THEORETICAL STUDIES ON MANY-BAND EFFECTS ON SUPERCONDUCTIVITY AND MAGNETISM IN MOLECULAR CRYSTALS

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Many bands play an important role of many gap superconductivity such as MgB_2 superconductor. Form viewpoint of the importance of many-band, we have theoretically investigated new physical properties such as superconducting gaps etc. arising from many-band. Our purposes of this project are (1) to investigate many-band effects on superconductivity, (2) to investigate many-band effects on magnetism and to discuss the possibility of new magnetic properties, and (3) to investigate many-band effects in molecular crystals. In this project, we have investigated many-band effects on superconductivity and magnetism by using Green's function techniques and some many-band models.In many-band effects on superconductivity, we have obtained that the superconducting gap in namy-band systems becomes more stable than that of single band systems. The transition temperature for many-band superconductivity becomes higher than that of single band superconductivity. An effective two-electron pair scattering … More process between bands has a very important role to apper many-band superconductivity. We also have investigated a cooperative mechanism for superconductivity with the pair electron scattering process and the effective electron interactins from electron-phonon interaction. In a single band model such as BCS model, the effective electron interaction should be negative. However, In many-band systems, many-band superconductivity appears, even if the effective electron interaction becomes positive or negative.In many-band effects on magnetism, we have found that effective electron-electron interaction becomes ferromagnetic at half-filling in many-band systems and that antiferromagnetic interaction appears by hole-doping into the many-band systems.We have investigated magnetic and superconducting properties of herringbone and two-layer square lattices in organic crystals. In the two-layer square lattice, we have obtained that we can expect interesting magnetic properties by many-band effects and have also found the possibilities of many-band superconductivity in such two-layer lattices. Less