Theoretical Study on Low-Energy Excitations in Alkali-Metal-Deped C_<60>

碱金属掺杂C60是分子晶体中超导转变温度极高的材料。虽然这种材料中超导电性的起源仍然存在争议，但我们认为超导的起源是电子-电子相互作用和电子-声子相互作用之间的竞争，其中动态的Jahn-Teller效应对后者克服前者起着至关重要的作用。在本研究中，我们试图从我们的角度来解释实验事实。我们首先用动力学平均场近似研究了碱金属掺杂C60的光电子能谱中观察到的反常行为。因此，我们发现，由于电子-电子相互作用和电子-声子相互作用的协同作用，A4C60是非磁性绝缘体，其中A是碱金属。非磁性的原因是电子-声子相互作用比电子-电子相互作用产生的Hund耦合强得多。此外，在碱金属掺杂的C60的红外反射光谱中观察到的一个峰结构是由该材料中的激子引起的。特别是，与通常的材料相比，在碱金属掺杂的C60中，自旋单重态激子的能量低于自旋三重态激子。我们得出结论，所有的特征性质都是由于Jahn-Teller效应

Alkali-metal-doped C60 is a material whose transition temperature of superconductivity is extraordinarily high among molecular crystals. Although the origin of superconductivity in this material is still controversial, we have claimed that the origin is the competition between the electron-electron interaction and the electron phonon interaction, where the dynamic Jahn-Teller effect plays an essential role for the latter interaction to overcome the former one. In the present study, we try to explain experimental facts from our point of view. We first investigate the anomalous behavior observed in the photoemission spectra in alkali-metal-doped C60 by using the dynamical mean field approximation. Consequently, we find that A4C60, where A is alkali metal, is a nonmagnetic insulator due to the cooperation between the electron-electron interaction and the electron-phonon interaction. The reason for the nonmagnetic nature is that the electron-phonon interaction is much stronger than the Hund coupling originated in the electron-electron interaction. Furthermore, a peak structure observed in the infrared reflection spectra of alkali metal-doped C60 is found to be originated in the exciton in this material. In particular, it is found that, in alkali-metal-doped C60, the spin-singlet exciton is energetically lower than the spin-triplet exciton in contrast with usual materials. We conclude that all the characteristic properties are due to the Jahn-Teller effect