POSSIBILITIES OF CHARGE-MEDIATED AND OR SPIN-MEDIATED SUPERCONDUCTORSAND PHOTO INDUCED SUPERCONDUCTORS IN THE INTERMEDIATE REGION OF METAL-INSULATOR TRANSITIONS/

Magnetism, conductivity, and superconductivity of molecular materials are discussed on the basis of extended Hubbard-Peierls Hamiltonians, w hich involve four parameters; transfer integral (T), electron-lattice interaction (W), and on-site and intersite Coulomb interactions (LI, V ). The four-component spinor is introduced to define order parameters which characterize four different electronic states (charge density wa ve, spin density wave, singlet and triplet superconductivities) in a u nified fashion. Several intermediate regions in these parameters are i nteresting and important from current theoretical reasons. Many experi mental results clearly show the importance of intermediate correlation regimes for active controls of these states by chemical modifications and by external fields such as high pressure. Several model Hamiltoni ans such as CT and t-J models are derived for elucidation of potential electronic properties of molecule-based materials. Recent computation al results based on the t-J model are utilized to rationalize our I mo del for spin-mediated superconductivity. Possibilities of magnetic con ductors, charge- and/or spin-mediated superconductors, and photoinduce d superconductors instead of the charge-mediated Little model are exam ined in the intermediate region of several metal-insulator transitions . Possible candidates are also proposed on the basis of various circum stantial experimental results. Implications of the calculated results are finally discussed in relation to the recent development of the int ersection area among conducting, magnetic, and optical molecular mater ials. (C) 1997 John Wiley & Sons, Inc.