Transport properties of #alpha# sec-phase organic conductors, (BEDT-TTF)_2CsHg(SCN)_4 and (BEDT-TTF)_2K_(1.4)Co(SCN)_4

摘要

Under pressure, the metal-insulator transition temperature, T_(MI) (210 K) of #alpha# sec-(BEDT-TTF)_2CsHg(SCN)_4 (bis(ethyl-enedithio)tetrathiafulvalene) decreases up to 7 kbar, whereas above this pressure the system becomes more insulating as the pressure increases. This behavior is interpreted in view of the mixed #beta# sec-like and #theta#-like characters of the #alpha#-phase. In this compound two-fold lattice modulation appears much above T_(MI), even at room temperature. This indicates that the lattice modulation in not the direct origin of the M-I transition. In #alpha# sec-(BEDT-TTF)_2CsHg(SCN)_4K_(1.4)Co(SCN)_4, T_(MI) (130 K) rises very slowly under pressure.