On superconductivity of hole-doped C60 and comparison with electron-doped X3C60 (X=alkali atom)

We undertake a quantitative analysis of critical temperatures in electron-doped X3C60 (with X an alkali atom or a combination of alkalis) and hole-doped C60, the latter realized recently (Nature 408 (2000) 549) by using strong electrostatic fields on thin films. The formalism employed involves indirect-exchange pairing between conduction electrons (quasi-particles) via diamagnetic Cooper-pair mediators, in the present case carbon–carbon double bonds of the C60 molecules. The same principle was previously applied in analyses of high-TC cuprates with oxygen anions O2− mediating Cooper-pair formation. The primary goals of the analysis are the interpretation of the established surprisingly high (52K)TC in hole-doped C60 with 3.2 holes per molecule C60, and the projected (same authors) TC beyond 100K which might be reached upon modest expansion of the lattice. It is found, on the basis of the indirect-exchange formalism, that the high TC values for hole-doped C60 are due to an intrinsic correlation between the two principal parameters (κ,β) of the formalism, which is lacking in electron-doped compounds. It is further conjectured that the projected TC beyond 100K upon expansion of the lattice is not realistic: instead of TC>100K at a (cubic) lattice parameter a=14.60Å we find TC=68.0K for that value of a, and a maximum TC(a) of 72.4K at a=15.02Å. The calculated TC(a) for electron-doped C60, evaluated along the same lines, are very similar to those obtained earlier (Chem. Phys. 176 (1993) 1), and are in quantitative agreement with experiment.