Whilst tight-binding bandstructure calculations are very successful in describing the Fermi-surface configuration in many quasi-two-dimensional organic molecular metals, the detailed topology of the predicted Fermi surface often differs from that measured in experiments. This is very significant when, for example, the formation of a density-wave state depends critically on details of the nesting of Fermi-surface sheets. These differences between theory and experiment probably result from the limited accuracy to which the -orbitals of the component molecules (which give rise to the transfer integrals of the tight-binding bandstructure) are known. In order to surmount this problem, we have derived a method whereby the transfer integrals within a tight-binding bandstructure model are adjusted until the detailed Fermi-surface topology is in good agreement with a wide variety of experimental data. The method is applied to the charge-transfer salt -(BEDT-TTF)2KHg(SCN)4, the Fermi surface of which has been the source of much speculation in recent years. The Fermi surface obtained differs in detail from previous bandstructure calculation findings. In particular, the quasi-one-dimensional component of the Fermi surface is more strongly warped. This implies that upon nesting of these sheets, significant parts of the quasi-one-dimensional sheets remain, leading to a complicated Fermi-surface topology within the low-temperature, low-magnetic-field phase. In contrast to previous models of this phase, the model for the reconstructed Fermi surface in this work can explain virtually all of the current experimental observations in a consistent manner.

Measurements of the temperature-dependent resistivity of high-mobility GaAs/GaAlAs heterojunctions are used to measure the effective mass of Composite Fermions (CF). The CF effective mass is found to increase approximately linearly with the effective field B * up to effective fields of 14 T. Data from all fractions around ν = 1/2 are unified by the single parameter B * for samples studied over a wide range of temperature. The energy gap is found to increase as √ B * at high fields. Hydrostatic pressure is used to reduce the value of the electron g-factor, and this is shown to have a large effect on the relative strengths of different fractions. By 13.4 kbar, where the Zeeman energy is only 1/4 of its value at 0 bar, fractions with odd numerators are found to be strongly suppressed, and new features with even numerators appear. The energy gaps measured for 5/3 as a function of carrier density and pressure are consistent with a g-factor equal to the bulk value enhanced by a factor of two due to exchange interactions.

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The temperature dependence ͑40 mKрTр1 K͒ of the oscillations in xx in a high-mobility GaAs-͑Ga,Al͒As heterojunction close to Landau-level filling factor ϭ 1 2 has been examined for many different values of , the angle between the normal to the sample and the magnetic field. It was found that the energy gaps associated with the fractional quantum Hall effect could be interpreted using the composite-fermion ͑CF͒ approach with a fixed CF effective mass at each . However, was found not to follow the dependence expected for a purely 2D system; i.e., the CF energy gaps at fixed grow markedly with increasing in-plane field. Comparisons with models based on the Fang-Howard variational wave function show that this effect is due to the compression of the electronic wave function caused by the in-plane component of the magnetic field.

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