A new aspect of superconductivity in A-15 compounds

We present a new aspect of superconductivity in A-15 compounds which is able to explain their exceptional role among the highT_c superconductors. The basic idea is that a strong energy dependence of the electronic density of states near the Fermi level may greatly reduce the repulsive part of the frequency dependent electron-phonon interaction. This leads to a large enhancement ofT_c which is a maximum when the Fermi energy is comparable to a typical phonon energy. Our findings are based on numerical solutions of the Eliashberg equations where both the retardation of the electron-phonon coupling and the energy dependence of the electronic density of states have been included. For the electronic density of states we use the models of Labbé und Friedel and of Cohen et al., while the shape of the Eliashberg function²F() is taken from the tunneling results of Shen. We compare our theory to experimental results for ternary A-15 compounds.