Bulk superconductivity in Pb-substituted ${\mathrm{BiS}}_2$-based compounds studied by hard x-ray spectroscopy

In this study, we investigate the bulk electronic structure of Pb-substituted ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ single crystals, using two types of hard x-ray spectroscopy. High-energy-resolution fluorescence-detected x-ray absorption spectroscopy revealed a spectral change at low temperatures. Using density functional theory (DFT) simulations, we find that the temperature-induced change originates from a structural phase transition, similar to the pressure-induced transition in ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. This finding suggests that the mechanism of bulk superconductivity induced by Pb substitution is the same as that under high pressure. Furthermore, a low-valence state with a mixture of divalent and trivalent Bi ions is discovered using hard x-ray photoemission spectroscopy with the aid of DFT calculations.

Figure 1

(a) Bi $L{\alpha }_1$ HERFD-XANES spectra for $x=0$, 0.05, and 0.09 at 200 K. Simultaneously measured conventional PFY spectra are also displayed. (b) Temperature dependence of Bi $L{\alpha }_1$ HERFD-XANES spectra for $x=0$ and 0.05.

Figure 2

Experimental and simulated Bi $L_3$-edge XANES spectra of ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{Bi}}_{1-x}{\mathrm{Pb}}_x{\mathrm{S}}_2$. (a) Temperature dependence of Bi $L{\alpha }_1$ HERFD-XANES spectrum for $x=0$.09. Simulated spectra for $x=0$ in the AP and HP phases are shown for comparison. The simulated spectra are broadened by Gaussian and Lorentzian functions representing the experimental energy-resolution and lifetime effects. (b) Details of the simulated spectra in (a). These are composed of unoccupied Bi $s$- and $d$-orbital pDOSs after multiplying the transition probabilities. The Bi $p$-orbital pDOS is also shown for reference.

Figure 3

Temperature dependence of Pb $L{\alpha }_1$ HERFD-XANES spectrum for $x=0$.09. Bi $L{\alpha }_1$ HERFD-XANES spectra (shifted) are also shown for comparison.

Figure 4

Bi and Pb $4f$ core-level HAXPES spectra of ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{Bi}}_{1-x}{\mathrm{Pb}}_x{\mathrm{S}}_2$. The spectra are normalized by the area under the curves after subtraction of the Shirley-type background [33]. (a) Bi $4f$ core-level HAXPES spectra, including the S $2p$ core levels, for $x=0$ and 0.09. The spectrum of ${\mathrm{LaOBiS}}_2$ is also shown. (b) The temperature dependence of the Pb $4f$ core-level HAXPES spectrum for $x=0$.09. (c) and (d) The results of the line-shape analysis for the HAXPES spectra.

Figure 5

VB-HAXPES spectra of ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{Bi}}_{1-x}{\mathrm{Pb}}_x{\mathrm{S}}_2$. The spectrum of ${\mathrm{LaOBiS}}_2$ is also shown. The inset shows the enlarged spectra near $E_{\mathrm{F}}$. The spectra are normalized by the area under the curves after subtraction of the Shirley-type background.

Figure 6

Total DOS and some pDOSs of ${\mathrm{LaOBiS}}_2$ and ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{Bi}}_{1-x}{\mathrm{Pb}}_x{\mathrm{S}}_2$. (a) Overall and (b) enlarged views.