Nonuniform site-charge distribution and fluctuations of charge order in the metallic state ofα−(BEDT-TTF)2I3

Abstract

We present the site-charge distribution in the charge-ordered and metallic states of $\alpha${}-(BEDT-TTF)2I3, based on the assignment of the CC stretching modes $\nu${}2, $\nu${}3, and $\nu${}27 of $\alpha${}-(BEDT-TTF)2I3 with the aid of C13- and deuterium-substituted compounds. The nonuniform site charges in both the metallic and insulating phases were consistent with those determined by a recent x-ray diffraction study. Comparing the line shapes of the charge-sensitive vibrational modes of $\alpha${}-(BEDT-TTF)2I3 with those of isostructural $\alpha${}-(BEDT-TTF)2NH4Hg(SCN)4, we propose a thermally activated fluctuation of charge order in the metallic phase of $\alpha${}-(BEDT-TTF)2I3. This fluctuation was considerably suppressed above 0.65 GPa. The optical conductivity in the metallic phase shows no Drude response.