Dr Moon-Sun Nam

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.

Abstract:

We show that Shubnikov-de Haas oscillations in the interlayer resistivity of the organic superconductor beta(")-(BEDT-TTF)2SF5CH2CF2SO3 become very pronounced in magnetic fields approximately 60 T. The conductivity minima exhibit thermally activated behavior 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.

Abstract:

We present a numerical simulation of angle dependent magnetoresistances oscillations (AMROs) in α-(BEDT-TTF)2KHg(SCN)4. The nesting vector of the density wave and the magnetic breakdowngap in the low-temperature, low-magnetic-field state (LTLF) are constrained by simulations of quasi-one-dimensional (Q1D) AMRO. Quasi-two-dimensional (Q2D) AMRO simulations for the high-temperature, high-magnetic-field state (HTHF) allow the precise derivation of an ellipticity of the Q2D Fermi surface. Simulations of the field dependent AMRO demonstrate the gradual evolution of the FS from LTLF to HTHF states toward the kink transition at 23 T.

Abstract:

The quasi-two-dimensional superconductor was studied via magnetic measurements. It was observed that the sample entered a Fulde-Ferrell-Larkin-Ovchinnikov state below 5.5 K in an exact in-plane magnetic field. The evidence of the transition of the sample was further studied.