THERMODYNAMIC EVIDENCE FOR A LOW-TEMPERATURE PHASE-TRANSITION IN THE ORGANIC CONDUCTOR ALPHA-(BEDT-TTF)(2)MHG(SCN)(4)(FOR M=K,RB)

We have measured the ac heat capacity of the organic conductor alpha-( BEDT-TTF)(2)MHg(SCN)(4) (for M = K, Rb) via a low addenda optically he ated calorimetric method. We find, contrary to previous reports, a ris e Delta C in the heat capacity near a transition temperature T-DW. The BCS parameter Delta C/gamma T-c for the transitions ranges from 7.5 t o 0.1 depending on different estimates of the electronic density of st ates. Nevertheless, the results are consistent with the opening of a d ensity wave gap at T-DW in the open orbit band, and a reconstruction o f the closed orbit band at lower temperatures.