Pressure dependence of superconductivity in Ba4C60

In BaxC60 superconductivity (sc) at T-c=(6.8+/-0.2) K is associated with the x=4 phase. Here we present pressure studies of T-c of a BaxC60 sample (dT(C)/dp = -1.9 K/GPa) which exhibits a large volume fraction of the x=4 phase. In the alkali-fullerides K3C60 and Rb3C60 T-C is governed by the electronic density of states N(E(F)) (T-C proportional to exp{-1/N(E(F))V}). The dependence of N(E(F)) on volume call be approximated by N(E(F))=N(0)d(eff)(3), where d(eff) is the surface to surface distance of neighboured C-60-molecules. This relationship is corroborated by (T-C, d)-data of various compounds of the form A(3-x)B(x)C(60) (A: K, Rb; B: K, Rb, Cs; x less than or equal to 2). Strong deviation from this behavior is found for the alkaline-fulleride superconductor Ba4C60. This deviation is attributed to the hybridization of alkaline-metal s-, d-states and C-60 pi-states.