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

多个能带在许多带隙超导中起着重要的作用，如MgB_2超导体。从多带的重要性出发，从理论上研究了多带所产生的超导带隙等新的物理性质。我们的目的是(1)研究超导电性的多带效应，(2)研究多带效应对磁性的影响，并讨论新的磁性性质的可能性，以及(3)研究分子晶体中的多带效应。在这个项目中，我们利用格林函数技术和一些多带模型研究了多带对超导电性和磁性的影响。在对超导电性的多带效应中，我们得到了Namy带系统的超导带隙比单带系统的超导带隙更稳定。多带超导的转变温度高于单带超导的转变温度。一种有效的双电子对散射…更多的带间过程对多带超导电性的实现具有非常重要的作用。我们还研究了双电子散射过程和电子-声子相互作用产生的有效电子相互作用对超导电性的协同机制。在单带模型中，如BCS模型，有效电子相互作用应该是负的。然而，在多带系统中，即使有效电子相互作用是正的或负的，也会出现多带超导电性。在多带对磁性的影响中，我们发现在多带系统中有效的电子-电子相互作用在半填充时变成铁磁性，反铁磁相互作用通过空穴掺杂出现在多带系统中。我们研究了有机晶体中人字形和两层正方形晶格的磁性和超导性质。在两层正方形晶格中，我们通过多带效应得到了有趣的磁性质，并发现了这种两层晶格中存在多带超导电性的可能性。较少

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