Robin Nicholas

Abstract:

There has been a high field magnet facility in Oxford since shortly after WW II. The heart of this installation used to be a 2MW rotary converter that powered water-cooled copper solenoids. The generator was retired some two years ago and the emphasis was switched to superconducting magnets and pulsed magnets. Recent success in attracting funding has enabled us to undertake several programmes of upgrading and re-equipping, which are described in this paper. They are;Converting the space occupied by the old generator, which will give us three pulsed magnet stations. The magnets will be in pits below floor level for safety. The maximum fields that we generate at present are over 60 T using magnets and high strength conductors developed in houseWe have acquired a new superconducting magnet that gives, 20T in a 40 mm bore at 4.2K (we believe that this is a first world-wide) and 21.5T in 40 mm at 2.2K. The magnet can also be configured to give up to 18T in a 110 mm bore by the removal of the innermost section.Using the new superconducting magnet we fan advance our programme of research and development of High T, insert coils for very high fields and we hope to have static fields of 25T before long.Development of "miniature" bore pulsed magnets and the associated cryogenics for fields >70T.

Abstract:

The influence of the interface states on the cyclotron resonance (CR) of the broken-gap two carrier InAs/GaSb DHET system has been investigated. Enhanced coupling between electron and hole levels is observed in the samples with a monolayer of InSb formed at one of the interfaces, leading to evidence of strong interband transitions close to that of the electron CR. This is believed to be the result of the asymmetry introduced into the structure. The results are consistent with the theoretical analysis from self-consistent k·p calculations.

Abstract:

The quantum Hall effect in InAs/GaSb heterojunctions at very high fields and low temperatures when both the electrons and holes contribute to the quantized Hall conductance was studied. InAs/GaSb were grown by metal organic vapor phase epitaxy and are known to possess a relatively low level of extrinsic doping so that the majority of charge carriers are created by intrinsic charge transfer from the GaSb layers to the InAs layer. The Hall conductance showed a digital sequence oscillating from 0-1-0 conductance quanta and the diagonal resistivity showed oscillatory insulating behavior due to the formation of a total gap in the energy spectrum.

Abstract:

We present a theoretical study of intersubband transitions (IST) in InAs/GaSb superlattices in the presence of a parallel magnetic field. Starting from a model describing a single electron in a single quantum well and extending it to include many-body effects and the non-parabolicity of the InAs conduction band we are able to explain the recent cyclotron resonance experiments on narrow and wide InAs/GaSb quantum wells.