968 research outputs found
Quantum transport: The link between standard approaches in superlattices
Theories describing electrical transport in semiconductor superlattices can
essentially be divided in three disjoint categories: i) transport in a
miniband; ii) hopping between Wannier-Stark ladders; and iii) sequential
tunneling. We present a quantum transport model, based on nonequilibrium Green
functions, which, in the appropriate limits, reproduces the three conventional
theories, and describes the transport in the previously unaccessible region of
the parameter space.Comment: 4 Page
Randomization algorithms for large sparse networks
In many domains it is necessary to generate surrogate networks, e.g., for hypothesis testing of different properties of a network. Generating surrogate networks typically requires that different properties of the network are preserved, e.g., edges may not be added or deleted and edge weights may be restricted to certain intervals. In this paper we present an efficient property-preserving Markov chain Monte Carlo method termed CycleSampler for generating surrogate networks in which (1) edge weights are constrained to intervals and vertex strengths are preserved exactly, and (2) edge and vertex strengths are both constrained to intervals. These two types of constraints cover a wide variety of practical use cases. The method is applicable to both undirected and directed graphs. We empirically demonstrate the efficiency of the CycleSampler method on real-world data sets. We provide an implementation of CycleSampler in R, with parts implemented in C.Peer reviewe
Integral Representations of Solutions in Isotropic Planar Elastostatics
In the field of plane elasticity, complexities arising from geometric
properties and applied forces often present substantial challenges to existing
methods. Some specific situations where these limitations emerge are when
considering problems with a non-vanishing traction, non-zero forces of specific
forms, and ring-shaped domains. This paper addresses these issues by developing
an approach that leverages established knowledge of the Neumann problem for the
inhomogeneous Cauchy-Riemann problem. We propose an integral representation
method for solutions applicable to domains that can be conformally mapped from
a unit disk or an annulus. We demonstrate the practicality and applicability of
this method through specific examples, including a notch problem in a cardioid
domain, a ring domain with shifted hole, and gear-like domain. We hope the
techniques presented in this work will find themselves useful for people
working on isotropic planar elastostatics problems
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
Sequential tunneling in doped superlattices: Fingerprints of impurity bands and photon-assisted tunneling
We report a combined theoretical and experimental study of electrical transport in weakly coupled doped superlattices. Our calculations exhibit negative differential conductivity at sufficiently high electric fields for all dopings. In low-doped samples the presence of impurity bands modifies the current-voltage characteristics substantially, and we find two different current peaks whose relative height changes with the electron temperature. These findings can explain the observation of different peaks in the current-voltage characteristics with and without external THz irradiation in low-doped samples. From our microscopic transport model we obtain quantitative agreement with the experimental current-voltage characteristics without using any fitting parameters. Both our experimental data and our theory show that absolute negative conductance persists over a wide range of frequencies of the free-electron laser source
Field-dependent exciton dissociation in organic heterojunction solar cells
In organic heterojunction solar cells, the generation of free charge carriers takes place in a multistep process which involves charge transfer (CT) states, that is, bound electron-hole pairs at the interface between donor and acceptor molecules. Past efforts to model the CT-state dissociation during solar cell operation were not able to consistently reproduce the experimentally observed field and temperature dependence. This discrepancy between model and experiment was partly due to the field-dependent free charge carrier collection process, which plays an important role in the widely used bulk heterojunction cell configuration and superimposes a possible field-dependent charge carrier generation process. In order to distinguish between generation and collection of free charge carriers, we propose the planar heterojunction cell configuration as a model system to study the field-dependent charge carrier generation process in organic heterojunction solar cells. We apply this model system to check current CT-state dissociation models against experimental data. Although the models can quantitatively account for the photocurrent's dependence on the applied voltage and the device thickness, they fail to account for the virtually negligible temperature dependence of the field-dependent charge-generation process. This discrepancy is traced back to a common feature of the models: an Arrhenius-like temperature dependence, distinctive of all processes involving a thermally activated jump over an energy barrier. As a solution to the problem, we introduce an exciton dissociation model based on a field-dependent tunnel process and demonstrate its consistency with the experimental observations. Our results indicate that the current microscopic picture of the charge-generation process in organic heterojunction solar cells being limited by the CT-state dissociation process needs to be reconsidered
Electronic transport in graphene-based structures: an effective cross section approach
We show that transport in low-dimensional carbon structures with finite
concentrations of scatterers can be modeled by utilising scaling theory and
effective cross sections. Our reults are based on large scale numerical
simulations of carbon nanotubes and graphene nanoribbons, using a tightbinding
model with parameters obtained from first principles electronic structure
calculations. As shown by a comprehensive statistical analysis, the scattering
cross sections can be used to estimate the conductance of a quasi-1D system
both in the Ohmic and localized regimes. They can be computed with good
accuracy from the transmission functions of single defects, greatly reducing
the computational cost and paving the way towards using first principles
methods to evaluate the conductance of mesoscopic systems, consisting of
millions of atoms.Comment: Submitted to Physical Review Letter
MASQOT: a method for cDNA microarray spot quality control
BACKGROUND: cDNA microarray technology has emerged as a major player in the parallel detection of biomolecules, but still suffers from fundamental technical problems. Identifying and removing unreliable data is crucial to prevent the risk of receiving illusive analysis results. Visual assessment of spot quality is still a common procedure, despite the time-consuming work of manually inspecting spots in the range of hundreds of thousands or more. RESULTS: A novel methodology for cDNA microarray spot quality control is outlined. Multivariate discriminant analysis was used to assess spot quality based on existing and novel descriptors. The presented methodology displays high reproducibility and was found superior in identifying unreliable data compared to other evaluated methodologies. CONCLUSION: The proposed methodology for cDNA microarray spot quality control generates non-discrete values of spot quality which can be utilized as weights in subsequent analysis procedures as well as to discard spots of undesired quality using the suggested threshold values. The MASQOT approach provides a consistent assessment of spot quality and can be considered an alternative to the labor-intensive manual quality assessment process
Are Infants and Toddlers with Moderate-to-severe Atopic Dermatitis Undertreated? Experiences of a Finnish Tertiary Care Hospital
Peer reviewe
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