N‐band Hubbard models II: Cooperative mechanisms of electron–phonon, electron correlation, and many‐band effects toward high‐Tc superconductors

Abstract

AbstractDiscoveries of superconductivity of MgB2 by Akimitsu et al. and high‐Tc cupurate superconductors by Bednorz and Muller have raised great interest for elucidation of the superconducting mechanism from both experimental and theoretical grounds. The transition temperature (Tc) was found to be 40 K for MgB2, while the very high‐Tc over 130 K was reported for doped copper oxides with layer structures. A crucial role of the electron–phonon (EP) interaction was pointed out as a common mechanism of the superconductivity of both materials. However, such high‐Tc superconductivity may indicate the possibility of cooperative mechanism of the EP interaction with others such as electron correlation (EC) and multiband (MB) effects discussed in part I (Int J Quantum Chem 1990, 37, 167) of this series. Here, as a continuation of part I, theoretical backgrounds are briefly described to select active orbital space for superconductivity and elucidate the nature of EP, EC, and MB effects for cooperative mechanisms. Next, molecular orbital calculations of MgmBn and cage‐type carbon cluster are carried out to elucidate contributions of the EP interaction using McMillan equation. The relative contributions of the EP and EC interactions are also discussed in relation to the screening of the Coulomb repulsion. Pair binding energies are calculated for cage‐type carbon clusters. Finally, the cooperative mechanisms of the EP, EC, and MB effects are discussed to realize the high‐Tc superconductivity in molecule‐based materials such as cage compounds and nanotubes. Several possible candidates are proposed on both experimental and theoretical grounds. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003