In-plane charge fluctuations in bismuth-sulfide superconductors

Abstract

The local atomic structure of the new nonmagnetic superconducting system ${\text{LaO}}_{1\ensuremath{-}x}{\text{F}}_{x}{\text{BiS}}_{2}$ is investigated using neutron diffraction and the pair density function analysis. Evidence for local charge fluctuations linked to a charge disproportionation of the Bi ions in the distorted lattice of superconducting ${\text{LaO}}_{1\ensuremath{-}x}{\text{F}}_{x}{\text{BiS}}_{2}$ is presented. In-plane short-range distortions of sulfur atoms up to 0.3 \AA{} in magnitude break site symmetry and create two distinct environments around Bi. Out-of-plane motion of apical sulfur brings it closer to the La-O/F doping layer with increasing $x$ that may lead to a charge transfer conduit between the doping layers and the superconducting ${\text{BiS}}_{2}$ planes. The mechanism for superconductivity may arise from the interplay between charge density fluctuations and an enhanced spin-orbit coupling suggested theoretically that induces spin polarization.