Context learning in the rodent hippocampus

We present a Bayesian statistical theory of context learning in the rodent hippocampus. While context is often defined in an experimental setting in relation to specific background cues or task demands, we advance a single, more general notion of context that suffices for a variety of learning phenomena. Specifically, a context is defined as a statistically stationary distribution of experiences, and context learning is defined as the problem of how to form contexts out of groups of experiences that cluster together in time. The challenge of context learning is solving the model selection problem: How many contexts make up the rodent's world? Solving this problem requires balancing two opposing goals: minimize the variability of the distribution of experiences within a context and minimize the likelihood of transitioning between contexts. The theory provides an understanding of why hippocampal place cell remapping sometimes develops gradually over many days of experience and why even consistent landmark differences may need to be relearned after other environmental changes. The theory provides an explanation for progressive performance improvements in serial reversal learning, based on a clear dissociation between the incremental process of context learning and the relatively abrupt context selection process. The impact of partial reinforcement on reversal learning is also addressed. Finally, the theory explains why alternating sequence learning does not consistently result in unique context-dependent sequence representations in hippocampus.     

A novel route to a polycrystalline silicon thin-film solar cell

An alternative approach is described for the fabrication of a polycrystalline silicon thin-film solar cell on inexpensive substrates. In a first step amorphous silicon is recrystallized in an aluminum-induced crystallization process forming a large-grained polycrystalline silicon layer on glass or metal substrates. In a second step this layer is used as a template for epitaxial growth of the absorber layer (2–3 μm thick) at T<600 °C using ion-assisted deposition techniques. The third step consists of the formation of an a-Si:H/c-Si heterojunction by depositing an a-Si:H emitter from the gas phase. It will be shown that each of these steps has been successfully developed and can now be implemented in a solar cell process.     

A calculus for modular loop acceleration and non-termination proofs

Loop acceleration can be used to prove safety, reachability, runtime bounds, and (non-)termination of programs. To this end, a variety of acceleration techniques have been proposed. However, so far all of them have been monolithic, i.e., a single loop could not be accelerated using a combination of several different acceleration techniques. In contrast, we present a calculus that allows for combining acceleration techniques in a modular way and we show how to integrate many existing acceleration techniques into our calculus. Moreover, we propose two novel acceleration techniques that can be incorporated into our calculus seamlessly. Some of these acceleration techniques apply only to non-terminating loops. Thus, combining them with our novel calculus results in a new, modular approach for proving non-termination. An empirical evaluation demonstrates the applicability of our approach, both for loop acceleration and for proving non-termination.

     

Photoelectric phenomena in ZnO(ITO)/a-Si:H(n)/c-Si(p)/Al solar cells

Solar cells Al/ZnO/a-Si:H(n)/c-Si(p)/Al and Al/ITO/a-Si:H(n)/c-Si(p)/Al were fabricated on single-crystal Si substrates. The photoelectric properties of these solar cells were investigated under exposure to natural and linearly polarized radiation at T=300 K. The polarization photosensitivity of the solar cells, which emerges under the conditions of oblique incidence of linearly polarized radiation, was observed. The origin of the induced photopleochroism of solar cells with ZnO and ITO antireflection coatings is ascertained. The oscillations in the spectrum of the induced photopleochroism, which are associated with interference phenomena in oxide films, are observed. The results obtained point to the possibility of using the solar cells as selective photosensors. At the same time, polarization spectroscopy can ensure effective monitoring of the antireflection coatings of solar cells.     

Bobillier's construction via three-pole theorem

Bobillier's construction of the center of curvature is derived by a simple geometric reasoning based upon the three-pole theorem.     

Defects and recombination in microcrystalline silicon

The paper addresses the defect structure of microcrystalline silicon, μc-Si:H. Electron spin resonance (ESR) techniques are employed to study the nature and energy distribution of paramagnetic states in a large variety of undoped and doped μc-Si:H samples prepared by various methods under different deposition conditions. A qualitative model for the density of states distribution in the energy gap is developed which is dominated by two kinds of dangling bonds and bandtail states at both band edges. In phosphorus-doped samples, ESR reveals a metal–insulator transition at a phosphorus concentration of 4×10¹⁸ cm⁻³. Light-induced ESR and electrically detected magnetic resonance show that at low temperatures recombination is dominated by tunneling transitions from the bandtail states into neutral dangling bonds. Apparently, at higher temperatures, direct capture from conducting states prevails. The results are discussed considering the heterogeneous structure of this material.     

A probabilistic model of bathing beach safety

An improved mathematical model for bathing beach safety is proposed. It is derived by joining the probability of infection from a given dose (Poisson distribution and the probability of acquiring such a dose (lognormal distribution).Even in the absence of better clinical and epidemiological data, the model permits an assessment of relative risk from certain hazards and the design of more meaningful bacteriological standards for individual beaches.     

NUMERICAL SOLUTION AND VALIDATION OF A DYNAMIC MODEL OF SOLAR COLLECTORS WORKING WITH VARYING FLUID FLOW RATE

A method to calculate the short-term dynamic behavior of solar collectors, working with varying fluid-flow rate, has been developed. It is based on a system of first-order partial differential equations. The method was applied to model a large unglazed collector used for heating a public outdoor swimming-pool in Marburg (Germany). A validation with data measured at this collector showed good short- and long-term accuracy of the model at constant and varying flow rates. Furthermore, the dynamic behavior of the detailed model was compared to that of several model approximations especially at varying flow rate. In the case of unglazed collectors a simple dynamic model, based on an ordinary differential equation, gives accurate results in most practical applications even at varying flow rate. The extension of the method to flat-plate collectors is demonstrated as well.