Inst Festkorperforsch Forschungszentrum Julich W-5170 Julich Germany
Max Planck Inst Festkorperforsch W-7000 Stuttgart 80 Germany

It is shown that experimental data on the critical magnetic fields slopes H(c1) and H(c2) for K3C60, taken together with local-density calculations for the density of states, are consistent with strong-coupling phonon mediated superconductivity. The renormalization factor for the specific heat jump at T(c) is about 4 (relative to the BCS model). This value does not depend on grain size, impurity concentrations etc. This is shown to be consistent with a popular hypothesis that intramolecular phonons couple with the electrons provided one assumes the existence of very soft (almost-equal-to 40 cm-1) rotational intermolecular modes with a relatively weak coupling. Such modes provide a moderate increase of T(c) but are responsible for the whole renormalization.
 

Solid State Communications, 81 935-938, 1992.