SEMIMETALLIC SPIN-DENSITY-WAVE PHASE IN (TMTSF)2X (X=ASF6 AND PF6) UNDER PRESSURE

In the spin density wave (SDW) phase of (TMTSF)2AsF6 and (TMTSF)2PF6, we measured the metal-insulator transition temperature T(MI) and the a ctivation energy E(A) in the SDW insulating phase under pressure. The ratio 2E(A)/k(B) T(MI) near the critical pressure approximately 10.5 k bar, above which the SDW phase does not exist, is examined and compare d with the mean-field theory developed by Yamaji. We conclude that T(M I) does not correspond to the normal-SDW transition temperature T(SDW) but is lower than T(SDW). Non-linear conduction with finite threshold was observed above T(MI).