Discrete states of conduction electrons bound to ionized acceptors are observed in intraband magneto-optical experiments on Be-doped GaAs/Ga0.67Al0.33As quantum wells. The electrons are bound to acceptors by a joint effect of the quantum well and an external magnetic field. The observed transition energies are successfully described using states of single magnetoacceptors. The energies show evidence for oscillatory screening of acceptor potentials. Also, two disorder modes of the cyclotron resonance related to acceptor potential fluctuations are observed at higher filling factors
This paper reports on an experimental study of the contact resistance of Hall bars in the Quantum Hall Effect regime while increasing the current through the sample. These measurements involve also the longitudinal resistance and they have been always performed before the breakdown of the Quantum Hall Effect. Our investigations are restricted to the i = 2 plateau which is used in all metrological measurements of the von Klitzing constant R K . A particular care has been taken concerning the configuration of the measurement. Four configurations were used for each Hall bar by reversing the current and the magnetic field polarities. Several samples with different width have been studied and we observed that the critical current for the contact resistance increases with the width of the Hall bar as previously observed for the critical current of the longitudinal resistance. The critical currents exhibit either a linear or a sublinear increase. All our observations are interpreted in the current understanding of the Quantum hall effect brekdown. Our analysis suggests that a heated region appears at the current contact, develops and then extends in the whole sample while increasing the current. Consequently, we propose to use the contact resistance as an electronic thermometer for the Hall fluid.
We have investigated experimentally and theoretically the effect of repulsive and attractive ionized impurities on the resistivity components ͑ xx and xy ͒ in the quantum Hall effect regime. GaAs/GaAlAs asymmetric modulation-doped quantum wells with additional delta doping ͑by Si donor atoms or Be acceptor atoms͒ in the GaAs channel or at the AlGaAs/GaAs interface has been grown using molecular beam epitaxy technique. Magnetotransport experiments, performed on samples doped with Si-attractive atoms, showed a plateau width increasing toward lower magnetic field at even filling factor. However, when samples were delta doped with Be repulsive atoms, the increase was observed in the opposite side. Part of the results was explained using a model based on the fifth Klauder's approximations where we demonstrate that the asymmetrical increase of the Hall plateaus with even filling factor ͑Landau gaps͒ is related to the asymmetry induced in the density of states by the additional impurities: the resulting disorder short range potential broadens and shifts the Landau levels but also creates impurity bands on the lower energy side of the Landau levels in the case of donors and on the upper energy side of the Landau levels in the case of acceptors. We notice that this asymmetrical behavior was not experimentally observed for odd filling factor plateaus ͑exchange gaps͒. We have also experimentally underscored the screening effect by free two-dimensional electrons of this disorder short range potential. Moreover, for delta-doped Be samples, the whole = 1 Hall plateau was shifted toward higher magnetic field with respect to the classical Hall effect. This shift, observed for all samples, cannot be explained by the asymmetry of the density of states but rather by a magnetic delocalization of electrons from the upper energy impurity band associated with the last Landau level ͑n =0͒ into the free n = 0 Landau states when this impurity band overtakes the Fermi level at the end of the = 2 plateau. This magnetic delocalization effect is the opposite effect of the magnetic freeze out.
Nonlinear dependence of the magnetophotoluminescence ͑MPL͒ energies in modulation-doped asymmetric GaAs/ Ga 0.67 Al 0.33 As quantum wells of different widths are investigated experimentally and theoretically as functions of an external magnetic field. The investigated structures have only one electric subband populated with electrons. Contrary to the theoretical descriptions existing in the literature and based on the oscillations of screening, the observed maxima of MPL energies do not occur at integer filling factors and do not change into minima for higher well widths. We interpret our observations assuming that the oscillations of MPL energies are due to an oscillatory electron transfer between a GaAs well and a reservoir outside the well. We obtain a very good description of the experimental data concerning both the maxima positions and the oscillation amplitudes for different well widths and electron densities. Our interpretation is corroborated by the quantum Hall data obtained on the same samples.
We calculate the density of states of a two-dimensional electron gas located at the interface of a strongly disordered GaAlAs/ GaAs heterojunction. The disorder potential is created by two delta doped layers. The first layer includes the parent donors which provides the well with electrons and creates a smooth disorder potential. The second layer is doped with either acceptor or donor impurities, and is located inside the well, thus creating a strongly disordered potential. We calculate the density of states in the presence of a magnetic field of arbitrary strength by taking into account all perturbative terms in the fifth Klauder's approximation. We find an anharmonic spectrum, strongly asymmetric, for the Landau level density of states. At low field, the attractive potential creates the well-known band tails or impurity bands. At higher field, we show that impurity bands are also created by repulsive potentials. We discuss the consequences of the anharmonicity and asymmetry on physical properties in the quantum Hall effect regime.
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