Quantum mechanical scattering investigation of the thermionic and field induced emission components of the dark current in quantum well infrared photodetectors

The thermionic emission and field induced emission components of the dark current in quantum well infrared photodetectors are investigated using a quantum mechanical scattering theory approach. Calculations are performed for an experimentally reported device. Using this as a standard, the device dimensions were altered in order to increase its detection wavelength to cover the mid- (MIR) and far-infrared (FIR) regions of the spectrum. The behavior of the scattering mechanisms that contribute to the thermionic emission and field induced emission components were studied. The results highlight the change in the dominating scattering mediator across the MIR and FIR bands. © 2002 American Institute of Physics

Wrinkling in engineering fabrics: a comparison between two different comprehensive modelling approaches

We consider two ‘comprehensive’ modelling approaches for engineering fabrics. We distinguish the two approaches using the terms ‘semi-discrete’ and ‘continuum’, reflecting their natures. We demonstrate a fitting procedure, used to identify the constitutive parameters of the continuum model from predictions of the semi-discrete model, the parameters of which are in turn fitted to experimental data. We, then, check the effectiveness of the continuum model by verifying the correspondence between semi-discrete and continuum model predictions using test cases not previously used in the identification process. Predictions of both modelling approaches are compared against full-field experimental kinematic data, obtained using stereoscopic digital image correlation techniques, and also with measured force data. Being a reduced order model and being implemented in an implicit rather than an explicit finite-element code, the continuum model requires significantly less computational power than the semi-discrete model and could therefore be used to more efficiently explore the mechanical response of engineering fabrics

Dynamic Consistency in Denmark: A Longitudinal Field Experiment

Evidence that individuals have dynamically consistent preferences is usually generated by studying the discount rates of the individual over different horizons, but where those rates are elicited at a single point in time. If these elicited discount rates vary by horizon the individual is typically claimed to have preferences that imply a dynamic inconsistency, although this inference requires additional assumptions such as intertemporal separability. However, what one really wants to know is if the same subject has the same discount rate function when that individual is asked at a later point in time. Such panel tests then require than one allow for possible changes in the states of nature that the subject faces, since they may confound any in-sample comparisons of discount rate functions at different points in time. We report the results of a large-scale panel experiment undertaken in the field that allows us to examine this issue. In June 2003 we elicited subjective discount rates from 253 subjects, representative of the adult Danish population. Between September 2003 and November 2004 we re-visited 97 of these subjects and repeated these tasks. In each visit we also elicited information on their individual characteristics, as well as their expectations about the state of their own economic situation and macroeconomic variables. We find evidence in favor of dynamic consistency.

Risk Attitudes, Randomization to Treatment, and Self-Selection Into Experiments

Randomization to treatment is fundamental to statistical control in the design of experiments. But randomization implies some uncertainty about treatment condition, and individuals differ in their preferences towards taking on risk. Since human subjects often volunteer for experiments, or are allowed to drop out of the experiment at any time if they want to, it is possible that the sample observed in an experiment might be biased because of the risk of randomization. On the other hand, the widespread use of a guaranteed show-up fee that is non-stochastic may generate sample selection biases of the opposite direction, encouraging more risk averse samples into experiments. We undertake a field experiment to directly test these hypotheses that risk attitudes play a role in sample selection. We follow standard procedures in the social sciences to recruit subjects to an experiment in which we measure their attitudes to risk. We exploit the fact that we know certain characteristics of the population sampled, adults in Denmark, allowing a statistical correction for sample selection bias using standard methods. We also utilize the fact that we have a complex sampling design to provide better estimates of the target population. Our results suggest that randomization bias is not a major empirical problem for field experiments of the kind we conducted if the objective is to identify marginal effects of sample characteristics. However, there is evidence that the use of show-up fees may have generated a sample that was more risk averse than would otherwise have been observed.

Estimating Risk Attitudes in Denmark

We estimate individual risk attitudes using controlled experiments in the field in Denmark. These risk preferences are elicited by means of field experiments involving real monetary rewards. The experiments were carried out across Denmark using a representative sample of 253 people between 19 and 75 years of age. Risk attitudes are estimated for various individuals differentiated by socio-demographic characteristics such as income and age. Our results indicate that the average Dane is risk averse, and that risk neutrality is an inappropriate assumption to apply. We also find that risk attitudes do vary significantly with respect to several important socio-demographic variables. These conclusions are robust to the use of relatively flexible specifications of risk preferences. When individual characteristics of the sample are ignored, relative risk aversion appears not to be constant over the domain of income considered here, and rises rapidly as income increases above "small" amounts. However, relative risk aversion appears to be constant when one corrects for individual heterogeneity, although there is considerable uncertainty in the characterization of risk attitudes for low stakesRisk preferences, field experiments, heterogeneity

Eliciting Risk and Time Preferences Using Field Experiments: Some Methodological Issues

We design experiments to jointly elicit risk and time preferences for the adult Danish population. The experimental procedures build on laboratory experiments that have been evaluated using traditional subject pools. The field experiments utilize field sampling designs that we developed, and procedures that were chosen to be relatively transparent in the field with non-standard subject pools. Our overall design was also intended to be a general template for such field experiments in other countries. We examine the characterization of risk over a wider domain for each subject than previous experiments, allowing more precise estimates of risk attitudes. We also examine individual discount rates over six time horizons, as the first stage in a panel experiment in which we revisit subjects to test consistency and stability of responses over time. Risk and time preferences are heterogeneous, varying by observable individual characteristics. On a methodological level, we implement a refinement of existing procedures which elicits much more precise estimates, and also mitigates framing effects.

A Cosmic Battery

We show that the Poynting-Robertson drag effect in an optically thin advection-dominated accretion flow around active gravitating objects generates strong azimuthal electric currents which give rise to astrophysically significant magnetic fields. Although the mechanism is most effective in accreting compact objects, it seems very promising to also account for the generation of stellar dipolar fields during the late protostellar collapse phase, when the star approaches the main sequence.Comment: 12 pages Latex, 1 postscript figure, to appear in the Astrophysical Journa

Observation of lobes near the X-point in resonant magnetic perturbation experiments on MAST

The application of non-axisymmetric resonant magnetic perturbations (RMPs) with a toroidal mode number n=6 in the MAST tokamak produces a significant reduction in plasma energy loss associated with type-I Edge Localized Modes (ELMs), the first such observation with n>3. During the ELM mitigated stage clear lobe structures are observed in visible-light imaging of the X-point region. These lobes or manifold structures, that were predicted previously, have been observed for the first time in a range of discharges and their appearance is correlated with the effect of RMPs on the plasma i.e. they only appear above a threshold when a density pump out is observed or when the ELM frequency is increased. They appear to be correlated with the RMPs penetrating the plasma and may be important in explaining why the ELM frequency increases. The number and location of the structures observed can be well described using vacuum modelling. Differences in radial extent and poloidal width from vacuum modelling are likely to be due to a combination of transport effects and plasma screening.Comment: 15 pages, 5 figure