High-field magnetotransport and Fermi-surface topology in the novel quasi-two-dimensional organic conductor bis(ethylenedithiolo)tetrathiafulvalenium mercuric postassium thiocyanate, (BEDT-TTF)2KHg(SCN)4

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.