COOPERATIVE EFFECTS OF ELECTRON CORRELATION AND CHARGE ORDERING ON THE METAL-INSULATOR-TRANSITION IN QUASI-ONE-DIMENSIONAL DEUTERATED (DME-DCNQI)(2)CU

We study the changes in the electronic structure of a quasi-one-dimens ional organic conductor, deuterated (DMe-DCNQI)(2)Cu, across the CDW/M -I (charge-density-wave/metal-insulator) transition at T-c=60 K, using temperature-dependent (30-300 K) high-resolution photoemission spectr oscopy. Above the mean-field (MF) transition temperature (T(MF)similar to 4T(c)), the system shows typical metallic behavior with small but finite density of states at E(F). The spectral weight in the vicinity of E(F) decreases systematically upon decreasing temperature below T-M F even in the metallic phase, resulting in a pseudogap formation above T-c. This behavior continues across T-c and causes the M-I transition . The spectral weight is transferred to energies much larger than the gap, indicating a cooperative effect of strong electron correlation an d structural changes which imply decreases in interchain coupling acro ss the M-I transition.