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Collective Excitations in the Charge-Ordered Phase of alpha-(BEDT-TTF)2I3

alpha-(BEDT-TTF)2I3 is a quasi-two-dimensional organic semimetal with a sharp phase transition to insulating state at TCO = 136 K. Below the transition a charge order is present in the BEDT-TTF layers as alternating charge-rich and charge-poor horizontal stripes.[1-3] We characterized the charge response of this charge-ordered state using dc resistivity, dielectric and optical spectroscopy in different crystallographic directions within the BEDT-TTF layer.[4] In this presentation we describe in detail the complex anisotropic dielectric response in the Hz-MHz range. We argue that it reveals two different kinds of excitations: a phason-like excitation alongside a soliton-like one. Possible ramifications are discussed within recent theoretical frameworks.[5] [1] Y. Takano et al., J. Phys. Chem. Solids 62, 393 (2001). [2] Organic Conductors, Special Topics Section, J. Phys. Soc. Jpn. 75, No. 5, May 2006. [3] T. Kakiuchi et al., J. Phys. Soc. Jpn. 76, 113702 (2007). [4] T. Ivek et al., arXiv:0912.1995, submitted to Phys. Rev. Lett. [5] R. T. Clay et al., J. Phys. Soc. Jpn. 71, 1816 (2002).

charge order; strongly correlated electron systems; dielectric loss and relaxation; conductivity; collective effects; quasi-2d

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