Robin Nicholas

Abstract:

We report studies of vertical superlattice transport in short period superlattices at high magnetic fields. This shows new magnetophonon resonances can occur in which LO phonons are resonantly scattered between high density of states regions of the superlattice leading to cycling of the carriers through high conductivity regions of the miniband.

Abstract:

We discuss the use of fixed wavelength infrared absorption measurements as a means for observing compositional changes in MOVPE process gas mixtures using relatively simple instrumentation consisting of an incandescent lamp, a narrow bandpass filter matched to the CH stretch modes of the alkyl precursors, and an infrared detector. An advantage of this setup is that the low energy of the infrared radiation does not induce photolysis reactions on the analyser cell windows in contrast to ultraviolet-based techniques. We present preliminary data on gas composition measurements obtained in the alkyl lines of an MOVPE reactor and show that useful information can readily be obtained on absolute gas concentrations, gas switching transients, and reactor line memory effects. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

The electrical transport properties of a bipolar InAs/GaSb system have been studied in a magnetic field. The resistivity oscillates between insulating and metallic behavior while the quantum Hall effect shows a digital character oscillating from 0 to 1 conductance quantum e(2)/h. The insulating behavior is attributed to the formation of a total energy gap in the system. A novel looped edge state picture is proposed associated with the appearance of a voltage between Hall probes which is symmetric on magnetic field reversal.

Abstract:

Self-assembled Sb-based quantum dots (QDs) were grown by metal-organic vapour phase epitaxy and assessed by means of atomic force microscopy, transmission electron microscopy and photoluminescence. Two series of InSb QDs in a GaSb matrix were grown at 490 +/- 10 degrees C and luminesced in the mid-infrared at about 1.7 mu m. Reductions in the III/V ratio and growth rate as used for the second series resulted in a change of the morphology of the InSb islands from hillocks without facets and a low level of order, to dumbbell shaped islands with distinct facets and a higher level of order. Self-assembled GaSb islands were grown on GaAs at 550 degrees C and assessed for comparison by means of AFM.

Abstract:

There has been a high field magnet facility in Oxford since shortly after WW It. The heart of this installation used to be a 2MW rotary converter that powered watercooled copper solenoids. The generator was retired some two years ago and the emphasis was switched to superconducting magnets and pulsed magnets, Recent success lu attracting funding has enabled us to undertake several programmes of upgrading and reequipping, which are described in this paper. They are; t Converting the space occupied by the old generator, which will give us three pulsed magmst stations. The magnets will be in pits below floor level for safety. The maximum fields that we generate at present arc over 60 T using magnets and high strength conductors developed In house + We have acquired a new superconducting magnet that gives, 2uT in a 40 mm bore at 4.2K (we believe that this is a first worldwide) and 21.5T in 40 mm at 2.2K. The magnet can also be configured to give up to 1ST in a 110 mm bore by the removal of the Innermost section. Using the new superconducting magnet we can advance our programme of research and development of High T<, insert colls 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, © 2000 IEEE.