3,048 research outputs found
Transport in semiconductor superlattices: from quantum kinetics to terahertz-photon detectors
Semiconductor superlattices are interesting for two distinct reasons: the
possibility to design their structure (band-width(s),doping, etc.) gives access
to a large parameter space where different physical phenomena can be explored.
Secondly, many important device applications have been proposed, and then
subsequently successfully fabricated. A number of theoretical approaches has
been used to describe their current-voltage characteristics, such as miniband
conduction, Wannier-Stark hopping, and sequential tunneling. The choice of a
transport model has often been dictated by pragmatic considerations without
paying much attention to the strict domains of validity of the chosen model. In
the first part of this paper we review recent efforts to map out these
boundaries, using a first-principles quantum transport theory, which
encompasses the standard models as special cases. In the second part, focusing
in the mini-band regime, we analyze a superlattice device as an element in an
electric circuit, and show that its performance as a THz-photon detector allows
significant optimization, with respect to geometric and parasitic effects, and
detection frequency. The key physical mechanism enhancing the responsivity is
the excitation of hybrid Bloch-plasma oscillations.Comment: 22 pages, 10 figures, uses lamuphys.sty (included); to appear in the
Proceedings of the XVI Sitges Conference, Statistical and Dynamical Aspects
of Mesoscopic Systems (Lecture Notes in Physics, Springer
Nonequilibrium Green's function theory for transport and gain properties of quantum cascade structures
The transport and gain properties of quantum cascade (QC) structures are
investigated using a nonequilibrium Green's function (NGF) theory which
includes quantum effects beyond a Boltzmann transport description. In the NGF
theory, we include interface roughness, impurity, and electron-phonon
scattering processes within a self-consistent Born approximation, and
electron-electron scattering in a mean-field approximation. With this theory we
obtain a description of the nonequilibrium stationary state of QC structures
under an applied bias, and hence we determine transport properties, such as the
current-voltage characteristic of these structures. We define two contributions
to the current, one contribution driven by the scattering-free part of the
Hamiltonian, and the other driven by the scattering Hamiltonian. We find that
the dominant part of the current in these structures, in contrast to simple
superlattice structures, is governed mainly by the scattering Hamiltonian. In
addition, by considering the linear response of the stationary state of the
structure to an applied optical field, we determine the linear susceptibility,
and hence the gain or absorption spectra of the structure. A comparison of the
spectra obtained from the more rigorous NGF theory with simpler models shows
that the spectra tend to be offset to higher values in the simpler theories.Comment: 44 pages, 16 figures, appearing in Physical Review B Dec 200
Microscopic modelling of perpendicular electronic transport in doped multiple quantum wells
We present a microscopic calculation of transport in strongly doped
superlattices where domain formation is likely to occur. Our theoretical method
is based on a current formula involving the spectral functions of the system,
and thus allows, in principle, a systematic investigation of various
interaction mechanisms. Taking into account impurity scattering and optical
phonons we obtain a good quantitative agreement with existing experimental data
from Helgesen and Finstad (J. Appl. Phys. 69, 2689, (1991)). Furthermore the
calculated spectral functions indicate a significant increase of the average
intersubband spacing compared to the bare level differences which might explain
the experimental trend.Comment: 10 pages 5 figure
Gain in quantum cascade lasers and superlattices: A quantum transport theory
Gain in current-driven semiconductor heterostructure devices is calculated
within the theory of nonequilibrium Green functions. In order to treat the
nonequilibrium distribution self-consistently the full two-time structure of
the theory is employed without relying on any sort of Kadanoff-Baym Ansatz. The
results are independent of the choice of the electromagnetic field if the
variation of the self-energy is taken into account. Excellent quantitative
agreement is obtained with the experimental gain spectrum of a quantum cascade
laser. Calculations for semiconductor superlattices show that the simple 2-time
miniband transport model gives reliable results for large miniband widths at
room temperatureComment: 8 Pages, 4 Figures directly included, to appear in Physical Review
High Multiplicity Searches at the LHC Using Jet Masses
This article introduces a new class of searches for physics beyond the
Standard Model that improves the sensitivity to signals with high jet
multiplicity. The proposed searches gain access to high multiplicity signals by
reclustering events into large-radius, or "fat," jets and by requiring that
each event has multiple massive jets. This technique is applied to
supersymmetric scenarios in which gluinos are pair-produced and then
subsequently decay to final states with either moderate quantities of missing
energy or final states without missing energy. In each of these scenarios, the
use of jet mass improves the estimated reach in gluino mass by 20 % to 50 %
over current LHC searches.Comment: 9 pages, 6 figures; v3 corrects a few small typo
14C contamination testing in natural abundance laboratories: a new preparation method using wet chemical oxidation and some experiences
Substances enriched with radiocarbon can easily contaminate samples and laboratories used for natural abundance measurements. We have developed a new method using wet chemical oxidation for swabbing laboratories and equipment to test for 14C contamination. Here, we report the findings of 18 months’ work and more than 800 tests covering studies at multiple locations. Evidence of past and current use of enriched 14C was found at all but one location and a program of testing and communication was used to mitigate its effects. Remediation was attempted with mixed success and depended on the complexity and level of the contamination. We describe four cases from different situations
On apparent breaking the second law of thermodynamics in quantum transport studies
We consider a model for stationary electronic transport through a
one-dimensional chain of two leads attached to a perturbed central region
(quantum dot) in the regime where the theory proposed recently by Capek for a
similar model of phonon transport predicts the striking phenomenon of a
permanent current between the leads. This result based on a rigorous but
asymptotic Davies theory is at variance with the zero current yielded by direct
transport calculations which can be carried out in the present model. We find
the permanent current to be within the error of the asymptotic expansion for
finite couplings, and identify cancelling terms of the same order.Comment: 5 pages, 3 figure
Density-matrix theory of the optical dynamics and transport in quantum cascade structures: The role of coherence
The impact of coherence on the nonlinear optical response and stationary
transport is studied in quantum cascade laser structures. Nonequilibrium
effects such as pump-probe signals, the spatio-temporally resolved electron
density evolution, and the subband population dynamics (Rabi flopping) as well
as the stationary current characteristics are investigated within a microscopic
density-matrix approach. Focusing on the stationary current and the recently
observed gain oscillations, it is found that the inclusion of coherence leads
to observable coherent effects in opposite parameter regimes regarding the
relation between the level broadening and the tunnel coupling across the main
injection barrier. This shows that coherence plays a complementary role in
stationary transport and nonlinear optical dynamics in the sense that it leads
to measurable effects in opposite regimes. For this reason, a fully coherent
consideration of such nonequilibrium structures is necessary to describe the
combined optical and transport propertiesComment: 14 pages, 11 figures; final versio
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