4,238 research outputs found
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
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
Simulation of Transport and Gain in Quantum Cascade Lasers
Quantum cascade lasers can be modeled within a hierarchy of different
approaches: Standard rate equations for the electron densities in the levels,
semiclassical Boltzmann equation for the microscopic distribution functions,
and quantum kinetics including the coherent evolution between the states. Here
we present a quantum transport approach based on nonequilibrium Green
functions. This allows for quantitative simulations of the transport and
optical gain of the device. The division of the current density in two terms
shows that semiclassical transitions are likely to dominate the transport for
the prototype device of Sirtori et al. but not for a recent THz-laser with only
a few layers per period. The many particle effects are extremely dependent on
the design of the heterostructure, and for the case considered here, inclusion
of electron-electron interaction at the Hartree Fock level, provides a sizable
change in absorption but imparts only a minor shift of the gain peak.Comment: 12 pages, 5 figures included, to appear in in "Advances in Solid
State Physics", ed. by B. Kramer (Springer 2003
High incidence of Angina pectoris in patients treated with 5-fluorouracil - A planned surveillance study with 102 patients
Objective: Angina pectoris, arrhythmic sudden death and myocardial infarction, all these cardiac events have occasionally been reported during 5-fluorouracil (5-FU) chemotherapy. Underlying mechanisms leading to these events are unknown; damage to the myocytes or vasospasms have been discussed. Methods: 102 consecutive and unselected patients were monitored with 12-lead ECG, echocardiography and radionuclide ventriculography prior to the first cycle of 5-FU chemotherapy and 3 months from baseline. Results: 19% of the patients developed reversible symptoms of angina pectoris during treatment which lasted up to 12 h after cessation of the infusion. Most of the 19 patients showed corresponding ECG changes. 6 out of the 19 patients with severe angina pectoris had subsequent coronary angiography. In none of these patients the coronary angiography showed coronary artery disease, but it showed low ventricular function (ejection fraction <50%) in 2 patients. The ejection fraction did not increase overtime. Arrhythmias were screened for with Holter monitoring during 5-FU chemotherapy. The frequency of bradycardia and ventricular extrasystoles increased significantly (p < 0.05) during treatment compared to arrhythmias in Holter monitoring 3 months later. Furthermore the Qtc time in the ECG 3 months later was significantly prolonged (p < 0.05) compared to baseline values. Conclusions:The incidence of angina pectoris in patients during 5-FU treatment seems higher than previously suspected. As myocardial ischemia can be fatal, attentiveness to these symptoms and immediate treatment are crucial. Copyright (C) 2003 S. Karger AG, Basel
Gain without inversion in a biased superlattice
Intersubband transitions in a superlattice under homogeneous electric field
is studied within the tight-binding approximation. Since the levels are
equi-populated, the non-zero response appears beyond the Born approximation.
Calculations are performed in the resonant approximation with scattering
processes exactly taken into account. The absorption coefficient is equal zero
for the resonant excitation while a negative absorption (gain without
inversion) takes place below the resonance. A detectable gain in the THz
spectral region is obtained for the low-doped -based superlattice and
spectral dependencies are analyzed taking into account the interplay between
homogeneous and inhomogeneous mechanisms of broadening.Comment: 6 pages, 4 figure
The Poker Face of Inelastic Dark Matter: Prospects at Upcoming Direct Detection Experiments
The XENON100 and CRESST experiments will directly test the inelastic dark
matter explanation for DAMA's 8.9? sigma anomaly. This article discusses how
predictions for direct detection experiments depend on uncertainties in
quenching factor measurements, the dark matter interaction with the Standard
Model and the halo velocity distribution. When these uncertainties are
accounted for, an order of magnitude variation is found in the number of
expected events at CRESST and XENON100.Comment: 5 pages, 3 figure
Tunneling through nanosystems: Combining broadening with many-particle states
We suggest a new approach for transport through finite systems based on the
Liouville equation. By working in a basis of many-particle states for the
finite system, Coulomb interactions are taken fully into account and correlated
transitions by up to two different contact states are included. This latter
extends standard rate equation models by including level-broadening effects.
The main result of the paper is a general expression for the elements of the
density matrix of the finite size system, which can be applied whenever the
eigenstates and the couplings to the leads are known. The approach works for
arbitrary bias and for temperatures above the Kondo temperature. We apply the
approach to standard models and good agreement with other methods in their
respective regime of validity is found.Comment: 9 pages, 5 figures included to tex
Poverty decompositions with counterfactual income and inequality dynamics
Traditional poverty accounting decomposes changes in a country's poverty headcount ratio into changes in income and inequality. We argue that this approach is unsatisfactory from the perspective of policy analysis because it compares a country in two points of time without taking the country's initial situation, and hence its potential for poverty reduction, into account. We thus suggest comparing traditional poverty decompositions with a counterfactual situation. This counterfactual indicates what a country starting from its initial situation could be expected to achieve in terms of income, inequality, and, hence, poverty developments. We construct those counterfactuals by modeling income and inequality trends characterized by convergence and a “Kuznets” relationship between inequality and development. Parameters in those relationships are estimated using PovcalNet survey data from 144 countries and we construct our counterfactual poverty predictions for 71 developing countries. While there is overall a tight relationship between actual developments and counterfactuals, we identify several cases, where both deviate from each other and discuss the policy implications. We also check for commonalities in differently performing countries and find that those who fell particularly short of expectations often underwent political transition and state fragility
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