Superconductivity and structural phase transition in A15 compounds

For the origin of the high-temperature superconductivity and its correlation with the structural phase transition in A15 compounds, an electron mechanism based on the Geilikman model (1965) is considered. The electronic band structure near the Fermi level epsilon F in these compounds consists of the very narrow, twofold degenerate Gamma 12 subbands referred to as the 'd bands' and other wide bands referred to as the 's bands'. The Coulomb interaction between the s and d electrons causes an indirect attractive interaction via the particle-hole pair excitations in the d-electron system. The s-electron system with the attractive interactions due to both the electron and phonon mechanisms is assumed to be responsible for the main part of the superconductivity, while the d-electron system is for the structural phase transition. The superconductivity of the s-electron system forces d electrons to form Cooper pairs. The two phase transitions interfere with each other, since the structural phase transition causes a decrease in the density of states near epsilon F of the d bands, which is unfavourable for superconductivity. The temperature dependences of the crystal distortion and of the elastic constants are calculated. This model can explain successfully the experimental results on V 3 Si and Nb 3 Sn.