Competition between Charge Ordering and Superconductivity in Layered Organic Conductors \(\alpha\)-(BEDT-TTF)\(_2M\)Hg(SCN)\(_4\) (M = K, NH\(_4\))

Abstract

While the optical properties of the superconducting salt \(\alpha\)-(BEDT-TTF)\(_2\)NH\(_4\)Hg(SCN)\(_4\) remain metallic down to 2 K, in the non-superconducting K-analog a pseudogap develops at frequencies of about 200 cm\(^{-1}\) for temperatures T < 200 K. Based on exact diagonalisation calculations on an extended Hubbard model at quarter-filling we argue that fluctuations associated with short range charge ordering are responsible for the observed low-frequency feature. The different ground states, including superconductivity, are a consequence of the proximity of these compounds to a quantum phase charge-ordering transition driven by the intermolecular Coulomb repulsion.