Robin Nicholas

Abstract:

The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by MBE on Gal? substrates has allowed the observation of heavy and light hole exciton transitions with values for the FWHM as narrow as 5 meV The 2s state of the heavy hole exciton is identified and exciton binding energies as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings; of the order of 10 meV for both the light and heavy hole exciton transitions appear in magneto-reflectivity spectra in magnetic fields up to 54 T. Large light hole exciton g-values of the order of four for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.

Abstract:

Magnetotransport measurements have been performed in strongly coupled two-dimensional electron-hole gases in an InAs Gash heterostructure in the presence of antidot potentials of different strengths. We have observed commensurability features in the magnetoresistance that we attribute to high mobility electrons. It is proposed that they can be observed due to magnetic breakdown, despite the fact that the electron-hole mixing is found to play an important role. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We report a study of cyclotron resonance (CR) in a bilayer-bipolar InAs/GaSb structure and the influence of the spatial separation between the electrons and holes. At low magnetic fields we find that the cyclotron mass of the electrons can double when the electrons and holes have a very small separation. At higher cyclotron energies we find that the CR lines are split by a series of resonances which are due to interband coupling, and which also increase rapidly in magnitude when the electron-hole separation is decreased. At very high fields, in the quantum limit a new series of couplings is observed, with the resonances being split by energies in the range of 3-12 meV. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We report a study of the spin polarisation of 2D electrons around an occupancy v = 1 for a series of quantum wells with widths ranging from 250 to 51 A in which the electron g-factor passes through zero, Our data are inconsistent with present theoretical pictures of the quantum Hall ferromagnet at v = 1 and suggest that a robust collective state forms in the region of v = 1 but that this state does not correspond to full spin polarisation of the system at values of g-factor less than similar to 0.5. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We examine the quantum Hall effect in an electron-hole system and its current-driven breakdown, We find that samples with closely matched electron and hole concentrations have vastly reduced critical cur-rents while those with many more electrons than holes show much larger critical currents, though still smaller than those reported for single-carrier type systems, The channel width dependence shows two regimes of behaviour. States with larger critical cur-rents ( > 5 muA) have linear width dependence. while for narrower channel widths where the critical current is smaller, the critical current has a superlinear relationship with the channel width. (C) 2002 Elsevier Science B.V. All rights reserved.