Overlap integrals between HOMO’s of two non-parallel BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) molecules have been calculated. As the dihedral angle between the molecular planes decreases from 180° (parallel) to 90° (perpendicular), the overlap integral increases and attains a maximum around 90°. This accounts for the “universal phase diagram” of the θ-phase; θ-salts vary from an insulator to a metal with decreasing the dihedral angle. The ratio of the lattice constants in the conducting plane, c/a, changes in proportion to the dihedral angle. Thus c/a can be used instead of the dihedral angle. A similar universal phase diagram is applicable to analogous phases like α and α″, and also to the corresponding phases of other donors. The properties of κ-phase salts are similarly scaled by c/a. As c/a increases, the intradimer overlap integral decreases owing to the increase of the intradimer spacing, and correlated insulator, superconductor, and simple-metal phases appear in succession. When c/a increases further, another insulating phase emerges due to the decrease of the interdimer overlaps. Chemical pressure in both θ- and κ-phases reduces c/a, and stabilizes the insulating state. Hydrostatic physical pressure gives the same influence in the θ-phase, but enhances the interdimer interaction in the κ-phase to result in the opposite effect. A diagram is proposed to illustrate which structures of β, β″, θ, and κ are favored by BEDT-TTF and other donors.

The behavior of the phase transitions is mapped out as a function of the dihedral angle (θ) or the transfer integral $\mathrm{}\left(t\right)\mathrm{}$ between donor columns for θ-type BEDT-TTF [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene, abbreviated as ET] salts involving the series $\theta -(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F}{)}_2{\mathrm{MM}}^{\prime }(\mathrm{SCN}{)}_4$ [$M=\mathrm{T}\mathrm{l},\mathrm{R}\mathrm{b},\mathrm{C}\mathrm{s},$ $M^{\prime }=\mathrm{C}\mathrm{o},\mathrm{Z}\mathrm{n}:$ abbreviated as $\theta {-MM}^{\prime }$], which we have recently prepared. The electronic correlation parameter $\mathrm{}(U/t)\mathrm{}$ 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, $U/t.\mathrm{}$ The series $\theta {-MM}^{\prime }$ 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.

The Shubnikov-de Haas (SdH) effect has been observed in the recently discovered ambient-pressure organic superconductor ${\mathrm{}\left[\mathrm{b}\mathrm{i}\mathrm{s}\right(\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{o}\mathrm{l}\mathrm{o}\left)\mathrm{t}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{a}\mathrm{f}\mathrm{u}\mathrm{l}\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}\right]}_2$Cu${\mathrm{}\left(\mathrm{NCS}\right)\mathrm{}}_2$. This is the first unambiguous report on the SdH effect in organic compounds. The SdH signal is observed below 1 K, above 8 T, and at angles $\theta$ between the crystal $a^{*}$-axis and a magnetic field of less than 30°. The period $0.0015cos\theta \left({\mathrm{T}}^{-1\mathrm{}}\right)$ corresponds to the cylindrical closed Fermi surface obtained by the tight-binding method based on the extended Hückel approximation, which is critically dependent on the crystal symmetry. The temperature dependence of the oscillation amplitude gives the effective mass at the Fermi surface to be about $3.5m_0$.