Electronic structure of single-crystalline ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ studied by angle-resolved photoemission spectroscopy

${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ is a new layered superconductor. We have studied the low-lying electronic structure of the single-crystalline ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ superconductor, whose superconducting transition temperature is 4.83 K, with angle-resolved photoemission spectroscopy. The Fermi surface consists of two small electron pockets around the $X$ point and shows little warping along the $k_z$ direction. Our results demonstrate the multiband and two-dimensional nature of the electronic structure. The comparison between the photoemission data and the band calculations gives the renormalization factor of $\sim$1.14, indicating the rather weak electron correlations in this material. Moreover, we found that the actual electron doping level and Fermi surface size are much smaller than what are expected from the nominal composition, which could be largely explained by the bismuth deficiency. The small Fermi pocket size and the weak electron correlations found here put strong constraints on theory, and suggest that the ${\mathrm{BiS}}_2$-based superconductors could be conventional BCS superconductors mediated by the electron-phonon coupling.

Figure 1

(a) Representative unit cell of ${\mathrm{NdO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ and its three-dimensional Brillouin zone. The dashed line represents the cleaved plane between the two ${\mathrm{BiS}}_2$ layers. (b) Temperature dependence of the electrical resistivity of ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. The inset presents the enlarged view of the same data. (c) The valence band structure around the $M$ point along the $M$-$X$ direction and corresponding angle-integrated energy distribution curves (EDCs). The inset is the enlarged view of the features near $E_F$. (d) The photoemission intensity map of ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ at $E_F$ over the projected two-dimensional Brillouin zone, which was obtained through mirroring the data with respect to $k_x$ and $k_y$ axes. The intensity was integrated over a window of ($E_F-15$ meV, $E_F+15$ meV). The Fermi surface sheets for the ${\alpha }_1$ and ${\alpha }_2$ bands are shown by the rectangles with different colors. The data were taken with 100 eV photons.

Figure 2

(a), (c), and (d) The photoemission intensity, corresponding EDCs, and MDCs along the #1 direction. (b), (e) and (f) The same as (a), (c), and (d), respectively, but taken along the #2 direction. The data were taken with 49 eV photons, corresponding to the same $k_z$ as that for 100 eV photons.

Figure 3

(a) The photoemission intensity of ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ in the $\Gamma ZRX$ plane. The intensity was integrated over a window of ($E_F-15$ meV, $E_F+15$ meV). Different $k_z$'s were accessed by varying the photon energy as indicated by the solid blue lines, where an inner potential of 15 eV is used to calculate $k_z$. Note that the weak streaklike features around the $\Gamma$ point originate from the background noise. (b) The photoemission intensities along or parallel to the $\Gamma$-$X$ direction, taken at different photon energies. (c) The photon energy dependence of the MDCs at $E_F$. (d) The same as (a), but in the XRAM plane. (e) and (f) The same as (b) and (c), respectively, but along or parallel to the $M$-$X$ direction.

Figure 4

(a) The calculated band structures without the spin-orbit coupling for ${\mathrm{NdO}}_{0.84}{\mathrm{F}}_{0.16}{\mathrm{BiS}}_2$ (the left side energy axis) and ${\mathrm{NdO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ (the right side one). (b) The comparison of the Fermi surface between the photoemission intensity map and the DFT for $x=0.5$ and 0.16 fluorine substitution levels. (c) and (d) The comparison of the low-lying band structure near $E_F$ between the photoemission data and the DFT calculations along the $\Gamma$-$X$ direction and the $M\text{−}X$ direction, respectively. The white open circles are the peak positions of the MDCs shown in Figs. 2 and 2 through Lorentzian fitting, and the green open circles are derived by scaling the white open circles by a factor of 1.14 in energy. The red solid lines are the band dispersions from DFT calculations. The ARPES data were taken with 40 eV photons.