Paul Goddard

Abstract:

The interlayer magnetoresistance sigma(zz) of the organic metal kappa-(BEDT-TTF)(2)Cu(NCS)(2) has been studied in fields B of up to 45 T and at temperatures T from 0.5 K to 50 K. The peak in rho(zz) seen in exactly in-plane fields, a definitive signature of interlayer coherence, remains to Ts exceeding the Anderson criterion for incoherent transport by a factor similar to 25. Angle-dependent magnetoresistance oscillations (AMROs) due to Fermi-surface orbits are suppressed by rising T, with a T-2 dependence suggesting electron-electron scattering.

Abstract:

TmAgGe is an antiferromagnet based on the ZrNiAl structure. At low temperatures the spins are confined to distorted kagome-like planes, wherein the magnetisation is strongly anisotropic. A previous study has shown that a series of stepped magnetic transitions are apparent in low, in-plane magnetic fields and can be explained using a three-fold Ising-like model. Here we present high-magnetic-field magnetisation experiments showing that further stepped transitions are observed when the field is directed out of the kagome planes. Angledependent measurements in fields of up to 55 T show that there are at least two distinct and separate energy scales present in this system; the weak exchange interactions and the strong crystalline electric field interactions. Simulations of the magnetisation using a three-dimensional, free-energy minimisation technique allow us to suggest the nature and hierarchy of the forces acting on the Tm3+ moments.

Abstract:

Magnetotransport measurements have been performed on single crystals Of kappa-(BEDT-TTF)(2)Cu(NCS)(2) in fields of up to 33 T at temperatures between 500 mK and 4.2K. Using a diamond anvil cell mounted on a goniometer, measurements of the angle and temperature dependence of the interlayer resistance, R-zz, under hydrostatic pressures between 1.1 kbar and 17.3 kbar were performed. For the first time we have been able to measure angle-dependent magnetoresistance oscillations under pressure due to both the 1D and 2D Fermi surfaces in addition to Shubnikov de Haas oscillations. The results show that the shape of the elliptical quasi-2D Fermi-pocket is more elongated under a hydrostatic pressure of 9.8 kbar compared with ambient pressure. When the magnetic field B is close to parallel to the highly conductive plane, bc, a peak in R-zz is observed with an angular width determined by the ratio of the maximum inter- and intra-layer Fermi velocities. The width of this peak is found to increase with pressure suggesting that the Fermi surface becomes more three-dimensional upon application of pressure.

Abstract:

AuZn undergoes a shape-memory transition at 67 K. The de Haas-van Alphen effect persists to 100 K enabling the observation of a change in the quantum oscillation spectrum indicative of a catastrophic Fermi surface reconstruction at the transition. The coexistence of both Fermi surfaces at low temperatures suggests an intrinsic phase separation in the bulk of the material. In addition, Dingle analysis reveals a sharp change in the scattering mechanism at a threshold cyclotron radius, attributable to the underlying microstructure driving the shape-memory effect.