Robin Nicholas

Abstract:

We report measurements of photoluminescence of GaAs/AlGaAs heterojunctions using hydrostatic pressure to reduce the Zeeman energy. A red-shift is observed near nu = 1 which falls with pressure, consistent with theoretical predictions of Skyrmionic exciton recombination energies. When the Zeeman energy is reduced below a critical value the redshift vanishes. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

InAs/GaSb heterojunctions offer the unique situation in a semiconductor of overlapping conduction and valence bands. A wavevector dependent minigap occurs due to the anticrossing of the electron and hole dispersion relations. As a result the resistivity increases strongly in very pure intrinsic structures due to the reduction in the electron group velocity. Applying a magnetic field parallel to the layers shifts the conduction and valence band relative to each other in k-space and the structure turns back to an indirect semimetal. As a result we can observe a giant negative magnetoresistance. Interband absorption is observed across the minigap, and this can also be removed by the parallel field which transforms the system to an indirect band gap alignment. In high perpendicular magnetic fields strong oscillations occur in the Hall coefficient, and at low temperatures a series of zero Hall resistance states are found in which a divergence of the diagonal resistivity occurs at the same point as a zero in the Hall resistance. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

We report large modulations in the far infrared absorption of semimetallic InAs/GaSb superlattices in parallel field. The strong absorption is attributed to direct transitions across the minigap at the point where the electron and hole dispersions anticross. The measured minigap energy is found to be in the range 3-10 meV depending on the structure of the sample. The optical results are compared to parallel field magnetoresistance measurements and also to theory using an 8 band k . p calculation. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

We have performed a magnetotransport study in both perpendicular and parallel magnetic fields for gated InAs/GaSb double heterostructures. These measurements provide a further proof for the existence of a minigap at the anti-crossing point between the electron and hole dispersion relations in this system. When a parallel magnetic field is increased from 0 to 12 T, the resistivity drops by 60% when the Fermi energy is in the middle of the minigap, but by less ;than 3% when the Fermi energy is positioned outside the minigap. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

We report magneto-luminescence studies of dense electron-hole plasmas in compressively strained GaInP/AlGaInP quantum wells. At carrier densities of order 10(13) cm(-2) many-body effects are investigated. The band gap is reduced by up to 36 meV, much less than expected from conventional random phase approximation calculations. However, much better agreement is obtained on comparison with recent density functional calculations which include the Hartree energy. The reduced mass is found to increase by up to 40% over values from low-excitation measurements. Comparison with other work suggests that the mass renormalisation is substantial in materials with wide band gaps. (C) 1998 Elsevier Science B.V. All rights reserved.