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Abstract:

The conducting properties of the pressure-induced, layered organic superconductor (BEDT-TTF)3Cl2·2±á2O have been studied at 13.5 and 14.0 kbar using low temperatures, high magnetic fields and two-axis rotation. An upper critical field that is significantly larger than that expected from the Pauli paramagnetic limit is observed when the field is applied parallel to the conducting layers. The angle-dependent magnetoresistance suggests incoherent transport between the conducting layers at both pressures and the observed negative magnetoresistance at 13.5 kbar can be explained by considering Anderson localization within the layers. Further application of pressure destroys the effects of localization.

Abstract:

Details of the Fermi-surface topology of deuterated κ(BEDT-TFF)2Cu(NCS)2 (where BEDT-TTF ≡ bis(ethylene-dithio)tetrathiafulvalene) 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.

Abstract:

Peaks in the magnetoresistivity of the layered superconductor kappa - (BEDT-TTF)2Cu(NCS)(2), measured in fields < or =45 T applied within the layers, show that the Fermi surface is extended in the interlayer direction and enable the interlayer transfer integral (t( perpendicular) approximately 0.04 meV) to be deduced. However, the quasiparticle scattering rate tau(-1) is such that Planck's over 2pi/tau approximately 6t( perpendicular), implying that kappa - (BEDT-TTF)2Cu(NCS)(2) meets the criterion used to identify interlayer incoherence. The applicability of this criterion to anisotropic materials is thus shown to be questionable.

Abstract:

Epitaxial Co/Cu superlattices were grown by molecular beam epitaxy on sapphire substrates and investigated by high-resolution x-ray diffraction at room temperature. Detailed analysis of x-ray scattering along the [10.L] reciprocal-lattice rows revealed that about 80% of Co was hexagonal close packed (h.c.p.) and only 20% was face-centred cubic (f.c.c.). A combination of modelling of x-ray scattering from superlattices with measurements of coherence lengths of individual components of the scattering suggests that the f.c.c. Co grows as a single block at the beginning of Co layers and the growth presumably starts from islands rather than from continuous Co layers. The h.c.p, structure is one-dimensionally disordered by stacking faults which appear after approximately every 12 layers of Co on average. Upon heating in ultrahigh vacuum to 700 °C the coherent superlattice structure was completely destroyed and the resulting structure consisted of relaxed layers of Co and Cu.