Electron-electron interaction and effective mass enhancement in BEDT-TTF salts

Summary form only given. Electron-electron interaction has often been considered to play an important role in the electronic properties of the BEDT-TTF salts, and the large difference between the transport and the cyclotron effective masses has often been used as an indication of a Hubbard model type enhancement. Using a Gutzwiller approximation for the Hubbard model in two dimensions, we have examined the behaviour of the effective mass enhancement as a function of electron density n and the effective electron-electron interaction U/t. Our results show that qualitatively, the effective mass has initially a strong decrease with decreasing effective electron-electron repulsion which levels off to a slower rate of change, qualitatively consistent with observations of the mass dependence with pressure in (BEDT-TTF)/sub 2/Cu(NCS)/sub 2/. The effects of further correlations in the approximations may give better quantitative behaviour, and will be discussed.