Magnetotransport measurements have been carried out on the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 at temperatures down to 500 mK and in hydrostatic pressures up to 16.3 kbar. The observation of Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependences of the area of the closed pocket of the Fermi surface and the carrier effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature Tc. The effective mass measured by the temperature dependence of the Shubnikov-de Haas oscillations is found to fall rapidly with increasing pressure up to a critical pressure Pc approximately=5 kbar. Above Pc a much weaker pressure dependence is observed; Tc also falls rapidly with pressure from 10.4 K at ambient pressure to zero at around Pc. This strongly suggests that the enhanced effective mass and the superconducting behaviour are directly connected in this organic superconductor. A simplified model of the band structure of kappa -(BEDT-TTF)2Cu(NCS)2 has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonance data and the effective masses measured in the present experiments indicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasiparticle interactions, with the electron-phonon interactions playing a secondary role. The dependence of Tc on the effective mass may be fitted satisfactorily to a suitably parametrized weak-coupling BCS expression, although this cannot be taken as a definitive proof of the nature of superconductivity in organic conductors.

The magnetoresistance of single crystals of the quasi-two-dimensional (Q2D) organic conductor has been studied at temperatures between 700 mK and 300 K in magnetic fields of up to 15 T and hydrostatic pressures of up to 20 kbar. Measurements of the resistivity using a direct-current van der Pauw technique at ambient pressure show that the material undergoes a metal-to-insulator transition at ; below this temperature the resistivity increases by more than five orders of magnitude as the samples are cooled to 4.2 K. If the current exceeds a critical value, the sample resistivity undergoes irreversible changes, and exhibits non-ohmic behaviour over a wide temperature range. Below 30 K, either an abrupt increase of the resistivity by two orders of magnitude or bistable behaviour is observed, depending on the size and/or direction of the measurement current and the sample history. These experimental data strongly suggest that the metal - insulator transition and complex resistivity behaviour are due to the formation of a charge-density wave (CDW) with a well-developed domain structure. The magnetotransport data recorded under hydrostatic pressure indicate that pressure has the effect of gradually reducing the CDW ordering temperature. At higher pressures, there is a pressure-induced transition from the CDW state to a metallic, superconducting state which occurs in two distinct stages. Firstly, a relatively small number of Q2D carriers are induced, evidence for which is seen in the form of the magnetoresistance and the presence of Shubnikov - de Haas oscillations; in spite of the low carrier density, the material then superconducts below a temperature of . Subsequently, at higher pressures, the CDW state collapses, resulting in Q1D behaviour of the magnetoresistance, and eventual suppression of the superconductivity.

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The magnetoresistance of (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and or K) has been studied under pressures of up to 14.8 kbar and for temperatures down to 0.7 K. The and Shubnikov - de Haas oscillations observed in the ambient pressure magnetoresistance of persist to the highest pressure while the other quantum oscillatory frequencies are removed under pressure. A strong second-harmonic component of the -frequency oscillations is observed on the raw data at 1 bar and is initially suppressed by pressure but returns above . Furthermore, in the salt a similar pronounced second harmonic of the quantum oscillations has been observed at the highest pressures. The origins of these features of the data are discussed in the context of current models of the Fermi surfaces of these materials.

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An experimental study and theoretical analysis of the Shubnikov-de Haas oscillations in Cd,-,~,Zn,Mn,As, single crystals is presented for samples with various compositions. T h e contribution of spin-dependent scattering of the conduction electrons on magnetic ions to t h e total relaxation time is deduced from the waveshape analysis. The formula lor the relaxation time corresponding to this scattering mechanism is derived and compared with the experimental data.

An anisotropy of the electron effective g-factor in highly degenerated tetragonal semimagnetic semiconductors (SMSC) alloyed with manganese has been found by us and by other workers when studying the Shubnikov-de Haas (SdH) effect for different orientations of the magnetic field with respect to the crystal axes. The detailed analysis of the experimental data by using a model based on an infinite Hamiltonian matrix, provides evidence for the anisotropy of the p-d exchange interaction in the materials of interest.