In-plane Charge Fluctuations in Bismuth Sulfide Superconductors

Abstract

Evidence for local charge fluctuations linked to a charge disproportionation of the Bi ions in the distorted lattice of superconducting LaO${}_{1-x}$F${}_x$ BiS${}_2$ is presented. In-plane short-range distortions of sulfur atoms up to 0.3 Å 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 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.

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