Soft x-ray spectroscopy experiments on the near K-edge of B in MB2 (M=Mg, Al, Ta, and Nb)

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Soft X-ray absorption and emission measurements are performed for the K- edge of B in MB₂ (M=Mg, Al, Ta and Nb). Unique feature of MgB₂ with a high density of B 2pxy(σ)-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 AlB₂ and TaB₂, 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 pσ-character in XES and XAS of AlB₂, which is due to the increase of three dimensionality in the pσ-band in AlB₂. The DOS of NbB₂ has a dip just below the Fermi energy. The present results indicate that the large DOS of B-2pσ states near the Fermi energy are crucial for the superconductivity of MgB₂.