Electronic structure of YbGa${}_{1.15}$Si${}_{0.85}$ and YbGa${}_x$Ge${}_{2-x}$ probed by resonant x-ray emission and photoelectron spectroscopies

We performed an x-ray spectroscopic study combining resonant x-ray emission spectroscopy (RXES) and photoelectron spectroscopy on the superconducting ternary silicide YbGa${}_{1.15}$Si${}_{0.85}$ and nonsuperconducting ternary germanide YbGa${}_x$Ge${}_{2-x}$ ($x$ = 1.0 and 1.1). The Yb valence for all three compounds is found to be about 2.3. In YbGa${}_{1.15}$Si${}_{0.85}$ no temperature dependence of the Yb valence is observed in the RXES spectra in the temperature range of 7–300 K, while the valence shows a drastic increase under pressure from the Yb${}^{2+}$ state partially including itinerant electrons to the localized Yb${}^{3+}$ state. Differences are observed in the valence-band spectra of the photoelectron spectroscopy between YbGa${}_{1.15}$Si${}_{0.85}$ and YbGa${}_x$Ge${}_{2-x}$, which may be attributed to the difference of crystal structure. We conclude that both the crystal structure of the planar GaSi layer in YbGa${}_{1.15}$Si${}_{0.85}$ and the resultant electronic structure may have a crucial role in the occurrence of superconductivity.

Figure 1

Crystal structures of (a) YbGaSi and (b) YbGaGe. (c) PFY-XAS spectra of YbGa${}_{1.15}$Si${}_{0.85}$ (solid line), and YbGaGe (dashed line) at 300 K.

Figure 2

(a) and (e) PFY-XAS spectra for YbGa${}_{1.15}$Si${}_{0.85}$ and YbGaGe at 300 K. (b) and (f) RXES spectra as a function of the incident photon energies. The vertical position of the each RXES spectra corresponds to the $E_{\mathit{in}}$ of PFY-XAS spectrum they were measured at in the panels (a) and (e), respectively. (c) and (g) Contour images of the RXES spectra. (d) and (h) Analyzed results of the Raman and fluorescence components as a function of the incident photon energies by the fitting procedure with the PFY-XAS spectra. Closed circles and closed squares correspond to the 2 +  and 3 +  components, respectively.

Figure 3

Temperature dependence of (a) the PFY-XAS spectra, (b) RXES spectra, and (c) valence for YbGa${}_{1.15}$Si${}_{0.85}$.

Figure 4

Pressure dependence of (a) the PFY-XAS spectra, (b) RXES spectra, and (c) valence for YbGa${}_{1.15}$Si${}_{0.85}$.

Figure 5

(a) The valence-band spectra of YbGa${}_{1.15}$Si${}_{0.85}$ and YbGa${}_{1.1}$Ge${}_{0.9}$ at the incident photon energy of $E_{\mathit{in}}=1100$ eV and 50 K. (b) Expanded view of the valence-band spectra for (a) around Yb${}^{3+}$ peaks. Yb${}^{3+}$ spectra based on the atomic model calculation are also shown. (c) Valence-band spectra at $E_{\mathit{in}}=1100$ eV and 50 K. (d) High-resolution valence-band spectra at $E_{\mathit{in}}=35$ eV and 7–9 K.

Figure 6

Resonant valence-band spectra for YbGa${}_{1.15}$Si${}_{0.85}$ at 50 K; (a)–(c) at Yb $3d$ absorption edge and (d) and (e) at Ga $2p_{3/2}$ absorption edge with total electron yield (TEY) spectra, corresponding x-ray absorption spectra (XAS). The vertical position of the resonant valence-band spectra in the left panel with regards to the absorption spectrum in the right panel corresponds to the incident energy they were collected at. The PFY-XAS spectra with wide incident energy range is also shown in the inset figures of (a) and (d). The intensity is normalized by the incident beam intensity in (b) and (e), and by the maximum intensity to see the Auger lines in (c). In (c) inset figure shows the incident energy ($E_{\mathit{in}}$) dependence of the binding energy and intensity of the Auger lines. Arrows in (c) are Auger lines.

Figure 7

Constant initial state (CIS) spectra at given binding energies as a function of the incident photon energies for YbGa${}_{1.15}$Si${}_{0.85}$ at 50 K, obtained from the data in Fig. 6, with XAS spectrum at Yb $3d$ absorption edge. Right-upper panel shows the extended figure for the spectra at the binding energies of 7.48 and 11.63 eV, corresponding Yb${}^{3+}$ peaks [see Fig. 5a].