Electronic structure of self-doped layered ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$ material revealed by x-ray absorption spectroscopy and photoelectron spectromicroscopy

We have studied the electronic structure of ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$ using a combination of Eu $L_3$-edge x-ray absorption spectroscopy (XAS) and space-resolved angle-resolved photoemission spectroscopy (ARPES). From the Eu $L_3$-edge XAS, we have found that the Eu in this system is in mixed valence state with coexistence of ${\mathrm{Eu}}^{2+}/{\mathrm{Eu}}^{3+}$. The bulk charge doping was estimated to be $\sim 0.3$ per Bi site in ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$, which corresponds to the nominal $x$ in a typical RE${\mathrm{O}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$ system (RE: rare-earth elements). From the space-resolved ARPES, we have ruled out the possibility of any microscale phase separation of Eu valence in the system. Using a microfocused beam we have observed the band structure as well as the Fermi surface that appeared similar to other compounds of this family with disconnected rectangular electronlike pockets around the $X$ point. The Luttinger volume analysis gives the effective carrier to be 0.23 electrons per Bi site in ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$, indicating that the system is likely to be in the underdoped region of its superconducting phase diagram.

Figure 1

(a) The crystal structure of ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$ as obtained from structural refinement of Ref. [8], the two nonequivalent crystal sites for Eu are shown. (b) The Eu $L_3$-edge x-ray absorption spectrum (red bubbles) shown along with the best fit (black solid line) composed of two Lorentzian components (blue solid lines) plus the background function (dashed line). Inset shows a sketch of the fluorescence yield (PFY) detection geometry.

Figure 2

(a) Photoelectron spectromicroscopy image of the cleaved single crystal of ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$. The photoemission intensity is integrated from $-3.6$ to 0.2 eV with respect to $E_F$. The red solid square is showing the region selected for high resolution images. (b) Photoemission spectra taken in the A and B region in (a). (c) Angle-integrated valence band photoemission (PES) of ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$. The inset shows the fine images of photoelectron spectromicroscopy in ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$ [the spatial region is depicted as a solid box in (a)], the three images are taken at different energies and shown as shaded regions underneath the PES spectrum.

Figure 3

(a) Fermi surface map and (b) photoemission intensity map at $E=-0.17$ eV of ${\mathrm{Eu}}_3{\mathrm{F}}_4{\mathrm{Bi}}_2{\mathrm{S}}_4$ taken at $h\nu =74$ eV photons. Both maps were obtained by integrating the spectra in the range of $\pm 80$ meV. Band dispersion near $E_F$ of (c) cut1 and (d) cut2 indicated in (b). (e) MDCs of cut1 and cut2, obtained by integrating $E=-0.25\pm 0.1$ eV as depicted in (c) and (d). Peak positions were extracted using two Gaussians for cut1 and four Gaussians for cut2. Fit results are also shown, and the peak positions were denoted by circles in (c), (d), and (e). (f) EDCs of cut1 and cut2. Peak positions of EDCs were indicated by squares in (c), (d), and (f). Using the peak positions of MDCs and EDCs, electronlike bands were determined by the fit using quadratic functions depicted in (c) and (d).

Figure 4

(a) Schematic diagram of the Fermi surfaces and Brillouin zone. (b) A sketch of the atomic arrangement in the ${\mathrm{BiS}}_2$ layer with tetragonal symmetry. (c) ARPES spectra (second derivative in the energy direction) measured along the high symmetry lines of the Brillouin zone as depicted in (a).