Proximity to Fermi-surface topological change in superconducting $\mathrm{La}{\mathrm{O}}_{0.54}{\mathrm{F}}_{0.46}\mathrm{Bi}{\mathrm{S}}_2$

The electronic structure of nearly optimally doped novel superconductor $\mathrm{La}{\mathrm{O}}_{1-x}{\mathrm{F}}_x\mathrm{Bi}{\mathrm{S}}_2(x=0.46)$ was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level $\left(E_{\mathrm{F}}\right)$, which are well reproduced by first-principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near $E_{\mathrm{F}}$ shows a squarelike distribution around the $\Gamma \left(Z\right)$ point in addition to electronlike Fermi-surface (FS) sheets around the $X\left(R\right)$ point, indicating that FS of $\mathrm{La}{\mathrm{O}}_{0.54}{\mathrm{F}}_{0.46}\mathrm{Bi}{\mathrm{S}}_2$ is in close proximity to the theoretically predicted topological change.

Figure 1

Valence band energy distribution curves (a) of $\mathrm{La}{\mathrm{O}}_{0.54}{\mathrm{F}}_{0.46}\mathrm{Bi}{\mathrm{S}}_2$ taken along approximately the $\Gamma \left(Z\right)-X\left(R\right)$ direction using photon energy of 70 eV, and their negative values of second derivatives [lower panel of (b)] as a function of binding energy and wave vector. Results of the band calculation for $x=0.46$ along the Γ-$X$ direction is also plotted in the lower panel of (b) as dashed orange lines. The upper panel of (b) shows a Brillouin zone of $\mathrm{La}{\mathrm{O}}_{1-x}{\mathrm{F}}_x\mathrm{Bi}{\mathrm{S}}_2$ and calculated FS for $x=0.46$ (orange lines). The blue line corresponds to the measured momentum region.

Figure 2

EDCs (a) and their second derivatives (b), and second derivatives of MDCs (c) in the vicinity of $E_{\mathrm{F}}$ taken along the $\Gamma \left(Z\right)-X\left(R\right)$ direction near the $X\left(R\right)$ point. The thick line in (a) and green arrow (b) show the energy position of a broad hump. The inset of (b) shows the Brillouin zone and the measured momentum region (blue line).

Figure 3

(a) ARPES intensity plot integrated over $\pm 40\mathrm{meV}$ with respect to $E_{\mathrm{F}}$ as a function of two-dimensional wave vectors taken at $h\nu =70\mathrm{eV}$, which is symmetrized according to the fourfold symmetry of the material. The original data is surrounded by gray dots. Red dashed lines denote Brillouin zone boundaries, and orange lines show calculated FS for $x=0.46$. The blue line shows the momentum cut of (c), while green lines show the measured momentum areas of (d). (b) is the same as (a) but taken with the use of $h\nu =880\mathrm{eV}$ light ($\pm 100\mathrm{meV}$ with respect to $E_{\mathrm{F}}$). (c) and (d) EDCs and intensity plots as a function of wave vector and binding energy taken in momentum cuts shown in (a). The half-transparent gray line in (c) is a guide for the eye, and the half-transparent red lines in (d) are calculated bands at corresponding momenta.