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Abstract

Hopping conductivity becomes activationless (metallic) when twice the intersite coupling supercedes the reorganization energy. Consistent with this idea, activationless electron pair transfer may be considered as a condition for the appearance of superconductivity. For electron pairs the condition is slightly more complicated than for single electrons. The activation energy depends on the relative energies between ionic states and a valence bond state. The latter serves as an intermediate state, contributing to the disappearance of the activation barrier. In A3C60, that pairing occurs because the 1Ag state of the negative ions Cmath image and Cmath image is stabilized by correlation effects and is the ground state. The correlation effects are primarily due to the closeness in energy and different parity of the tlu and tlg orbitals, which are partly occupied in the negative ions, but empty in neutral C60. The small interelectronic repulsion and the polarization energy from the environment also contribute to the stability of the evenly negative ions. © 1993 John Wiley & Sons, Inc.