Direct observation of the peaked density of states in high-TcA15superconductors by electron energy-loss spectroscopy

Abstract

Using transmission electron energy-loss spectroscopy, we have studied the structures above core excitation edges in various $A15\mathrm{}$ compounds. These structures reflect the unoccupied parts of the electronic density-of-states (DOS) curves. For all materials investigated (${\mathrm{Nb}}_3$Sn, ${\mathrm{Nb}}_3$Ge, and ${\mathrm{Nb}}_3$Al) we have observed pronounced peaks immediately above $E_F$—evidence for the high-DOS regions in a narrow energy range around the Fermi energy typical for all high-$T_c$ $A15\mathrm{}$ compounds. Our spectra agree very well with tight-binding DOS curves based on the augmented-plane-wave band-structure work of Klein et al. [Phys. Rev. B 18, 6411 (1978)]. The peak heights decrease very much for nonstoichiometric, low-$T_c$ samples of these compounds.