Temperature dependence of the electronic structure of α-(BEDT-TTF) ₂MHg(SCN)₄ (M = NH₄, K, Rb)

By using the reported temperature dependence of the lattice constants for organic metals α-(BEDT-TTF)₂MHg(SCN)₄ (M = NH₄, K, Rb), where BEDT-TTF is bis(ethylenedithio)tetrathia fulvalene, the temperature dependence of the transfer integrals have been calculated. In addition the derivatives of each transfer integral with respect to each lattice constant are calculated. The transfer integrals are sensitive to the change of the lattice constants within the conducting sheet. The gap between the open and closed orbits in the momentum space is principally generated by the difference of the transfer integrals, c1 and c2; the magnitude of the intrastack dimerization determines the separation between the open and closed orbits. Among the various salts, the NH₄ salt shows an exceptionally large shrinkage of the lattice constant c, which gives rise to large c1 - c2, a large gap, and results in a comparatively small cross section of the closed Fermi surface.