On superconductivity of hole-doped C-60 and comparison with electron-doped X3C60 (X = alkali atom) - Quantitative analysis on the basis of indirect-exchange pairing

摘要

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 C-60, 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 O-2-mediating Cooper-pair formation. The primary goals of the analysis are the interpretation of the established surprisingly high (52 K) T-C in hole-doped C-60 with 3.2 holes per molecule C-60, and the projected (same authors) T-C 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 C-60 are due to an intrinsic correlation between the two principal parameters (kappa, beta) 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 T-C > 100 K at a (cubic) lattice parameter a = 14.60 Angstrom we find T-C = 68.0 K for that value of a, and a maximum T-C(a) of 72.4 K at a = 15.02 Angstrom. The calculated T-C(a) for electron-doped C-60, evaluated along the same lines, are very similar to those obtained earlier (Chem. Phys. 176 (1993) 1), and are in quantitative agreement with experiment. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 22]