Extended Hückel tight-binding study of the effect of pressure and uniaxial stress on the electronic structure of α-(BEDT-TTF)2KHg(SCN)4and κ-(BEDT-TTF)2Cu(NCS)2

We have performed extended H\"uckel tight-binding calculations to model the effects of pressure and uniaxial stress on the electronic band structure of the anisotropic organic metals \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$KHg(SCN${)}_{4}$ (a) and \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$ (b). The calculated changes in the Fermi-surface topology are in excellent agreement with the experimental values determined from Shubnikov--de Haas measurements. In (a) the area of the closed orbits increases with pressure and decreases with uniaxial stress, while in (b) the closed orbits increase both with pressure and stress. We also report predictions of the effects of uniaxial stress in the a and c directions, and discuss the behavior of the effective mass and magnetic breakdown probability. \textcopyright{} 1996 The American Physical Society.