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Magnetoplasma and impurity excitations in GaAs studied by resonant Raman scattering in high magnetic fields

Andrzej Wysmolek 

University of Warsaw, Institute of Experimental Physics (IFDUW), Hoża 69, Warsaw 00-681, Poland
Grenoble High Magnetic Field Laboratory, CNRS (GHMFL), 25 Avenue Des Martyrs BP166, Grenoble 38042, France


Plasmonic and shallow donor excitations in n-type GaAs are studied in extreme magnetic-fields (up to 28T) using the method of inelastic light scattering (Raman scattering). [1] The strength of magnetic field was sufficient to made cyclotron frequency comparable to all important frequencies in the system, namely the plasma frequency, and the frequencies of longitudinal (LO) and transverse (TO) optical phonons. The physics of mixed plasma and phonon excitations under these particular and interesting conditions has never been fully explored in Raman scattering experiments.

Experimental data are analyzed using a standard, dielectric function theory. Results obtained for samples with high electron concentration are well understood in terms of observation of longitudinal, plasmon-phonon excitations. A strong interaction of coupled LO-phonon-plasmon modes with the collective cyclotron resonance excitations (Bernstein modes) is observed.

In samples with lower electron concentration, the unexpected feature in the vicinity of the undressed optical phonon is observed at high magnetic fields. This effect is explained in terms of transverse excitations, which can appear in the Raman spectrum due to disorder-activated selection-rule breaking. A field induced metal-insulator transition GaAs is shown to be traced with Raman scattering experiments in samples with the lowest electron concentration. In the insulating state, the repulsion (magnetopolaron effect) is observed between the intradonor 1s-2p excitation and the LO-phonon mode. This is explained in terms of Fröhlich interaction. Interestingly, the analogous effect involving the cyclotron resonance excitation is not observed in the metallic phase. Our observation may give some additional insight into the physics of the resonant magnetopolaron effect, which has been recently reviewed in two-dimensional systems.

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Presentation: Invited oral at E-MRS Fall Meeting 2008, Symposium A, by Andrzej Wysmolek
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-07-03 14:53
Revised:   2009-06-07 00:48