CHARACTERIZATION OF LOW-TEMPERATURE ELECTRONIC STATES OF THE ORGANIC CONDUCTORS ALPHA-(BEDT-TTF)(2)MHG(SCN)(4) (M=K, RB, AND NH4) BY SPECIFIC-HEAT MEASUREMENTS

The low-temperature specific heat of the organic conductors alpha-(BEDT-TTF)(2)MHg(SCN)(4) (M = K, Rb, and NH4) has been measured at temperatures down to 0.25 K and under magnetic field up to 8 T The electronic specific-heat coefficient gamma is estimated at 7.0 +/- 0.6 mJ/mol K-2 for both M = K and Rb salts and 25-26 mJ/mol K-2 for M = NH4 salt. The smaller gamma values for M = K and Rb than that for M = NH4 imply that the electron density of states of the low-temperature phase of the former two salts is suppressed down to about 35% of that of the high-temperature phase. This profile of specific heat remains unchanged up to 8 T The superconducting NH4 salt shows a very broad specific-heat peak around the transition temperature, reflecting a strong two-dimensional nature of the superconductivity.