Prof Arzhang Ardavan

Abstract:

We have used a novel millimetre-wave magneto-optical technique to study the angle-dependence of the high-frequency conductivity of the molecular superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2). The data, strongly suggest that the superconducting gap has nodes directed along the b and c directions of the crystal, in agreement with recent theoretical predictions. This supports the idea that the superconductivity in kappa-(BEDT-TTF)(2)Cu(NCS)(2) is d-wave in nature, and is mediated by spin fluctuations.

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 alpha-(BEDT-TTF)(2)KHg(SCN)(4). (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

The FELIX free-electron laser is continuously tuneable over the wavelength range 5-100 mu m and can provide great power and the possibility of time-resolved experiments in the ns to mu s range. We describe a pulsed magnet specifically designed for semiconductor experiments using FELIX, which provides fields of up to 45 T, with a pulse-shape tailored to the macropulses of the FELIX source. The magnet consists of a conventional outer coil providing fields of up to 30 T with a pulse length of several ms, plus a single-layer inner coil with a rise-time of similar to 10 mu s, matched to duration of the FELIX macropulses. The magnet is to be extended to fields of up to 60 T. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

We have performed the first cyclotron resonance studies of high mobility two-dimensional hole systems (2DHS) under pressure. Experiments were carried out using millimetre wave radiation on GaAs-(Ga, Al)As heterojunctions. Pressures were applied using a clamp cell specially designed for millimetre wave measurements. Our data at ambient pressure show collectively coupled resonances arising from interactions between the two hole systems with different effective masses formed by the spin-split halves of the lowest heavy-hole subband. The application of pressure appears to completely suppress the collective effect and only a resonance corresponding to the lighter cyclotron mass from the heavy-hole spin-split subband is observed. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

Measurements of the magnetic-field dependent millimetre-wave response of Cs2CuCl4, a quasi-one-dimensional Heisenberg antiferromagnet, are presented. The evolution of the electron spin resonance between high temperatures (the paramagnetic regime) and temperatures below T-N, at which there is an onset of long-range magnetic order (the antiferromagnetic regime), is studied for a range of crystal orientations. The magneto-optical data suggest that there is an onset of local magnetic order at temperatures considerably higher than T-N as determined from neutron scattering measurements, and that the evolution towards a long-range ordered state is gradual. A phenomenological model that describes the general features is suggested. (C) 1998 Elsevier Science B.V. All rights reserved.