NUMERICAL-MODEL OF QUANTUM OSCILLATIONS IN QUASI-2-DIMENSIONAL ORGANIC METALS IN HIGH MAGNETIC-FIELDS

Departures from standard Lifshitz-Kosevich behavior observed in the os cillatory magnetization and magnetoresistance of bis(ethylenedithio)te trathiafulvalene (BEDT-TTF) charge-transfer salts in high magnetic fie lds are investigated using a numerical model of the Landau levels in a quasi-two-dimensional metal. The numerical model enables oscillations in the chemical potential to be treated, as well as the effects of fi nite temperature, Landau level broadening, and the presence of additio nal quasi-one-dimensional Fermi surface sheets. The numerical calculat ions reproduce experimental magnetization data successfully, and allow several phenomena observed in the experiments to be investigated. It is found that pinning of the chemical potential to the Landau levels i s responsible for the apparent anomalously low effective masses of the higher harmonics of the de Haas-van Alphen oscillations observed in r ecent experiments. In addition, the quasi-one-dimensional components o f the Fermi surface are found to have a pronounced influence on the wa ve form of the oscillations in the model, providing a means by which t heir density of states can be estimated from experimental results. Whi lst the magnetization is a thermodynamic function of state, calculatio ns of the behavior of the magnetoresistance are much more model depend ent. In this paper, recent theoretical models for the longitudinal mag netoresistance in semiconductor superlattices have been modified for u se with the BEDT-TTF salts and are shown to successfully reproduce the form of the experimental data. The strongly peaked structure of the m agnetoresistance, which comes about when the chemical potential is sit uated in or close to the gap between adjacent Landau levels, is found to be responsible for the apparent strong increase of the effective ma ss which has recently been reported in high field transport measuremen ts.