Electronic Structure of B-2<i>p</i>State in AlB₂Single Crystal: Direct Observation of<i>p</i>σ and<i>p</i>π Density of States

Abstract

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