Quantum criticality in reduced π-donor system (MeDH-TTP)2AsF6

The temperature-pressure (T-P) phase diagram of the reduced pi-donor system (MeDH-TTP)(2)AsF6 is studied using resistivity measurement, where MeDH-TTP stands for 2-methyl-5-(1,3-dithiolan-2-yliden)-1,3,4,6-tetrathiapentalene. The insulating phase is suppressed completely beyond P-c = 2.4 GPa. Above P-c, we find two distinct metallic states that are regarded as the Fermi liquid (FL) state at lower temperatures and the bad metal at higher temperatures. Around P-c, remarkable enhancements in the residual resistivity rho(0) as well as in the T-2-term coefficient A associated with FL behavior are found. We discuss the origin of the insulating phase and the possible quantum critical point at P-c in (MeDH-TTP)(2)AsF6.