Character of charge carriers in a low-carrier superconductor Ag$_{5}$Pb$_{2}$O$_{6}$ revealed by core-level photoemission

Identifying an element-specific orbital character of electrons near the Fermi level is important to understand electronic properties of a metallic solid, as manifested in high-$T_{c}$ superconducting cuprates in which doped-hole and --electron characters are different from each other. Photoemission spectroscopy (PES) may serve as an ideal tool for that purpose by varying photon energies. However, in the case of a low-carrier superconductor Ag$_{5}$Pb$_{2}$O$_{6}$, distinguishing between Ag 5$s$ and Pb 6$s$ conduction bands is difficult by the technique. Here, we present another method utilizing core-level PES. The Pb 4$f$ spectrum exhibits a quite asymmetric line shape unlike the Ag 3$d$ spectrum. The asymmetry can be attributed to electron-hole pair excitations across the Fermi level in the Pb 6$s$ band affected by strong local attractive Coulomb potential due to the Pb 4$f$ hole. However, the Pb 4$f$ spectrum is not explained by the well-known Doniach-\v{S}unji\'{c} line shape for a simple metal. By employing Pb 6$s$ PDOS from first-principle calculations, the line shape is successfully generated, thus confirming that the main charge-carrier character in Ag$_{5}$Pb$_{2}$O$_{6}$ is Pb 6$s$ electrons.