Abstract
Using transmission electron energy loss spectroscopy, we have studied the structures near core excitation edges in various A15 compounds. These structures reflect the unoccupied parts of the electronic densities of states N( E). In all materials investigated (Nb 3Sn, Nb 3Ge and Nb 3Al), we have observed pronounced peaks immediately above E F. The peak heights decrease very much for nonstoichiometric samples with low superconducting transition temperatures T c. Our spectra agree well with tight-binding N( E) curves based on the APW bandstructure work of Klein et al. In particular, we observe larger peaks in Nb 3Al and Nb 3Ge than in Nb 3Sn. This result is consistent with the theoretical prediction that E F shifts from the top part of the high N( E) region in Nb 3Sn towards the center and bottom parts in Nb 3Ge and Nb 3Al.
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