Charge fluctuations in a disordered superconductor, LaO$_{1-x}$F$_{x}$BiS$_{2}$

LaO1-xFxBiS2 is a disordered, non-magnetic superconductor with a transition temperature of 10.8 K at x $=$ 0.5. The parent compound, LaOBiS2, is a band insulator with a layered tetragonal structure. The evolution of the crystal structure and nano-scale atomic fluctuations are investigated as a function of temperature and composition using neutron scattering. Even though the symmetry remains unchanged with doping, lattice strain develops along the c-axis and buckling of the BiS2 plane changes orientation. In addition, strong local distortions are observed around the Bi ion that are in response to charge fluctuations. Two distinct Bi-S plaquettes are present due to atomic displacement of in-plane sulfur because the Bi ion undergoes a charge disproportionation. The charge fluctuations along with spin-orbit coupling most likely play important roles in the mechanism of superconductivity in this system.