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

Abstract

The behavior of the phase transitions is mapped out as a function of the dihedral angle (\ensuremath{\theta}) or the transfer integral $(t)$ between donor columns for \ensuremath{\theta}-type BEDT-TTF [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene, abbreviated as ET] salts involving the series $\ensuremath{\theta}\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}{\mathrm{MM}}^{\ensuremath{'}}(\mathrm{SCN}{)}_{4}$ [$M=\mathrm{T}\mathrm{l},\mathrm{R}\mathrm{b},\mathrm{C}\mathrm{s},$ ${M}^{\ensuremath{'}}=\mathrm{C}\mathrm{o},\mathrm{Z}\mathrm{n}:$ abbreviated as $\ensuremath{\theta}{\ensuremath{-}MM}^{\ensuremath{'}}$], which we have recently prepared. The electronic correlation parameter $(U/t)$ increases by increasing the dihedral angle (\ensuremath{\theta}). In the phase diagram of \ensuremath{\theta}-ET salts the ground state varies from insulating, to superconducting, to metallic with decreasing \ensuremath{\theta}, namely, $U/t.$ The series $\ensuremath{\theta}{\ensuremath{-}MM}^{\ensuremath{'}}$ 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.