Theoretical Study on Electron-Electron and Electron-phonon Interactions in Fullerene Compounds

在碱金属掺杂C_<60>中，电子-电子和电子-声子相互作用起着非常重要的作用。我们已经发现，这两种相互作用共同打开了A_2C_<60>和A_4C_<60>中的带隙，使这些材料绝缘。相反，A_3C_<60>（介于A_2C_<60>和A_4C_<60>之间）仍然是金属相，尽管电子-电子和电子-声子相互作用在该材料中也应该很强。到目前为止，这个问题还没有解决。在本研究中，我们使用与研究A_2C_<60>和A_4C_<60>相同的模型，研究了A_3C_<60>尽管有很强的电子-电子和电子-声子相互作用，但仍然保持金属性的原因。结果，我们发现，在A_3C_<60>中，电子-电子和电子-声子相互作用相互竞争，因此，这种材料仍然是金属的。这种竞争导致了电荷的波动。特别地，我们发现在A_3C_<60>的光发射光谱中，电荷波动是一个肩的起源。此外，我们用量子蒙特卡罗方法研究了该模型，发现肩在高温下消失。这一结果与光发射光谱的温度依赖性实验结果吻合较好。本文的结果对于理解A_3C_<60>的低能激发具有重要意义。最后，我们研究了A_xC_<60> (x=1,-,5)的相图，发现A_1C_<60>, A_3C_<60>, A_5C_<60>中存在顺磁性金属，超导体，轨道极化金属，自旋和轨道极化金属，而A_2C_<60>和A_4C_<60>中存在顺磁性金属，超导体，轨道极化绝缘体。

In alkali-metal-doped C_<60>, the electron-electron and electron-phonon interactions play very important roles. We have already found that the two interactions open the band gaps in A_2C_<60> and A_4C_<60> cooperatively and make these materials insulating. On the contrary, A_3C_<60>, which is a phase between A_2C_<60> and A_4C_<60>, remains metallic although the electron-electron and electron-phonon interactions should also be strong in this material. This has been unsolved so far. In the present study, we have studied the reason why A_3C_<60> remains metallic in spite of the strong electron-electron and electron-phonon interactions by using the same model that has been used for studying A_2C_<60> and A_4C_<60>. As a result, we have found that, in A_3C_<60>, the electron-electron and electron-phonon interactions compete with each other and, consequently, this material remains metallic. This competition results in the charge fluctuation. In paticular, we have found that the charge fluctuation is the origin of a shoulder observed in the photoemission spectra in A_3C_<60>. Furthermore, we have studied the present model by the quantum Monte Carlo method and found that the shoulder disappears at high temperature. This result is in good agreement with experimental temperature dependence of the photoemission spectra. The present results are very important for understanding low energy excitation in A_3C_<60>. Finally, we have studied the phase diagrams of A_xC_<60> (x=1,-,5) and found that there exist paramagnetic metal, superconductor, orbital-palarized metal, and spin- and orbital-polarized metal in A_1C_<60>, A_3C_<60>, and A_5C_<60> while there exist paramagnetic metal, superconductor, orbital-polarized insulator in A_2C_<60> and A_4C_<60>.