51,549 research outputs found
Deducing effective light transport parameters in optically thin systems
We present an extensive Monte Carlo study on light transport in optically
thin slabs, addressing both axial and transverse propagation. We completely
characterize the so-called ballistic-to-diffusive transition, notably in terms
of the spatial variance of the transmitted/reflected profile. We test the
validity of the prediction cast by diffusion theory, that the spatial variance
should grow independently of absorption and, to a first approximation, of the
sample thickness and refractive index contrast. Based on a large set of
simulated data, we build a freely available look-up table routine allowing
reliable and precise determination of the microscopic transport parameters
starting from robust observables which are independent of absolute intensity
measurements. We also present the Monte Carlo software package that was
developed for the purpose of this study
Aging concrete structures: a review of mechanics and concepts
The safe and cost-efficient management of our built infrastructure is a challenging task considering the expected service life of at least 50 years. In spite of time-dependent changes in material properties, deterioration processes and changing demand by society, the structures need to satisfy many technical requirements related to serviceability, durability, sustainability and bearing capacity. This review paper summarizes the challenges associated with the safe design and maintenance of aging concrete structures and gives an overview of some concepts and approaches that are being developed to address these challenges
From the Hands of an Early Adopter's Avatar to Virtual Junkyards: Analysis of Virtual Goods' Lifetime Survival
One of the major questions in the study of economics, logistics, and business
forecasting is the measurement and prediction of value creation, distribution,
and lifetime in the form of goods. In "real" economies, a perfect model for the
circulation of goods is impossible. However, virtual realities and economies
pose a new frontier for the broad study of economics, since every good and
transaction can be accurately tracked. Therefore, models that predict goods'
circulation can be tested and confirmed before their introduction to "real
life" and other scenarios. The present study is focused on the characteristics
of early-stage adopters for virtual goods, and how they predict the lifespan of
the goods. We employ machine learning and decision trees as the basis of our
prediction models. Results provide evidence that the prediction of the lifespan
of virtual objects is possible based just on data from early holders of those
objects. Overall, communication and social activity are the main drivers for
the effective propagation of virtual goods, and they are the most expected
characteristics of early adopters.Comment: 28 page
Nondiffusive spin dynamics in a two-dimensional electron gas
We describe measurements of spin dynamics in the two-dimensional electron gas in GaAs=GaAlAs quantum wells. Optical techniques, including transient spin-grating spectroscopy, are used to probe the relaxation rates of spin polarization waves in the wave vector range from zero to 6 x 104 cm-1. We find that the spin polarization lifetime is maximal at a nonzero wave vector, in contrast with expectations based on ordinary spin diffusion, but in quantitative agreement with recent theories that treat diffusion in the presence of spin-orbit couplin
Energy Barriers and Activated Dynamics in a Supercooled Lennard-Jones Liquid
We study the relation of the potential energy landscape (PEL) topography to
relaxation dynamics of a small model glass former of Lennard-Jones type. The
mechanism under investigation is the hopping betweem superstructures of PEL
mimima, called metabasins (MB). From the mean durations \tauphi of visits to
MBs, we derive effective depths of these objects by the relation
\Eapp=\d\ln\tauphi/\d\beta, where \beta=1/\kB T. Since the apparent
activation energies \Eapp are of purely dynamical origin, we look for a
quantitative relation to PEL structure. A consequence of the rugged nature of
MBs is that escapes from MBs are not single hops between PEL minima, but
complicated multi-minima sequences. We introduce the concept of return
probabilities to the bottom of MBs in order to judge whether the attraction
range of a MB was left. We then compute the energy barriers that were
surmounted. These turn out to be in good agreement with the effective depths
\Eapp, calculated from dynamics. Barriers are identified with the help of a
new method, which accurately performs a descent along the ridge between two
minima. A comparison to another method is given. We analyze the population of
transition regions between minima, called basin borders. No indication for the
mechanism of diffusion to change around the mode-coupling transition can be
found. We discuss the question whether the one-dimensional reaction paths
connecting two minima are relevant for the calculation of reaction rates at the
temperatures under study.Comment: 17 pages, 16 figure
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