Quantum spin dynamics

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:

The Fermi-surface topology of the organic superconductor λ-(BETS)2GaCl4 has been determined using the Shubnikov-de Haas and magnetic breakdown effects and angle-dependent magnetoresistance oscillations. The former experiments were carried out in pulsed fields of up to 60 T, whereas the latter employed quasistatic fields of up to 30 T. All of these data show that the Fermi-surface topology of λ-(BETS)2GaCl4 is very similar to that of the most heavily studied organic superconductor, κ-(BEDT-TTF)2Cu(NCS)2 (BEDT-TTF ≡ bis(ethylene-dithio)tetrathiafulvalene), except in one important respect: the interplane transfer integral of λ-(BETS)2GaCl4 is a factor ∼5 larger than that of κ-(BEDT-TTF)2Cu(NCS)2. The increased three-dimensionality of λ-(BETS)2GaCl4 is manifested in radio-frequency penetration-depth measurements, which show a clear dimensional crossover in the behaviour of Hc2(T). The radio-frequency measurements have also been used to extract the Labusch parameter determining the fluxoid interactions as a function of temperature, and to map the flux-lattice melting curve.