Comparative study of the electronic structure ofMgB2andZrB2

X-ray photoemission spectroscopy is used to compare the electronic structures of ${\mathrm{MgB}}_2$ and ${\mathrm{ZrB}}_2.$ The B $1s$ core levels in high-quality ${\mathrm{MgB}}_2$ and ${\mathrm{ZrB}}_2$ exhibit a single asymmetric peak typical of a metallic boride. The Mg $2p$ core level shows a single peak with negligible intensity in charge-transfer satellites and no correlation effects. The Mg $2p$ and B $1s$ core-level spectra exhibit a broad bulk plasmon feature centered at about 22 eV from the main peak, in good accord with calculations. The measured valence bands are consistent with band-structure calculations indicating a higher density of states (DOS) at $E_F$ for ${\mathrm{MgB}}_2$ compared to ${\mathrm{ZrB}}_2.$ The high ${\mathrm{T}}_c$ in ${\mathrm{MgB}}_2$ is due to p-derived DOS, while ${\mathrm{ZrB}}_2$ is dominated by d-derived states at $E_F.$