Characterization of low-temperature electronic states of the organic conductors α-(BEDT-TTF${)}_2$MHg(SCN${)}_4$ (M=K, Rb, and ${\mathrm{NH}}_4$) by specific-heat measurements

The low-temperature specific heat of the organic conductors α-(BEDT-TTF${)}_2$MHg(SCN${)}_4$ (M=K, Rb, and ${\mathrm{NH}}_4$) has been measured at temperatures down to 0.25 K and under magnetic field up to 8 T. The electronic specific-heat coefficient γ is estimated at 7.0±0.6 mJ/mol ${\mathrm{K}}^2$ for both M=K and Rb salts and 25–26 mJ/mol ${\mathrm{K}}^2$ for M=${\mathrm{NH}}_4$ salt. The smaller γ values for M=K and Rb than that for M=${\mathrm{NH}}_4$ 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 ${\mathrm{NH}}_4$ salt shows a very broad specific-heat peak around the transition temperature, reflecting a strong two-dimensional nature of the superconductivity.