110 research outputs found
Conduction electrons localized by charged magneto-acceptors A in GaAs/GaAlAs quantum wells
A variational theory is presented of A and A centers, i.e. of a
negative acceptor ion localizing one and two conduction electrons,
respectively, in a GaAs/GaAlAs quantum well in the presence of a magnetic field
parallel to the growth direction. A combined effect of the well and magnetic
field confines conduction electrons to the proximity of the ion, resulting in
discrete repulsive energies above the corresponding Landau levels. The theory
is motivated by our experimental magneto-transport results which indicate that,
in a heterostructure doped in the GaAs well with Be acceptors, one observes a
boil-off effect in which the conduction electrons in the crossed-field
configuration are pushed by the Hall electric field from the delocalized Landau
states to the localized acceptor states and cease to conduct. A detailed
analysis of the transport data shows that, at high magnetic fields, there are
almost no conducting electrons left in the sample. It is concluded that one
negative acceptor ion localizes up to four conduction electrons.Comment: 8 pages, 5 figure
Heating process in the pre-Breakdown regime of the Quantum Hall Efect : a size dependent effect
Our study presents experimental measurements of the contact and longitudinal
voltage drops in Hall bars, as a function of the current amplitude. We are
interested in the heating phenomenon which takes place before the breakdown of
the quantum Hall effect, i.e. the pre-breakdown regime. Two types of samples
has been investigated, at low temperature (4.2 and 1.5K) and high magnetic
field (up to 13 T). The Hall bars have several different widths, and our
observations clearly demonstrate that the size of the sample influences the
heating phenomenon. By measuring the critical currents of both contact and
longitudinal voltages, as a function of the filling factor (around ), we
highlight the presence of a high electric field domain near the source contact,
which is observable only in samples whose width is smaller than 400 microns.Comment: 4 pages, 5 igures, 7th International Symposium of Research in High
Magnetic Fields, to be published in physica
Zener Tunneling Between Landau Orbits in a High-Mobility Two-Dimensional Electron Gas
Magnetotransport in a laterally confined two-dimensional electron gas (2DEG)
can exhibit modified scattering channels owing to a tilted Hall potential.
Transitions of electrons between Landau levels with shifted guiding centers can
be accomplished through a Zener tunneling mechanism, and make a significant
contribution to the magnetoresistance. A remarkable oscillation effect in weak
field magnetoresistance has been observed in high-mobility 2DEGs in
GaAs-AlGaAs heterostructures, and can be well explained by the
Zener mechanism.Comment: 5 pages, 4 figures. Text slightly shortened, figures resize
Electric-Field Breakdown of Absolute Negative Conductivity and Supersonic Streams in Two-Dimensional Electron Systems with Zero Resistance/Conductance States
We calculate the current-voltage characteristic of a two-dimensional electron
system (2DES) subjected to a magnetic field at strong electric fields. The
interaction of electrons with piezoelectric acoustic phonons is considered as a
major scattering mechanism governing the current-voltage characteristic. It is
shown that at a sufficiently strong electric field corresponding to the Hall
drift velocity exceeding the velocity of sound, the dissipative current
exhibits an overshoot. The overshoot of the dissipative current can result in a
breakdown of the absolute negative conductivity caused by microwave irradiation
and, therefore, substantially effect the formation of the domain structures
with the zero-resistance and zero-conductance states and supersonic electron
streams.Comment: 5 pages, 4 figure
Electronic Processes at the Breakdown of the Quantum Hall Effect
Microscopic processes giving the energy gain and loss of a two-dimensional
electron system in long-range potential fluctuations are studied theoretically
at the breakdown of the quantum Hall effect in the case of even-integer filling
factors. The Coulomb scattering within a broadened Landau level is proposed to
give the gain, while the phonon scattering to give the loss. The energy balance
equation shows that the electron temperature T_e and the diagonal conductivity
sigma_{xx} exhibit a bistability above the lower critical electric field
E_{c1}. Calculated values of E_{c1} as well as T_e and sigma_{xx} at E_{c1} are
in agreement with the observed values in their orders of magnitude.Comment: 4 pages, 2 Postscript figures, submitted to the Journal of the
Physical Society of Japa
Nonlinear effects in microwave photoconductivity of two-dimensional electron systems
We present a model for microwave photoconductivity of two-dimensional
electron systems in a magnetic field which describes the effects of strong
microwave and steady-state electric fields. Using this model, we derive an
analytical formula for the photoconductivity associated with photon- and
multi-photon-assisted impurity scattering as a function of the frequency and
power of microwave radiation. According to the developed model, the microwave
conductivity is an oscillatory function of the frequency of microwave radiation
and the cyclotron frequency which turns zero at the cyclotron resonance and its
harmonics. It exhibits maxima and minima (with absolute negative conductivity)
at the microwave frequencies somewhat different from the resonant frequencies.
The calculated power dependence of the amplitude of the microwave
photoconductivity oscillations exhibits pronounced sublinear behavior similar
to a logarithmic function. The height of the microwave photoconductivity maxima
and the depth of its minima are nonmonotonic functions of the electric field.
It is pointed to the possibility of a strong widening of the maxima and minima
due to a strong sensitivity of their parameters on the electric field and the
presence of strong long-range electric-field fluctuations. The obtained
dependences are consistent with the results of the experimental observations.Comment: 9 pages, 6 figures Labeling of the curves in Fig.3 correcte
Longitudinal impedance of a two-dimensional electron in the quantum Hall effect regime
International audienceWe have measured the longitudinal impedance of a two dimensional electron gas in the quantum Hall effect regime. We have found smooth resonances of impedance for the middle of quantized plateaus and large resonances at the beginning and the end of the plateaus. Those resonances can be partially described using a simplified electrical circuit model. The analysis of the different experimental results shows a great influence of the experimental setup on the experimental measurements
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