Soft x-ray spectroscopy experiments on the nearK-edge of B inMB2(M=Mg,Al, Ta, and Nb)

Abstract

Soft x-ray-absorption and emission measurements are performed for the K-edge of B in $M{\mathrm{B}}_2$ $(M=\mathrm{Mg},$ Al, Ta, and Nb). Unique feature of ${\mathrm{MgB}}_2$ with a high density of the B ${2p}_{\mathrm{xy}}\left(\sigma \right)$ state below and above the Fermi edge, which extends to 1 eV above the edge, is confirmed. In contrast, the B $2p$ density of states in ${\mathrm{AlB}}_2$ and ${\mathrm{TaB}}_2,$ both of occupied and unoccupied states, decreased linearly towards the Fermi energy and showed a dip at the Fermi energy. Furthermore, there is a broadening of the peaks with $p\sigma$ character in x-ray emission spectra and x-ray-absorption sepctra of ${\mathrm{AlB}}_2,$ which is due to the increase of three dimensionality in the $p\sigma$ band in ${\mathrm{AlB}}_2.$ The density of states (DOS) of ${\mathrm{NbB}}_2$ has a dip just below the Fermi energy. The present results indicate that the large DOS of $\mathrm{B}-2p\sigma$ states near the Fermi energy are crucial for the superconductivity of ${\mathrm{MgB}}_2.$