Proximity of the layered organic conductors alpha-(BEDT-TTF)2MHg(SCN)4 (M = K, NH4) to a charge-ordering transition.
While the optical properties of the superconducting salt alpha-(BEDT-TTF)(2)-NH4Hg(SCN)(4) remain metallic down to 2 K, in the nonsuperconducting K analog a pseudogap develops at frequencies of about 200 cm(-1) for temperatures T<200 K. We show that the optical conductivity calculated with exact-diagonalization techniques on an extended Hubbard model at quarter filling is consistent with the observed low-frequency feature. We argue that the different optical responses observed are a consequence of the proximity of these compounds to a charge-ordering transition driven by the intermolecular Coulomb repulsion.
1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany.
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Dressel
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Drichko
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Schlueter
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Merino