Comparative study of the electronic structure of MgB₂ and ZrB₂

X-ray photoemission spectroscopy is used to compare the electronic structures of MgB₂ and ZrB₂. The B 1s core levels in high-quality MgB₂ and ZrB₂ 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 MgB₂ compared to ZrB₂. The high T_c in MgB₂ is due to p-derived DOS, while ZrB₂ is dominated by d-derived states at E_F.