PRESSURE EFFECTS ON THE ELECTRONIC-STRUCTURE AND LOW-TEMPERATURE STATES IN THE ALPHA-(BEDT-TTF)(2)MHG(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 alpha-(BEDT-TTF)(2)MHg( SCN)(4) [where BEDT-TTF is alpha-bis(ethylenedithio) tetrathiafulvalen e-M (=potassium, rubidium, thallium, or ammonium)-mercury-thiocyanate] . Extended Huckel tight-binding calculations show that this isostructu ral class of materials has a quasi-two-dimensional Fermi surface with both open and closed orbit bands. Both superconducting (M=NH4) and den sity-wave (M=K, Tl, and Rb) states in these materials have been invest igated. 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 O.5%/kbar, respectively. We observe pressure-induced changes in t he nesting condition of the open-orbit band that allow new small close d orbits on the Fermi surface. The onset of quasi-three-dimensional be havior with increasing pressure is observed in some cases. In those ma terials with density-wave states, the associated resistive anomalies a re removed between 6 and 8 kbar. In the superconducting member (M=NH4) , pressure decreases the superconducting transition temperature T-c as dT(c)/dP approximate to-0.25 K/kbar, with a corresponding reduction i n the effective mass. From analysis of the pressure dependence of T-c we find that the interaction term in the mean-field expression for sup erconductivity is very sensitive to pressure. We note that the pronoun ced pressure dependence of the electronic properties of these material s provides fertile ground for future studies of low-dimensional phenom ena.