Robin Nicholas

Abstract:

Transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL) with and without additional magnetic fields (magneto-PL) were employed to study metal organic vapour phase epitaxy (MOVPE) grown, self-assembled InSb rich quantum dots (QDs) embedded in InAs and GaSb matrices. Depending on the growth conditions, coherently strained, partly relaxed, and completely relaxed InSb rich agglomerates were observed by means of TEM; AFM delivered more reliable QD number densities and accurate heights of InSb islands on GaSb. For the case of InSb embedded in GaSb, PL showed QD emission at 1.69 mum and wetting layer (WL) emission at 1.61 mum.

Abstract:

We describe the properties of a strongly interacting two-dimensional (2D) electron gas in high magnetic fields whose properties can be described in terms of the formation of composite fermions at fractional Landau level occupancy. For a Landau level occupancy nu = 1 the excitations correspond to spin waves for high g-factors and magnetic fields, but when the g-factor is reduced close to zero by the use of hydrostatic pressure there is strong evidence for the formation of skyrmions. Studies of composite fermions in the limit of vanishing Zeeman energy also suggest that skyrmionic excitations can occur for composite fermions.

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 tilted field magnetotransport measurements of semimetallic InAs/GaSb structures. It has been observed that superlattices behave in a distinctly different manner to single quantum well samples. We show that the observation of Shubnikov-de-Haas (SdH) oscillations requires that carriers are able to tunnel across the gaps between the unconnected Fermi surface islands. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

We report photoluminescence measurements on In0.25Ga0.75As/GaAs quantum well and dots grown on (1 1 1)B GaAs substrate in high magnetic fields up to 45 Tesla. A well-defined PL line with full width at half maximum of approximately 5.5 meV is observed. From an analysis of the zero field transition energy, we point out the importance of an internal piezoelectric field. By analyzing the diamagnetic shift of the PL in both Faraday and Voigt configurations, the optical characteristics of a quasi-zero dimensional exciton are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.