Superconductivity at very strong coupling

A numerical treatment of the eliashberg equations reveals that mcmillan's equation fails for lamda>1.5. For very large lamda, tc scales as (lamda(omega2))12/, while for lamda<1.0, mcmillan's equation is reasonably accurate. The authors give a new approximate tc equation which satisfies these limits, and show that the maximum tc occurs at infinite lamda. A new tunnelling measurement for amorphous pb0.45bi0.55 gives lamda=2.59 and a tc in excellent agreement with numerical solutions.