanged between nonsuperconducting and superconducting states either by F doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.