Professor Stephen Blundell

Abstract:

We experimentally demonstrate a mechanism for a new kind of magnetic resonance, the Fermi-surface traversal resonance (FTR). This is caused by the periodic traversal of carriers across quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in an external magnetic field. Owing to the warping of the Q1D Fermi sheets, the real space velocities of the carriers oscillate as they cross the FS, generating resonances in the high frequency conductivity of the material. The results contain information about the FS, including the direction and harmonic content of the warping components. Using a rotating resonant-cavity system, FTRs have been observed in α-(BEDT-TTF)2 KHg(SCN)4. © 1998 The American Physical Society.

Abstract:

We have performed muon-spin rotation (mu SR) measurements on the organic superconductor K-(BEDT-TTF)(2)CU(SCN)(2) as a function of the angle between the applied field and the superconducting planes. The measurements are found to have much in common with those on the anisotropic cuprate superconductor Bi2Sr2CaCu2O8 divided by delta. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

A non-linear model of solid-state amorphization (SSA) is proposed which takes into account the interaction of the structural elements of the amorphous phase via a stress field. For the experimental parameters which correspond to semiconductors undergoing SSA by high-pressure phase retention, we obtain the solution in the form of a self-sustaining wave with a constant velocity v. This solution exists in the region v < vc, where the critical velocity vc is controlled by the thermal and concentration diffusivities DT and DN . A simple scaling relation is proposed for vc: vc(DN, DT) ∼ DT/(DN)0.5. It is found that the characteristic correlation length of the amorphous phase formed by SSA strongly depends on the interplay between the parameters v, DN and DT.

Abstract:

We present the results of a muon-spin rotation/relaxation (μSR) study in zero and longitudinal magnetic fields and temperatures down to 40 mK of para-cyanophenyl nitronyl nitroxide (p-CNPNN). This layered material is expected to show ferromagnetic interactions on the basis of susceptibility measurements. In a zero-field μSR experiment, the spins of muons which are implanted in diamagnetic states will precess in any local internal fields. Hence a study of precession frequencies and/or relaxation rates can be used to follow the magnetic order parameter as a function of temperature. We find strongly temperature dependent muon relaxation below ∼0.17 K similar to that found recently in 1-NAPNN, indicating a magnetic transition at this temperature.

Abstract:

We report the first muon-spin relaxation study of an organic spin-Peierls system, the linear-chain compound MEM(TCNQ)2. Our results show a crossover from a Gaussian relaxation to an exponential relaxation as the temperature is lowered below the spin-Peierls transition. We associate this behaviour with the slowing down of the electronic fluctuations resulting from the opening of a gap in the magnetic excitation spectrum.