SYSTEMATIC STUDY OF THE ELECTRONIC-STATE IN THETA-TYPE BEDT-TTF ORGANIC CONDUCTORS BY CHANGING THE ELECTRONIC CORRELATION

The behavior of the phase transitions is mapped out as a function of t he dihedral-angle (theta) or the transfer integral (t) between donor c olumns for theta-type BEDT-TTF [BEDT-TTF: bis(ethylenedithio)tetrathia fulvalene, abbreviated as ET] salts involving the series theta-(BEDT-T TF)(2)MM'(SCN)(4) [M=Tl,Rb,CS, M' =Co,Zn: abbreviated as theta-MM'], w hich we have recently prepared. The electronic correlation parameter(U /t) increases by increasing the dihedral angle (theta). In the phase d iagram of theta-ET salts the ground state varies from insulating, to s uperconducting, to metallic with decreasing theta,namely, U/t. The ser ies theta-MM' is located at the center of the phase diagram where meta llic, paramagnetic insulating, and singlet states are observed at low temperatures. With applied pressure, the metal-insulator transition te mperature rises because the dihedral angle increases,which is related to the enhancement of the electronic correlation. [S0163-1829(98)03919 -8].