Charge ordering and phase transition in θ-(BDT-TTP)2Cu(NCS)2

θ-(BDT-TTP)2Cu(NCS)2 (BDT-TTP=2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathia-pentalene) is a highly correlated organic conductor with a quasi-two-dimensional electronic structure. This compound undergoes a second-order phase transition at 250 K, which accompanies a disproportionation of charge. The optical spectrum and magnetic susceptibility suggest that the separated charge forms a vertical stripe along the b-axis. The ESR experiment shows that the ground state is a non-magnetic state, which suggests the doubling of b-axis below 5 K. The properties which characterize the CO and magnetic phase transition were compared with those of θ-(BEDT-TTF)2RbZn(SCN)4.