Spectroscopic evidence of the existence of substantial $\text{Ca}3d$ derived states at the Fermi level in the Ca-intercalated graphite superconductor ${\text{CaC}}_6$

We have performed soft x-ray photoemission studies of Ca-intercalated graphite superconductor ${\text{CaC}}_6\left(T_{\text{c}}=11.2\text{K}\right)$. The valence-band spectrum shows six main structures, with a peak at the Fermi level $\left(E_{\text{F}}\right)$, which correspond to those of calculated DOS. The intensity of the $E_{\text{F}}$ peak is resonantly enhanced at the $\text{Ca}2p\text{−}3d$ threshold, providing spectroscopic evidence for the existence of $\text{Ca}3d$ electrons at $E_{\text{F}}$, which is confirmed experimentally for the first time. The $\text{Ca}2p$ core-level spectrum has a very large asymmetric line shape, which is further possible suggestion of the existence of $\text{Ca}3d$ derived conduction electrons at Ca sites. These results strongly support the picture where $\text{Ca}3d$ derived interlayer band plays a crucial role for the superconductivity of this material with relatively high $T_{\text{c}}$.

Figure 1

(Color online) Valence-band SXPES spectra from ${\text{CaC}}_6$ (red/gray circles) and HOPG (blue/dark gray circles) using 1100 eV, together with calculated DOS. Green line is total DOS. Broken lines are partial DOS from $\text{C}2s$ (orange/light gray) and $\text{C}2p$ (blue/dark gray). Thin lines are partial DOS from $\text{Ca}4s$ (sky blue/light gray) and $\text{Ca}3d$ (red/gray).

Figure 2

RPES using the $\text{Ca}2p_{3/2}$ $\left(L_3\right)$ absorption edge. (a) The $\text{Ca}2p_{3/2}$ absorption edge of ${\text{CaC}}_6$. The labels on the spectra (1–6) correspond to photon energies for photoemission. (b) Valence band spectra of ${\text{CaC}}_6$ measured using photon energies near $\text{Ca}2p_{3/2}$ absorption edge. The arrows indicate the Auger transition.

Figure 3

(Color online) Core-level SXPES spectra of (a) $\text{C}1s$ of ${\text{CaC}}_6$ and HOPG as well as (b) $\text{Ca}2p$ of ${\text{CaC}}_6$. Circles are the experimental data, blue/dark gray thick lines are the fitting results, green/light green thin lines are the components used for the fittings, and black broken lines are background.