2,199 research outputs found
Inelastic quantum transport in superlattices: success and failure of the Boltzmann equation
Electrical transport in semiconductor superlattices is studied within a fully
self-consistent quantum transport model based on nonequilibrium Green
functions, including phonon and impurity scattering. We compute both the drift
velocity-field relation and the momentum distribution function covering the
whole field range from linear response to negative differential conductivity.
The quantum results are compared with the respective results obtained from a
Monte Carlo solution of the Boltzmann equation. Our analysis thus sets the
limits of validity for the semiclassical theory in a nonlinear transport
situation in the presence of inelastic scattering.Comment: final version with minor changes, to appear in Physical Review
Letters, sceduled tentatively for July, 26 (1999
10 MVA thyristor converters for the active damping of generator shaft oscillations at ASDEX Upgrade
Modeling the instantaneous response of glaciers after the collapse of the Larsen B Ice Shelf
Following the disintegration of the Larsen B Ice Shelf, Antarctic Peninsula, in 2002, regular surveillance of its ∼20 tributary glaciers has revealed a response which is varied and complex in both space and time. The major outlets have accelerated and thinned, smaller glaciers have shown little or no change, and glaciers flowing into the remnant Scar Inlet Ice Shelf have responded with delay. In this study we present the first areawide numerical analysis of glacier dynamics before and immediately after the collapse of the ice shelf, combining new data sets and a state‐of‐the‐art numerical ice flow model. We simulate the loss of buttressing at the grounding line and find a good qualitative agreement between modeled changes in glacier flow and observations. Through this study, we seek to improve confidence in our numerical models and their ability to capture the complex mechanical coupling between floating ice shelves and grounded ice
Effects of impurity scattering on electron-phonon resonances in semiconductor superlattice high-field transport
A non-equilibrium Green's function method is applied to model high-field
quantum transport and electron-phonon resonances in semiconductor
superlattices. The field-dependent density of states for elastic (impurity)
scattering is found non-perturbatively in an approach which can be applied to
both high and low electric fields. I-V curves, and specifically electron-phonon
resonances, are calculated by treating the inelastic (LO phonon) scattering
perturbatively. Calculations show how strong impurity scattering suppresses the
electron-phonon resonance peaks in I-V curves, and their detailed sensitivity
to the size, strength and concentration of impurities.Comment: 7 figures, 1 tabl
Conceptual design of the power supply system for the in-vessel saddle coils for MHD control in ASDEX Upgrade
Biased quasi ballistic spin torque magnetization reversal
We explore the fundamental time limit of ultra fast spin torque induced
magnetization reversal of a magnetic memory cell. Spin torque precession during
a spin torque current pulse and free precessional magnetization ringing after
spin torque pulse excitation is detected by time resolved magneto transport.
Adapting the duration of the spin torque excitation pulse to the spin torque
precession period allows suppression of the magnetization ringing and thus
coherent control of the final orientation of the magnetization. In the presence
of a hard axis bias field such coherent control enables an optimum ultra fast,
quasi ballistic spin torque magnetization reversal by a single precessional
turn directly from the initial to the reversed equilibrium state.Comment: 13 pages 3 Figure
A novel way of constraining WIMPs annihilations in the Sun: MeV neutrinos
Annihilation of dark matter particles accumulated in the Sun would produce a
flux of high-energy neutrinos whose prospects of detection in neutrino
telescopes and detectors have been extensively discussed in the literature.
However, for annihilations into Standard Model particles, there would also be a
flux of neutrinos in the MeV range from the decays at rest of muons and
positively charged pions. These low-energy neutrinos have never been considered
before and they open the possibility to also constrain dark matter annihilation
in the Sun into e+e-, mu+mu- or light quarks. Here we perform a detailed
analysis using the recent Super-Kamiokande data in the few tens of MeV range to
set limits on the WIMP-nucleon scattering cross section for different
annihilation channels and computing the evaporation rate of WIMPs from the Sun
for all values of the scattering cross section in a consistent way.Comment: 26 pages, 7 figures. Substantial additions with further explanations.
Two significant improvements: in the calculation of the evaporation and
capture rates; and in the analysis of the SK data. Minor changes in the
conclusions. It matches published versio
Zener transitions between dissipative Bloch bands. II: Current Response at Finite Temperature
We extend, to include the effects of finite temperature, our earlier study of
the interband dynamics of electrons with Markoffian dephasing under the
influence of uniform static electric fields. We use a simple two-band
tight-binding model and study the electric current response as a function of
field strength and the model parameters. In addition to the Esaki-Tsu peak,
near where the Bloch frequency equals the damping rate, we find current peaks
near the Zener resonances, at equally spaced values of the inverse electric
field. These become more prominenent and numerous with increasing bandwidth (in
units of the temperature, with other parameters fixed). As expected, they
broaden with increasing damping (dephasing).Comment: 5 pages, LateX, plus 5 postscript figure
Intersubband gain in a Bloch oscillator and Quantum cascade laser
The link between the inversion gain of quantum cascade structures and the
Bloch gain in periodic superlattices is presented. The proposed theoretical
model based on the density matrix formalism is able to treat the gain mechanism
of the Bloch oscillator and Quantum cascade laser on the same footing by taking
into account in-plane momentum relaxation. The model predicts a dispersive
contribution in addition to the (usual) population-inversion-dependent
intersubband gain in quantum cascade structures and - in the absence of
inversion - provides the quantum mechanical description for the dispersive gain
in superlattices. It corroborates the predictions of the semi-classical
miniband picture, according to which gain is predicted for photon energies
lower than the Bloch oscillation frequency, whereas net absorption is expected
at higher photon energies, as a description which is valid in the
high-temperature limit. A red-shift of the amplified emission with respect to
the resonant transition energy results from the dispersive gain contribution in
any intersubband transition, for which the population inversion is small.Comment: 10 pages, 6 figure
Wind vane correction during yaw misalignment for horizontal-axis wind turbines
This paper investigates the accuracy of wind direction measurements for horizontal-axis wind turbines and their impact on yaw control. The yaw controller is crucial for aligning the rotor with the wind direction and optimizing energy extraction. Wind direction is conventionally measured by one or two wind vanes located on the nacelle, but the proximity of the rotor can interfere with these measurements. The authors show that the conventional corrections, including low-pass filters and calibrated offset correction, are inadequate to correct a systematic overestimation of the wind direction deviation caused by the rotor misalignment. This measurement error can lead to an overcorrection of the yaw controller and, thus, to an oscillating yaw behaviour, even if the wind direction is relatively steady. The authors present a theoretical basis and methods for quantifying the wind vane measurement error and validate their findings using computational fluid dynamics simulations and operational data from two commercial wind turbines. Additionally, the authors propose a correction function that improves the wind vane measurements and demonstrate its effectiveness in two free-field experiments. Overall, the paper provides new insights into the accuracy of wind direction measurements and proposes solutions to improve the yaw control for horizontal-axis wind turbines.</p
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