Robin Nicholas

Abstract:

We investigate intersubband relaxation rates above the optical phonon energy in a InAs/GaSb superlattice using saturation spectroscopy. A high-intensity free-electron laser tuned to the intersubband transition energy is used to saturate the absorption process revealing picosecond relaxation rates. The effects of the parallel magnetic field and laser energy on the relaxation processes are explored. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

Magnetic field orientated parallel to the superlattice layers enables intersubband absorption to occur for normally incident light. We investigate semi-metallic InAs/GaSb superlattices in this configuration, presenting data over a wide range of InAs well widths. For narrow wells, the reduced absorption coefficient means that an alternative waveguide configuration, where the light makes about 4 or 5 passes through the sample is used. In this second configuration we see further activation of the intersubband resonance by the parallel field. The intersubband energy and absorption characteristics are studied over a large range of well widths and compared with results from 8-band k.p calculations. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

We have studied the temperature dependence of the magnetoresistance of semimetallic InAs/GaSb structures with magnetic field applied parallel to the layers. We present the first unambiguous evidence for the presence of a mini-gap at the crossing point between the electron and hole dispersion relations. The resistivity is found to change from semiconductor-like behaviour with a strong temperature dependence at low parallel magnetic fields to that of a semimetal with a weak temperature dependence at high field. Furthermore, the magnetoresistance, for intrinsic samples, is found to decrease with field by as much as 70% at low temperatures. As the parallel magnetic field is increased the centres of the electron and hole dispersions are shifted apart in k-space and at sufficiently high field the mini-gap is destroyed and the bands overlap fully. Finally, a theoretical model allows us to estimate that the mini-gap is of order 7 meV. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

We present a simple theoretical model which explains why intrinsic InAs/GaSb superlattices show semimetallic behaviour despite the presence of an energy mini-gap at the Fermi energy. In addition, the model also explains the new negative magnetoresistance which has recently been observed in terms of the formation of this mini-gap and its consequent reduction of the carrier group velocities. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

Measurements of the fractional quantum Hall effect energy gaps at high pressures are presented that provide evidence for the existence of composite Skyrmions. Just as charged spin textures known as Skyrmions are thought to be the lowest lying excitations for electrons at v = 1, we show that composite Skyrmions can be formed at v = 1/3 when the electron g-factor is sufficiently small. (C) 1998 Elsevier Science B.V. All rights reserved.