Structural Genealogy of BEDT-TTF-Based Organic Conductors III. Twisted Molecules: δ and α′ Phases

Abstract

Abstract δ-phase (β-PF6 type) BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene)-based organic conductors are characterized by the twisted overlap mode in the stack. Since the twisted mode has large intermolecular orbital overlap, the δ-phase is regarded as a twisted dimer structure. This type of overlap is, however, insensitive to small changes of the structure. On the contrary, an oblique interaction alters the warping of the open Fermi surface, to control the metal-insulator transition temperature, TMI. Through the change of this oblique interaction, TMI is scaled by the axis ratio for a family of salts with the same stacking pattern. As a general summary of BEDT-TTF salts, an empirical rule is proposed to predict, from the number of crystallographically independent molecules, whether an insulating state of BEDT-TTF salts will be paramagnetic or nonmagnetic. This rule is based on hypothetical pair formation of spins located on each dimer, and universally applies to all BEDT-TTF salts. This rule is extended to metallic salts, and among the potentially nonmagnetic salts, a dimerized dimer structure is the necessary condition of superconductivity. The potentially paramagnetic superconductors have strongly dimerized structures like β and κ phases. From this we can make a reasonable prediction as to superconducting phases.