Energy gaps and transition temperatures of the strong-coupling superconductors, lead and mercury, as functions of the mean free paths of the electrons

Abstract

The energy gaps and the transition temperatures of the strong-coupling superconductors lead and mercury are measured as functions of the mean free paths of the electrons. The energy gap of lead increases with decreasing mean free path; the transition temperature is found to be constant. The energy gap of mercury runs through a maximum value with decreasing mean free path whereas the transition temperature decreases. The energy gap of pure mercury is extrapolated to the value 1.58 meV and yields the ratio 2Δ0/kTc=4.4. The experimental results confirm the theoretical prediction that the attractive electron-electron interaction in disordered superconductors is increased. But in addition, the strong-coupling effect appears to become more important in disordered superconductors. A comparison with similar experiments for weak-coupling superconductors is made.