Fullerene-based Molecular Materials: an Ongoing Voyage into the Unexpected
Kosmas Prassides 教授
（東北大学 原子分子材料科学高等研究機構（AIMR） 教授）
Superconductivity in classical elemental metals and intermetallics is clearly understood in terms of coupling between the electrons and lattice vibrations to produce Cooper pairs, which can move without electrical resistance. It is now well-established that in systems where the bands in which the electrons move are narrow−i.e. the energies associated with interelectron repulsion are comparable to the electronic bandwidth−, these correlation effects need to be taken into account in understanding the superconductivity mechanisms. These concepts were developed primarily in d-electron systems such as the cuprates where doping, with the associated structural disorder and distribution of metal charge states and d-electron counts, is required to produce superconductivity by suppressing magnetically ordered states. In addition, recent theoretical work indicates that the competition between correlation and delocalization energies is critically controlled by the orbital degeneracy, but this cannot be investigated in d-electron superconductors as their low-symmetry crystal structures remove the degeneracy. An ideal material for understanding the interactions producing superconductivity in structurally and chemically complex correlated electron systems should allow the isolation of the influence of purely electronic factors without the complications of disorder, structural transitions and low dimensionality, while maintaining the site symmetry required for orbital degeneracy in all competing electronic ground states. I will argue that the fullerenes occupy a unique position in terms of their ability to test and develop the most advanced aspects of the theoretical understanding of the ground states accessible to electrons in the presence of structural (lattice symmetry) and electronic (orbital degeneracy) factors that control their energies.