High magnetic field optical studies of electron systems confined to two or fewer dimensions.

Summary of EPSRC Grant GR/K 39202
Dr AS Plaut and Dr A Usher,
Semiconductor Physics Group, School of Physics, University of Exeter, EX4 4QL

The aims of this research were to use optical techniques to investigate the phase transitions that occur at low temperatures and high magnetic field in two-dimensional (2D) semiconductor systems and to make magneto-luminescence measurements on quantum wires and quantum dots fabricated using etching and strain techniques.

The work has resulted in 15 papers and a number are either planned or in preparation. We have presented our results at 10 international conferences and invited talks.

These aims have been fulfilled by the following principal achievements:

• Optical detection of the fractional and integer quantum Hall effects in 2D hole systems (2DHSs).
• Attainment of unprecedentedly low 2D hole concentrations by optically induced density depletion; this should enable us to examine the intriguing regime in which Wigner crystallisation is predicted to occur in zero magnetic field.
• A world-first in the derivation of the first experimental phase diagram for a 2D electron system (2DES) in the extreme magnetic quantum limit, obtained from photoluminescence measurements (highlighted in the 1993 'Forward Look' Report of the Materials Commission of SERC). This result showed the competition between the fluid state responsible for fractional quantum Hall effect and an insulating phase thought to be the Wigner solid.
• First milli-Kelvin photoluminescence excitation measurements of 2DHSs.
• Optical detection of spatial charge transfer in a delta-doped 2DES.
• Determination of the total ground state energy of interacting electrons in quantum dots -- the first photoluminescence results from 0DESs.
• The realisation of quantum wires and dots produced by strain-confining electron gases, and the detection of quantum confined energy levels in the smallest dots.
• Development of a theoretical model for the reduction in 2D electron density in single heterojunctions under illumination. This model enables us to fine-tune the design of new heterostructures in which we may use the density-reduction to probe previously inaccessible regions of the Wigner-solid phase diagram.

Publications

Optically-induced density depletion of the two-dimensional electron system in GaAs/AlGaAs heterojunctions M Hayne, A Usher, A S Plaut, K Ploog, Physical Review B50 17208 (1994)

Transport and photoluminescence studies of 2D hole systems in the extreme quantum limit P J Rodgers, B L Gallagher, M Henini, Y Ponomarev, A Usher, G Hill, in ÒHigh Magnetic Fields in the Physics of SemiconductorsÓ, ed. D. Heiman, p452. World Scientific, New Jersey (1995)

Studies of the extreme quantum limit of 2D hole systems B L Gallagher, P J Rodgers, C J G M Langerak, P A Crump, M Henini, G Hill, S A J Wiegers, J A A J Perenboom, Y Ponomarev, A Usher, Physica B 211 417 (1995)

An elegant verification of the negative charge-state of the DX-centre from mobility measurements of 2D electrons M Hayne, A Usher, J J Harris, C T Foxon, Surface Science 362 574 (1996)

Milli-Kelvin magneto-optical studies of two-dimensional hole systems Y V Ponomarev, A Usher, P J Rodgers, B L Gallagher, M Henini, G Hill, Physical Review B 54 13891 (1996)

Milli-Kelvin magneto-transport studies of two-dimensional hole systems under illumination Y V Ponomarev, A Usher, M Hayne, P J Rodgers, B L Gallagher, M Henini, G Hill, Solid State Communications 102 53 (1997)

Conditions for optical detection of the quantum Hall effect in two-dimensional hole systems C M Townsley, Y V Ponomarev, A Usher, B L Gallagher, M Henini, G Hill, in ÒHigh Magnetic Fields in the Physics of Semiconductors IIÓ, ed. G. Landwehr and W. Ossau, p601. World Scientific, Singapore (1997)

Charge transfer in optical and transport experiments on delta-doped GaAs/Al_xGa_1-xAs heterojunctions M Hayne, M G Greally, A Usher, A S Plaut, S P Beaumont, M Holland, C R Stanley, Submitted to Physical Review B.

Optical detection of charge redistribution in a d modulation-doped GaAs-Al_xGa_1-xAs heterojunction M G Greally, G C Kerridge, M Hayne, S Patel, A Usher, A S Plaut, M C Holland, C R Stanley, Submitted to Physical Review B

Quantum versus semi-classical density of states of a two-dimensional electron gas in a magnetic field M Hayne, A Usher, J J Harris, C T Foxon, Submitted to Physical Review B

Photoluminescence excitation studies of two-dimensional hole systems in the quantum Hall regime C M Townsley, A Usher, B L Gallagher, M Henini, G Hill, Proc. Int. Workshop on Novel Physics in Low-Dimensional Electron Systems, Dresden Germany 1997 (Physica E, in press)

Magneto-transport studies of localised and extended states of a 2DEG in low magnetic fields M Hayne, A Usher, J J Harris, C T Foxon, Proc. Int. Workshop on Novel Physics in Low-Dimensional Electron Systems, Dresden Germany 1997 (Physica E, in press)

Magneto-optics of electron gases confined in GaAs quantum dots S Patel, A S Plaut, P Hawrylak, H Lage, P Grambow, D Heitmann, K von Klitzing, J P Harbison, L T Florez, Solid State Communications 101 865 (1997)

Strain-induced quantum confinement of electron gases W G Stallard, A S Plaut, S Thoms, M C Holland, S P Beaumont, C R Stanley, M Hopkinson, Proc. Eighth Int. Conf. on Modulated Semiconductor Structures, Santa Barbara USA 1997 (Physica E, in press)