1,961 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
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
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
Elastic constants of nematic liquid crystals of uniaxial symmetry
We study in detail the influence of molecular interactions on the Frank
elastic constants of uniaxial nematic liquid crystals composed of molecules of
cylindrical symmetry. A brief summary of the status of theoretical development
for the elastic constants of nematics is presented. Considering a pair
potential having both repulsive and attractive parts numerical calculations are
reported for three systems MBBA, PAA and 8OCB. For these systems the
length-to-width ratio is estimated from the experimentally proposed
structure of the molecules. The repulsive interaction is represented by a
repulsion between hard ellipsoids of revolution (HER) and the attractive
potential is represented by the quadrupole and dispersion interactions. From
the numerical results we observe that in the density range of nematics the
contribution of the quadrupole and dispersion interactions are small as
compared to the repulsive HER interaction. The inclusion of attractive
interaction reduces the values of elastic constants ratios. The temperature
variation of elastic constants ratios are reported and compared with the
experimental values. A reasonably good agreement between theory and experiment
is observed
Complete nucleotide sequences and genome organization of a cherry isolate of cherry leaf roll virus
The complete nucleotide sequence of cherry leaf roll virus (CLRV, genus Nepovirus) from a naturally infected cherry tree (Prunus avium cv. Bing) in North America was determined. RNA1 and RNA2 consist of 7,893 and 6,492 nucleotides, respectively, plus a poly-(A) tail. Each RNA encodes a single potential open reading frame. The first 657 nucleotides of RNA1 and RNA2 are 99% identical and include the 5′-UTR and the first 214 deduced amino acids of the polyproteins following the first of two in-frame start codons. Phylogenetic analysis reveals close relationships between CLRV and members of subgroup C of the genus Nepovirus
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
Lowering the energy threshold in COSINE-100 dark matter searches
COSINE-100 is a dark matter detection experiment that uses NaI(Tl) crystal
detectors operating at the Yangyang underground laboratory in Korea since
September 2016. Its main goal is to test the annual modulation observed by the
DAMA/LIBRA experiment with the same target medium. Recently DAMA/LIBRA has
released data with an energy threshold lowered to 1 keV, and the persistent
annual modulation behavior is still observed at 9.5. By lowering the
energy threshold for electron recoils to 1 keV, COSINE-100 annual modulation
results can be compared to those of DAMA/LIBRA in a model-independent way.
Additionally, the event selection methods provide an access to a few to sub-GeV
dark matter particles using constant rate studies. In this article, we discuss
the COSINE-100 event selection algorithm, its validation, and efficiencies near
the threshold
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