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

Coulomb scattering with remote continuum states in quantum dot devices

Electron capture and emission by Coulomb scattering in self-assembled quantum dot (QD) devices is studied theoretically. While the dependence of the Coulomb scattering (Auger) rates on the local wetting layer electron density has been a topic of intense research, we put special interest on the remote scattering between QD electrons and continuum electrons originating from a quantum well, doped bulk layers or metal contacts. Numerical effort is made to include all microscopic transitions between the Fermi distributed continuum states. The remote Coulomb scattering is investigated as a function of the electron density, the distance from the QDs and the temperature. Our results are compared with experimental observations, considering lifetime limitations in QD memory structures as well as the electron emission in pn-diodes

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

Nearly Supersymmetric Dark Atoms

Theories of dark matter that support bound states are an intriguing possibility for the identity of the missing mass of the Universe. This article proposes a class of models of supersymmetric composite dark matter where the interactions with the Standard Model communicate supersymmetry breaking to the dark sector. In these models supersymmetry breaking can be treated as a perturbation on the spectrum of bound states. Using a general formalism, the spectrum with leading supersymmetry effects is computed without specifying the details of the binding dynamics. The interactions of the composite states with the Standard Model are computed and several benchmark models are described. General features of non-relativistic supersymmetric bound states are emphasized.Comment: 39 pages, 2 figure

The Health Belief Model and preventive health behavior: an analysis of alternative models of causal relationships

Three areas of preventive health behavior using data from the National Survey of Preventive Health Practices and Consequences were investigated. First the latent constructs of preventive health behaviors were examined for fourteen preventive health behaviors. A confirmatory factor analysis using LISREL was conducted on three different hypothesized models, each representing a different way to conceptualize health behaviors. The model based on a common approach to grouping health behavior resulted in a model that was empirically unsound suggesting a need to identify the mechanisms that facilitate the clustering of preventive health actions;The second research question investigated the Health Belief Model along with demographic variables was modifying factors in an attempt to account for varying degrees of involvement in preventive health behavior. Health behaviors of physical activity, preventive medical care, and risk avoidance behavior across three different adult age groups (20-35, 36-54, 55-64) were analyzed using structural equation modeling. Results indicated that vulnerability, health concern cues, perception of health status, and barriers to preventive health varied in their power to explain preventive health behavior in each of the three age groups. Preventive health behavior was influenced by the modifying factors, particularly education in young adults, income in older adults, and gender in all age groups;The third research question employed six different two-wave, two-variable models to investigate the crosslag, stability, and contemporaneous effect of health status and health habits. Results indicated that health habits and health status at time 1 were strong predictors of those behaviors one year later. The crosslag effect of health status at time 1 on health habits at time 2 was the strongest crosslag effect; the relationship was small in magnitude due to the strong stability effects. Implications for future research and panel data analyses are discussed

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 $GaAs$-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

Disentangling Dark Matter Dynamics with Directional Detection

Inelastic dark matter reconciles the DAMA anomaly with other null direct detection experiments and points to a non-minimal structure in the dark matter sector. In addition to the dominant inelastic interaction, dark matter scattering may have a subdominant elastic component. If these elastic interactions are suppressed at low momentum transfer, they will have similar nuclear recoil spectra to inelastic scattering events. While upcoming direct detection experiments will see strong signals from such models, they may not be able to unambiguously determine the presence of the subdominant elastic scattering from the recoil spectra alone. We show that directional detection experiments can separate elastic and inelastic scattering events and discover the underlying dynamics of dark matter models.Comment: 7 pages, 5 figures, references and figures update