Superconductivity in Fullerides

Experimental studies of superconductivity properties of fullerides are briefly reviewed. Theoretical calculations of the electron-phonon coupling, in particular for the intramolecular phonons, are discussed extensively. The calculations are compared with coupling constants deduced from a number of different experimental techniques. It is discussed why the A_3 C_60 are not Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates of the Coulomb pseudopotential $\mu^*$, describing the effect of the Coulomb repulsion on the superconductivity, as well as possible electronic mechanisms for the superconductivity are reviewed. The calculation of various properties within the Migdal-Eliashberg theory and attempts to go beyond this theory are described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to Review of Modern Physics, more information at http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm

Superconductivity at 18 K in potassium-doped C60

The synthesis of macroscopic amounts of C60 and C70 (fullerenes) has stimulated a variety of studies on their chemical and physical properties. We recently demonstrated that C60 and C70 become conductive when doped with alkali metals. Here we describe low-temperature studies of potassium-doped C60 both as films and bulk samples, and demonstrate that this material becomes superconducting. Superconductivity is demonstrated by microwave, resistivity and Meissner-effect measurements. Both polycrystalline powders and thin-film samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor.

Superconductivity at 28 K in RbxC60

Meissner-effect and microwave-absorption measurements on bulk samples show that RbxC60 is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only Ba0.6K0.4BiO3 and the cuprate superconductors have higher transition temperatures.