Electrodynamic response of the charge ordering phase: Dielectric and optical studies of α-(BEDT-TTF)2I3

We report on the anisotropic response, the charge and lattice dynamics of normal and charge ordered phases with horizontal stripes in single crystals of the organic conductor alpha-(BEDT-TTF)(2)I-3 determined by dc resistivity and dielectric and optical spectroscopy. An overdamped Drude response and a small conductivity anisotropy observed in optics is consistent with a weakly temperature-dependent dc conductivity and anisotropy at high temperatures. The splitting of the molecular vibrations nu(27)(B-u) evidences the abrupt onset of static charge order below T-CO = 136 K. The drop of optical conductivity measured within the ab plane of the crystal is characterized by an isotropic gap that opens at approximately 75 meV with several phonons becoming pronounced below. Conversely, the dc conductivity anisotropy rises steeply, attaining at 50 K a value 25 times larger than at high temperatures. The dielectric response within this plane reveals two broad relaxation modes of strength Delta epsilon(LD) approximate to 5000 and Delta epsilon(SD) approximate to 400, centered at 1 kHz < nu(LD) < 100 MHz and nu(SD) approximate to 1 MHz. The anisotropy of the large-mode (LD) mean relaxation time closely follows the temperature behavior of the respective dc conductivity ratio. We argue that this phasonlike excitation is best described as a long-wavelength excitation of a 2k(F) bond charge density wave expected theoretically for layered quarter-filled electronic systems with horizontal stripes. Conversely, based on the theoretically expected ferroelectriclike nature of the charge ordered phase, we associate the small-mode (SD) relaxation with the motion of domain-wall pairs, created at the interface between two types of domains, along the a and b axes. We also consider other possible theoretical interpretations and discuss their limitations.