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 ({θ}) or the transfer integral $(t)$ between donor columns for {θ}-type BEDT-TTF [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene, abbreviated as ET] salts involving the series θ-(BEDT-TTF)₂MM^'(SCN)₄ [M=Tl,Rb,Cs, M^'=Co,Zn: abbreviated as θ-MM^'], which we have recently prepared. The electronic correlation parameter $(U/t)$ increases by increasing the dihedral angle ({θ}). In the phase diagram of {θ}-ET salts the ground state varies from insulating, to superconducting, to metallic with decreasing {θ}, namely, $U/t.$ The series θ-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.