Electronic Structure of B-2p State in AlB2 Single Crystal: Direct Observation ofp��and p��Density of States

X-ray emission (XES) and absorption (XAS) spectra near the B-$K$ edge were measured on single-crystalline AlB$_2$ compound which is an isostructural diboride of superconducting MgB$_2$. The partial density of states (PDOS) of B-2$p��$ and $p��$ orbitals were derived from the polarization dependence of XES and XAS spectra. There are considerable amounts of PDOS near the Fermi energy in AlB$_2$ similarly to that in MgB$_2$, but there are almost no PDOS in $p��$ orbitals of AlB$_2$ near the Fermi energy, i.e., a pseudo-gap in $p��$ state and a broad metallic state in $p��$ state are observed. The present result indirectly supports scenarios that the $p��$ holes play an important role in the occurrence of superconductivity in MgB$_2$. The overall features of PDOS were found to be in good agreement with the result of band calculation of AlB$_2$, but a small discrepancy in the Fermi energy is observed, which is attributed to the Al vacancy in the compounds, i.e., the estimated concentration is Al$_{0.93}$B$_2$.

LaTeX files and 4 eps figures, to be published in J. Phys. Soc. Jpn