Quantum spin dynamics

Abstract:

We report a study of the angle-dependent millimetre-wave magnetoconductivity of the p-wave triplet-paired perovskite superconductor Sr2RuO4. We find two harmonic series. We assign the first to interlayer cyclotron resonance of the beta-Fermi surface and its harmonics, yielding a cyclotron effective mass of m(beta)(cr) = 4.25 m(e). We assign the second series, which contains only odd harmonics, to cyclotron resonance of the gamma-Fermi surface, yielding a cyclotron effective mass of m(gamma)(cr) = 12.4 m(e). In addition, we find a very strong absorption mode in the presence of a magnetic field component parallel to the quasi-two-dimensional (Q2D) planes of the sample. Its dependence on the orientation of the magnetic field cannot be described in the context of conventional Q2D cyclotron resonance, and the origin of this mode is not yet clear.

Abstract:

The conducting properties of the pressure-induced, layered organic superconductor (BEDT-TTF)3Cl2·2H2O 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.