Paul Goddard

Abstract:

We present results of magnetization and electron paramagnetic resonance experiments on the spin-dimer system BaCu Si2 O6. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian lies in magnetic dipolar interactions. Axial symmetry breaking is on a very small energy scale of ≤11 mK, confirming Bose-Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point. © 2006 The American Physical Society.

Abstract:

This article reviews high magnetic field magnetization measurements of the shape memory alloy AuZn. These measurements elucidate the underlying electronic mechanism driving this effect. The application of high magnetic fields (up to 65 T), in combination with exceptional sample purity and light electronic effective masses enables the observation of magnetic quantum oscillations over an unprecedented range of temperatures (up to 100 K); thus the electronic band structure can be tracked across the shape memory phase transition (at T=67 K) and the Fermi surface nesting event driving it identified. © 2006 Elsevier Ltd. All rights reserved.

Abstract:

We describe instrumentation designed to perform millimeter-wave conductivity measurements in pulsed high magnetic fields at low temperatures. The main component of this system is an entirely nonmetallic microwave resonator. The resonator utilizes periodic dielectric arrays (photonic band-gap structures) to confine the radiation, such that the resonant modes have a high Q factor, and the system possesses sufficient sensitivity to measure small samples within the duration of a magnet pulse. As well as measuring the sample conductivity to probe orbital physics in metallic systems, this technique can detect the sample permittivity and permeability allowing measurement of spin physics in insulating systems. We demonstrate the system performance in pulsed magnetic fields with both electron paramagnetic resonance experiments and conductivity measurements of correlated electron systems. © 2006 American Institute of Physics.

Abstract:

After a brief introduction to crystalline organic superconductors and metals, we shall describe two recently-observed exotic phases that occur only in high magnetic fields. The first involves measurements of the non-linear electrical resistance of single crystals of the charge-density-wave (CDW) system (Per)2Au(mnt)2 in static magnetic fields of up to 45 T and temperatures as low as 25 mK. The presence of a fully gapped CDW state with typical CDW electrodynamics at fields higher than the Pauli paramagnetic limit of 34 T suggests the existence of a modulated CDW phase analogous to the Fulde-Ferrell-Larkin-Ovchinnikov state. Secondly, measurements of the Hall potential of single crystals of α-(BEDT-TTF)2KHg(SCN) 4, made using a variant of the Corbino geometry in quasi-static magnetic fields, show persistent current effects that are similar to those observed in conventional superconductors. The longevity of the currents, large Hall angle, flux quantization and confinement of the reactive component of the Hall potential to the edge of the sample are all consistent with the realization of a new state of matter in CDW systems with significant orbital quantization effects in strong magnetic fields. © 2005 Elsevier Ltd. All rights reserved.

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.