# Element-Specific Orbital Character in a Nearly-Free-Electron Superconductor Ag$_5$Pb$_2$O$_6$ Revealed by Core-Level Photoemission

## TL;DR

This study uses core-level photoemission to reveal that Pb 6$s$ electrons dominate near the Fermi level in Ag$_5$Pb$_2$O$_6$, providing insight into its nearly-free-electron superconductivity and orbital contributions.

## Contribution

The paper introduces a novel core-level photoemission method to identify orbital contributions, confirming Pb 6$s$ dominance in a nearly-free-electron superconductor.

## Key findings

- Pb 6$s$ electrons dominate near the Fermi level.
- Pb 4$f_{7/2}$ spectrum asymmetry indicates local attractive interactions.
- Pb 6$s$ partial density of states explains spectral features.

## Abstract

Ag$_5$Pb$_2$O$_6$ has attracted considerable attention due to its novel nearly-free-electron superconductivity, but its electronic structure and orbital character of the Cooper-pair electrons remain controversial. Here, we present a method utilizing core-level photoemission to show that Pb 6$s$ electrons dominate near the Fermi level. We observe a strongly asymmetric Pb 4$f_{7/2}$ core-level spectrum, while a Ag 3$d_{5/2}$ spectrum is well explained by two symmetric peaks. The asymmetry in the Pb 4$f_{7/2}$ spectrum originates from the local attractive interaction between conducting Pb 6$s$ electrons and a Pb 4$f_{7/2}$ core hole, which implies a dominant Pb 6$s$ contribution to the metallic conduction. In addition, the observed Pb 4$f_{7/2}$ spectrum is not explained by the well-known Doniach-\v{S}unji\'{c} lineshape for a simple metal. The spectrum is successfully generated by employing a Pb 6$s$ partial density of states from local density approximation calculations, thus confirming the Pb 6$s$ dominant character and free-electron-like density of states of Ag$_5$Pb$_2$O$_6$.