In-plane charge fluctuations in bismuth-sulfide superconductors

The local atomic structure of the new nonmagnetic superconducting system ${\text{LaO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ 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 ${\text{LaO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ 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 ${\text{BiS}}_2$ 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.

Figure 1

(a) The crystal structure of ${\text{LaO}}_{1-x}{\text{F}}_x{\text{BiS}}_2$ with the BiS tetrahedra. (b) The neutron powder diffraction pattern (black symbols) of $\text{LaO}{\text{BiS}}_2$ at 6 K was collected using NOMAD at the Spallation Neutron Source of Oak Ridge National Laboratory. The red, green, and blue solid lines represent the calculated intensity, background, and difference between the observed and calculated intensities, respectively. (c) The neutron powder diffraction pattern of ${\text{LaO}}_{0.5}{\text{F}}_{0.5}{\text{BiS}}_2$ at 2 K has its intensity considerably reduced in comparison to the parent compound of (b). At the same time, the background (green solid line) has increased. (d) The comparison of the (110) and (114) Bragg peaks between the $x=0$ and $x=0.5$ compositions shows significant changes of the nonzero $l$ peaks with $x$.

Figure 2

(a) A plot of the composition dependence of the data corresponding to the local structures of $x=0$, 0.2, 0.3, and 0.5. The data were collected at 6 K. (b) The PDF of the local structure for the parent compound at 6 K (black symbols) is compared to the average (dashed blue line) and local (solid red line) model PDFs. The agreement factor $\left(A_{\mathrm{factor}}\right)$ between the local model and data is 0.156 while between the average model and data it is 0.283. (c) A schematic of the ${\text{BiS}}_2$ plane showing the antiferrodistortive type mode. (d) A schematic of the ${\text{BiS}}_2$ plane showing the ferrodistortive type mode. The displacements of S1 are either in the $x$ or $y$ direction.

Figure 3

(a) The PDF of the local structure for $x=0.5$ at 2 K (black symbols) is compared to the average (dashed blue line) and local (solid red line) model PDFs. The $A_{\mathrm{factor}}$ between the local model and data is 0.18 while between the average model and data it is 0.239. The inset is a plot of the partial PDFs for Bi-S1 and Bi-S2 for $x=0$ (top) and $x=0.5$ (bottom). The partial of Bi-S1 for both the average and local models is shown while the partial of Bi-S2 is only shown for the local model. (b) A schematic of the Bi-S tetrahedron with the bond lengths determined from the refinement of the local structures of $0\le x\le 0.5$. (c) Stripes of charge fluctuations in the two ${\text{BiS}}_2$ planes of the crystal structure. The $\left(x,-x\right)$ and $\left(x,-y\right)$ refer to the S1 coordinates in the $z\sim 1/3$ and $z\sim 2/3$ planes, respectively.

Figure 4

(a) The temperature dependence of the short (square) and long (circle) local Bi-S1 bond lengths is contrasted to the Bi-S1 average bond length (triangle). (b) The temperature dependence of the S2 atom from the oxygen plane obtained from fitting the data and from the Rietveld refinement of the average structure.