Systematic study of the electronic state in θ-type BEDT-TTF organic conductors by changing the electronic correlation

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