1,868 research outputs found
Parabolic Anderson model with a finite number of moving catalysts
We consider the parabolic Anderson model (PAM) which is given by the equation
with , where is the diffusion constant,
is the discrete Laplacian, and
is a space-time random environment that drives the equation. The solution of
this equation describes the evolution of a "reactant" under the influence
of a "catalyst" . In the present paper we focus on the case where is
a system of independent simple random walks each with step rate
and starting from the origin. We study the \emph{annealed} Lyapunov exponents,
i.e., the exponential growth rates of the successive moments of w.r.t.\
and show that these exponents, as a function of the diffusion constant
and the rate constant , behave differently depending on the
dimension . In particular, we give a description of the intermittent
behavior of the system in terms of the annealed Lyapunov exponents, depicting
how the total mass of concentrates as . Our results are both a
generalization and an extension of the work of G\"artner and Heydenreich 2006,
where only the case was investigated.Comment: In honour of J\"urgen G\"artner on the occasion of his 60th birthday,
25 pages. Updated version following the referee's comment
Micropatterned Electrostatic Traps for Indirect Excitons in Coupled GaAs Quantum Wells
We demonstrate an electrostatic trap for indirect excitons in a field-effect
structure based on coupled GaAs quantum wells. Within the plane of a double
quantum well indirect excitons are trapped at the perimeter of a SiO2 area
sandwiched between the surface of the GaAs heterostructure and a
semitransparent metallic top gate. The trapping mechanism is well explained by
a combination of the quantum confined Stark effect and local field enhancement.
We find the one-dimensional trapping potentials in the quantum well plane to be
nearly harmonic with high spring constants exceeding 10 keV/cm^2.Comment: 21 pages, 6 figures, submitted to Phys. Rev.
Drift mobility of long-living excitons in coupled GaAs quantum wells
We observe high-mobility transport of indirect excitons in coupled GaAs
quantum wells. A voltage-tunable in-plane potential gradient is defined for
excitons by exploiting the quantum confined Stark effect in combination with a
lithographically designed resistive top gate. Excitonic photoluminescence
resolved in space, energy, and time provides insight into the in-plane drift
dynamics. Across several hundreds of microns an excitonic mobility of >10^5
cm2/eVs is observed for temperatures below 10 K. With increasing temperature
the excitonic mobility decreases due to exciton-phonon scattering.Comment: 3 pages, 3 figure
Generalized Shortest Path Kernel on Graphs
We consider the problem of classifying graphs using graph kernels. We define
a new graph kernel, called the generalized shortest path kernel, based on the
number and length of shortest paths between nodes. For our example
classification problem, we consider the task of classifying random graphs from
two well-known families, by the number of clusters they contain. We verify
empirically that the generalized shortest path kernel outperforms the original
shortest path kernel on a number of datasets. We give a theoretical analysis
for explaining our experimental results. In particular, we estimate
distributions of the expected feature vectors for the shortest path kernel and
the generalized shortest path kernel, and we show some evidence explaining why
our graph kernel outperforms the shortest path kernel for our graph
classification problem.Comment: Short version presented at Discovery Science 2015 in Banf
Patrones composicionales de hierbas ruderales en Santiago, Chile
Fragmentation of the natural environment is a consequence of urbanisation. It impacts the biodiversity of native flora thatcharacterises a region. This study focused on characterising the diversity, composition and distribution of native and alienruderal species present in different suburbs of Santiago de Chile. We found that plant assemblages of ruderal species werecharacterised by a higher proportion of alien species (69 taxa), whose original distribution corresponds to the MediterraneanBasin (46 taxa), and a low representation of native species (14 taxa). The results show that the spatial distribution of weedswithin Santiago was not random, because two clusters were found based on patterns of compositional similarity. Furtherresearch should be undertaken to determine the cause of this phenomenon that probably obeys historical and ecologicalfactors such as the past use of soils or urban landscape ornamentation programs.La expansión de los centros urbanos tiene como consecuencia la fragmentación de los ambientes naturales y elconsecuente impacto en la biodiversidad de la flora nativa que caracteriza a una región. El presente estudio se enfocó enla caracterización de la diversidad, composición y distribución de especies ruderales nativas e introducidas presentes endiferentes comunas de Santiago de Chile. Se encontró que los ensambles de especies ruderales están dominados por lapresencia de especies introducidas (69 taxa), cuya distribución original corresponde a la Cuenca Mediterránea (46 taxa),con una baja representación de especies nativas (14 taxa). Los resultados indican además que la distribución espacial demalezas en la ciudad de Santiago no es aleatoria, pues dos conglomerados fueron encontrados en función de los patrones desimilitud composicional. Las causas de este fenómeno deben ser investigadas, pero probablemente obedecen a un complejode factores entre los que se pueden mencionar el modo de uso de suelo, y/o programas de ornamentación del paisaje urbano
Ten steps toward a better personality science - How quality may be rewarded more in research evaluation
This target article is part of a theme bundle including open peer commentaries (https://doi.org/10.5964/ps.9227) and a rejoinder by the authors (https://doi.org/10.5964/ps.7961). We point out ten steps that we think will go a long way in improving personality science. The first five steps focus on fostering consensus regarding (1) research goals, (2) terminology, (3) measurement practices, (4) data handling, and (5) the current state of theory and evidence. The other five steps focus on improving the credibility of empirical research, through (6) formal modelling, (7) mandatory pre-registration for confirmatory claims, (8) replication as a routine practice, (9) planning for informative studies (e.g., in terms of statistical power), and (10) making data, analysis scripts, and materials openly available. The current, quantity-based incentive structure in academia clearly stands in the way of implementing many of these practices, resulting in a research literature with sometimes questionable utility and/or integrity. As a solution, we propose a more quality-based reward scheme that explicitly weights published research by its Good Science merits. Scientists need to be increasingly rewarded for doing good work, not just lots of work
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