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We investigated theoretically the electronic structure of a new C 60 based superconductor, Na x H y C 60 where x ∼3.6 and y ∼1, to provide criteria that enable one to identify which of two possible phases, Na 3 HC 60 and Na 4 HC 60 , is important to the superconductivity. Studying these two phases by using the first-principles calculations, we found remarkable differences in the electronic structures between them, especially in the partial densities of states and the nuclear-quadrupole interaction of 23 Na. We propose that these quantities can work as the criteria for the identification.

We present the anomalies at ∼200 K in electrical resistivity, transmittance and reflectance in the far-infrared region, and the magnetic susceptibility of α'-(BEDT-TTF) 2 IBr 2 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]. This phase transition is investigated by infrared, Raman, and far-infrared spectroscopies. Through the spectroscopic investigation, we found that the positive charges on BEDT-TTF molecules are strongly localized owing to a strong Coulomb interaction in both low-temperature (LT) and high-temperature (HT) phases. We show the spectroscopic data that suggest the charge order in the LT phase and the charge disorder in the HT phase. The phase transition at ∼200 K in α'-(BEDT-TTF) 2 IBr 2 is the charge order–disorder phase transition not accompanied by structural changes. On the basis of the localized picture, we qualitatively interpreted the experimental results above and below the phase transition. The effect of hydrostatic pressure was investigated by electrical resistivity measuremen...