154 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
Anharmonicity and asymmetry of Landau levels for a two-dimensional electron gas
We calculate the density of states of a two dimensional electron gas located
at the interface of a GaAlAs/GaAs heterojunction. The disorder potential which
is generally created by a single doping layer behind a spacer, is here enhanced
by the presence of a second delta doped layer of scatterers which can be
repulsive or attractive impurities. We have calculated the density of states by
means of the Klauder's approximation, in the presence of a magnetic field of
arbitrary strength. At low field either band tails or impurity bands are
observed for attractive potentials, depending on the impurity concentration. At
higher field, impurity bands are observed for both repulsive and attractive
potentials. We discuss the effect of such an asymmetrical density of states on
the transport properties in the quantum Hall effect regime.Comment: 22 pages, 12 figures. submitted to Phys. Rev.
Plant-soil interactions during the native and exotic range expansion of an annual plant
We thank the greenhouse and technical staff at UC Santa Cruz and NIOO-KNAW for facilities and plant care, especially Jim Velzy and Sylvie Childress, and Renske Jongen and Freddy C. ten Hooven. We are grateful to Colby Cole, Asa Conover, Kelsey Songer, and Andrew Lopez for planting, harvesting, and root washing. Many thanks to Matthew Hartfield, Josie Borden, and Dante Park for their fieldwork assistance. We appreciate the permitting agencies that allowed us to collect seed and soil for this study: US Forest Service, Santa Clara County Parks, Midpeninsula Regional Open Space District, Don Edwards San Francisco Bay National Wildlife Refuge, and the Santa Clara Valley Water District. Seeds were collected following the Nagoya Protocol (French certificate of compliance TREL2302365S/653) and imported into the United States using APHIS permit P37-18-01389. This research was funded by the United States Department of Agriculture, National Institute of Food and Agriculture (Agriculture and Food Research Initiative Grant 2020-67013-31856 to I.M.P). N.L. acknowledges support from the Swiss National Science Foundation (Early.Postdoc mobility fellowship P2EZP3_178481), Natural Environment Research Council (Standard Grant NE/W006553/1), and the UKRI Horizon Europe Guarantee Research Scheme (Marie-Sklodowska-Curie European Fellowship EP/X023362/1). T.M.R.C. was funded by ENS de LyonPeer reviewe
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
Application of the TRL calibration technique for HEMT's microwave characterization at temperatures down to 77 K.
For the possible application to the cryogenically cooled low noise HEMT amplifier, this paper presents scattering parameter measurements of a HEMT chip at room and liquid nitrogen temperatures and in the 1-20 GHz frequency range. The measurement method which uses the well known TRL calibration technique will be fully discussed. S parameters and noise parameters measured at room temperature by means of this technique have been used to design a single stage low noise amplifier for the K band. Without any tuning this amplifier has an overall noise figure value of 1.6 dB with a 7 dB associated gain at 18 GHz which is in good agreements with predictions
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