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.

Magnetic breakdown phenomena have been investigated in the longitudinal magnetoresistance of the quasi-two-dimensional (Q2D) superconductor in magnetic fields of up to 50 T, well above the characteristic breakdown field. The material is of great interest because its relatively simple Fermi surface, consisting of a closed Q2D pocket and an open Q1D band, is almost identical to the initial hypothetical breakdown network proposed by Pippard. Two frequencies are expected to dominate the magnetoresistance oscillations: the frequency, corresponding to orbits around the closed pocket, and the frequency, corresponding to the simplest classical breakdown orbit. However, a frequency is in fact found to be the dominant high-frequency oscillation in the magnetoresistance. Numerical simulations, employing standard theories for calculating the density of states, indicate that a significant presence of the frequency (forbidden in the standard theories) can result simply from the frequency-mixing effects associated with the pinning of the chemical potential in a quasi-two-dimensional system. While this effect is able to account for the previous experimental observation of frequency oscillations of small amplitude in the magnetization, it cannot explain why such a frequency dominates the high-field magnetotransport spectrum. Instead we have extended the numerical simulations to include a quantum interference model adapted for longitudinal magnetoresistance in a quasi-two-dimensional conductor. The modified simulations are then able to account for most of the features of the experimental magnetoresistance data.

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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Magnetoresistance measurements have been made on a number of single-crystal samples of the metallic charge-transfer salt P"-(BEDT-TTF}, AuBr"using magnetic fields up to 50 T. The experiments have been carried out for a wide range of orientations of the sample with respect to the magnetic field and for temperatures ranging between 80 mK and 4.2 K. The magnetoresistance exhibits a complex series of Shubnikov -de Haas oscillations, an anisotropic angle dependence, and, below 1 K, hysteresis. Both the hysteresis in the magnetoresistance and frequency mixing effects observed in the Shubnikov -de Haas spectrum can be explained by the effects of Shoenberg magnetic interaction, and this mechanism has been successfully used to model the observed Fourier spectrum of the magnetoresistance. The complex Shubnikov-de Haas frequency spectrum of P"-(BEDT-TTF}zAuBrz is proposed to result from the effects of a spin-density wave on the band structure, which alters the original Fermi surface to produce three two-dimensional carrier pockets. The angle dependence of the Shubnikov-de Haas oscillation amplitudes has been used to deduce the approximate shapes and orientations of these pockets, which are found to be in good qualitative agreement with the proposed model.

High-field magnetoresistance oscillations in nbis(ethylenedithio) tetrathiafulvalene &KHg(SCN) 4. The efFects of magnetic breakdown, exchange interactions, and Fermi-surface reordering

We repon angle-dependent magnetoresistance experiments on the charge-transfer salt cr-ET2KHgW.S)r using fields of up to 30 T. The data indicale tJnI the widely reported high-field 'kink' observed in the magnetoresistance of Ute a-ET$Wig(NCS)4 (M = K, TI, Rb) family of charge-bansfer salts represents a transition from an imperfectly nested spin-density wave gmund state, characterired by a quasi-onedimensional Fermi surface sheet tilted with re~peet to Ihe b'c plane, to a state with a cylindrical two-dimensional Fermi surface. Charge-transfer salts of the molecule ET (bis(ethy1enedithio)tetrathiafulvalene or BEDT

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