Pressure effects on the electronic structure and low-temperature states in the alpha -(BEDT-TTF)2MHg(SCN)4 organic-conductor family (M=K, Rb, Tl, NH4).

We have used the magnetoresistance and the Shubnikov--de Haas effect to study the pressure-dependent ground states of \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$MHg(SCN${)}_{4}$ [where BEDT-TTF is alpha-bis(ethylenedithio) tetrathiafulvalene-M (=potassium, rubidium, thallium, or ammonium)-mercury-thiocyanate]. Extended H\"uckel tight-binding calculations show that this isostructural class of materials has a quasi-two-dimensional Fermi surface with both open and closed orbit bands. Both superconducting (M=${\mathrm{NH}}_{4}$) and density-wave (M=K, Tl, and Rb) states in these materials have been investigated. We report the effects of pressure on the electronic structure (Fermi surface), the density-wave state, and the superconducting state. We find (where M=Tl, for example) that the fundamental closed-orbit and Brillouin-zone areas increase with pressure at a rate of 1.3%/kbar and 0.5%/kbar, respectively. We observe pressure-induced changes in the nesting condition of the open-orbit band that allow new small closed orbits on the Fermi surface. The onset of quasi-three-dimensional behavior with increasing pressure is observed in some cases. In those materials with density-wave states, the associated resistive anomalies are removed between 6 and 8 kbar. In the superconducting member (M=${\mathrm{NH}}_{4}$), pressure decreases the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ as ${\mathit{dT}}_{\mathit{c}}$/dP\ensuremath{\approxeq}-0.25 K/kbar, with a corresponding reduction in the effective mass. From analysis of the pressure dependence of ${\mathit{T}}_{\mathit{c}}$ we find that the interaction term in the mean-field expression for superconductivity is very sensitive to pressure. We note that the pronounced pressure dependence of the electronic properties of these materials provides fertile ground for future studies of low-dimensional phenomena.