  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.
