Single crystals of the layered organic type II superconductor, κ-(BEDT-TTF) 2 Cu(NCS) 2 , have been studied in magnetic fields of up to 33 T and at temperatures between 0.5 K and 11 K using a compact differential susceptometer. When the magnetic field lies precisely in the quasi-two-dimensional planes of the material, there is strong evidence for a phase transition from the superconducting mixed state into a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, manisfested as a change in the rigidity of the vortex system. The behaviour of the transition as a function of temperature is in good agreement with theoretical predictions.There has been recent renewed interest in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) 1 state in superconductors subjected to high magnetic fields (for a summary, see Refs.2,3 and references therein). In a metal in a magnetic field, the normal quasiparticles have separate 1

We have performed detailed studies of the angle-and temperature-dependent resistive upper critical fields in the layered organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 . With the magnetic field lying in the conducting planes, our measurements show an upper critical field which comfortably exceeds the Pauli-paramagnetic limit in this material. We find no azimuthal angle dependence of the critical field, in spite of recent evidence that this material has gap nodes characteristic of d-wave superconductivity. We propose that the large critical fields may be due to a Fulde-Ferrell-Larkin-Ovchinnikov state which can exist in exactly in-plane fields because of the nature of the Fermi surface of κ-(BEDT-TTF) 2 Cu(NCS) 2 .

Abstract. Details of the Fermi-surface topology of deuterated κ-(BEDT-TTF) 2 Cu(NCS) 2 have been measured as a function of pressure, and compared with equivalent measurements of the undeuterated salt. We find that the superconducting transition temperature is much more dramatically suppressed by increasing pressure in the deuterated salt. It is suggested that this is linked to pressure-induced changes in the Fermi-surface topology, which occur more rapidly in the deuterated salt than in the undeuterated salt as the pressure is raised. Our data suggest that the negative isotope effect observed on deuteration is due to small differences in Fermi-surface topology caused by the isotopic substitution.

The shape of the Fermi surface of organic metals can be measured by recording angle-dependent magnetoresistance oscillations. We review this technique and develop a model for parametrizing the shape of the quasi-two-dimensional Fermi surface sections which often appear in organic metals. Using this model, we show that it is possible to extract more detail about the quasi-two-dimensional pocket shape from angle-dependent magnetoresistance oscillations than in the traditional approximation which assumes an elliptical Fermi surface shape. We also consider the implications for cyclotron resonance experiments.

We show that Shubnikov-de Haas oscillations in the interlayer resistivity of the organic superconductor β ′′ -(BEDT-TTF)2SF5 CH2CF2SO3 become very pronounced in magnetic fields ∼ 60 T. The conductivity minima exhibit thermally-activated behaviour that can be explained simply by the presence of a Landau gap, with the quasi-one-dimensional Fermi surface sheets contributing negligibly to the conductivity. This observation, together with complete suppression of chemical potential oscillations, is consistent with an incommensurate nesting instability of the quasi-one-dimensional sheets.PACS numbers: 74.70. Kn, 78.20.Ls, 71.20.Rv The quantizing effect of a magnetic field on a chargecarrier system is well known [1]. In metals, this leads to oscillations of the free energy and quasiparticle density of states as Landau levels cross the Fermi energy [2]. The effect is very pronounced in quasi-two-dimensional (Q2D) metals containing Fermi surfaces (FSs) that are approximately cylindrical [3]. Recently there has also been interest in analogous effects in quasi-one-dimensional (Q1D) FS sections, which can lead to magnetic-field-induced quantization [4] and localization [5].In this paper we describe the magnetoresistance of the organic superconductor. Bandstructure calculations suggest that this material possesses a FS comprising a Q2D cylinder and a pair of Q1D sheets [6]. However, Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) measurements reveal that the Q2D cylinder has only one third the expected cross-section [7,8,9]. Angle-dependent magnetoresistance oscillation (AMRO) [8,10] and millimetre-wave magnetoconductivity experiments [11] show that the cross-section of this Q2D pocket resembles an elongated diamond. The same experimental techniques find no evidence for the presence of Q1D Fermi sheets at low temperatures [8,10,11], unlike the situation in other Q2D organic metals [3]. By contrast, in order to explain the observation of a fixed chemical potential µ in the dHvA effect [7], Wosnitza et al. proposed Q1D states which have an enormous density of states, exceeding the estimates from bandstructure calculations by at least an order of magnitude [7]. Moreover, using a simple formula for the background magnetoresistance, Wosnitza et al. suggested that the Q1D sheets become localised in a magnetic field [12]. In the present paper, we show that magnetoresistance data suggest a much simpler explanation. The thermally activated behaviour of the data at integer Landau level filling factors is explained entirely in terms of a Landau gap. Moreover, the failure of the Q1D sheets to contribute to the conductivity together with their ability to fix µ is explained by their nesting to form an incommensurate density-wave ground state. This mechanism is supported by the temperature dependence of the resistivity at B = 0.Single crystals (∼ 1.0 × 0.5 × 0.2 mm 3 ) of β ′′ -(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 were prepared using standard electrochemical techniques [6]. Contacts were applied using 12.5 µm Pt wires and graphite pain...