Electron-phonon superconductivity near charge-density-wave instability in LaO_0.5F_0.5BiS₂: Density-functional calculations

We discuss the electronic structure, lattice dynamics, and electron-phonon interaction of the newly discovered superconductor LaO_0.5F _0.5BiS₂ using density-functional-based calculations. A strong Fermi surface nesting at k=(π,π,0) suggests a proximity to charge-density-wave instability and leads to imaginary harmonic phonons at this k point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant 0.85, prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials.