On superconductivity of hole-doped C60 and comparison with electron-doped X3C60 (X=alkali atom): Quantitative analysis on the basis of indirect-exchange pairing

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 (52 K) TC in hole-doped C60 with 3.2 holes per molecule C60, and the projected (same authors) TC beyond 100 K 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 100 K upon expansion of the lattice is not realistic: instead of TC>100 K at a (cubic) lattice parameter a=14.60 Å we find TC=68.0 K for that value of a, and a maximum TC(a) of 72.4 K at a=15.02 Å. 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.