4,592 research outputs found
Kondo insulators in the periodic Anderson model: a local moment approach
The symmetric periodic Anderson model is well known to capture the essential
physics of Kondo insulator materials. Within the framework of dynamical
mean-field theory, we develop a local moment approach to its single-particle
dynamics in the paramagnetic phase. The approach is intrinsically
non-perturbative, encompasses all energy scales and interaction strengths, and
satisfies the low-energy dictates of Fermi liquid theory. It captures in
particular the strong coupling behaviour and exponentially small quasiparticle
scales characteristic of the Kondo lattice regime, as well as simple
perturbative behaviour in weak coupling. Particular emphasis is naturally given
to strong coupling dynamics, where the resultant clean separation of energy
scales enables the scaling behaviour of single-particle spectra to be obtained.Comment: 15 pages, 10 postscript figures, accepted for publication in EPJ B;
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The corotating hollow vortex pair: steady merger and break-up via a topological singularity
The shapes of two steadily rotating, equal circulation, two-dimensional hollow vortices are determined and their properties examined. By means of a numerical scheme that accounts for the doubly connected nature of the fluid domain, it is shown that a one-parameter family of solutions exists that is a continuation of a corotating point vortex pair. Withb= 2 set as the distance between the vortex centroids we find that each vortex reaches a maximum possible area of 0.796 corresponding toa/b= 0.260 where a is a measure of the vortex core radius proposed by Meunieret al[Phys. Fluids,14, (2002)]. Results are compared to those of a previous study by Saffman & Szeto [Phys. Fluids,23, (1980)] in which two corotating patches of uniform vorticity are considered in place of the hollow vortices studied here. The general behaviour of the two systems is seen to be similar but some differences are highlighted, especially when the vortices become close to touching due to the accumulation of vorticity in thin extended fingers emanating from each of the vortices. The numerical scheme captures the family of equilibria very close to a critical configuration where these fingers tend to touch at the centre of rotation corresponding to a/b≈0.283. By a simple adaptation of the numerical scheme to compute 2-fold rotationally symmetric equilibria for a single rotating hollow vortex we then show that its limiting configuration is one where a thin waist forms leading to two separate parts of its single boundary drawing close together. We give evidence that the limit of this single vortex configuration coincides with the limit of the two-vortex configuration. The limiting configuration itself turns out not to be physically admissible leading to what we refer to as a topological singularity since no physical quantities blow up, indeed they appear to be continuous as the limiting state is approached from the two topologically distinct directions
Temporary Memory Neuron for the Leaky Integrate and Fire Neuron Model
Low-level terrain-following systems require the ability to rapidly and accurately respond to the environment to prevent inadvertent actions. Catastrophic and fatal results could occur if missed cues or latency issues in data processing are encountered. Spiking neural networks (SNNs) have the computational ability to continuously process spike trains from rapid sensory input. However, most models of SNNs do not retain information from the spike train of a previous time step because the membrane potential is rapidly reset to a resting potential after activation. A novel approach is presented, allowing the spike train of a previous time step to be \u27remembered.\u27 Results are presented showing rapid onset of a membrane potential that exceeds the threshold and spikes in the presence of the same continuous spike train without the latency of increasing the membrane potential from its resting state
Characterization of doping levels in heteronuclear, gas-phase, van der Waals clusters and their energy absorption from an intense optical field
A simple mass spectrometric method has been developed to quantify dopant
levels in heteronuclear clusters in the gas phase. The method is demonstrated
with reference to quantification of the water content in supersonic beams of
water-doped argon clusters. Such doped clusters have assumed much importance in
the context of recently-reported doping-induced enhancement in the emission of
energetic charged particles and photons upon their interaction with intense
laser pulses. We have also measured the energy that a doped cluster absorbs
from the optical field; we find that energy absorption increases with
increasing level of doping. The oft-used linear model of energy absorption is
found to be quantitatively inadequate.Comment: To appear in Chemical Physics Letter
Coarsening and Slow-Dynamics in Granular Compaction
We address the problem of the microscopic reorganization of a granular medium
under a compaction process in the framework of Tetris-like models. We point out
the existence of regions of spatial organization which we call domains, and
study their time evolution. It turns out that after an initial transient, most
of the activity of the system is concentrated on the boundaries between
domains. One can then describe the compaction phenomenon as a coarsening
process for the domains, and a progressive reduction of domain boundaries. We
discuss the link between the coarsening process and the slow dynamics in the
framework of a model of active walkers on active substrates.Comment: Revtex 4 pages, 4 figures, in press in PRL. More info
http://axtnt3.phys.uniroma1.it/Tetri
Coulomb Interactions and Nanoscale Electronic Inhomogeneities in Manganites
We address the issue of endemic electronic inhomogeneities in manganites
using extensive simulations on a new model with Coulomb interactions amongst
two electronic fluids, one localized (polaronic), the other extended
(band-like), and dopant ions. The long range Coulomb interactions frustrate
phase separation induced by the strong on site repulsion between the fluids. A
single quantum phase ensues which is intrinsically and strongly inhomogeneous
at a nano-scale, but homogeneous on meso-scales, with many characteristics
(including colossal responses)that agree with experiments. This, we argue, is
the origin of nanoscale inhomogeneities in manganites, rather than phase
competition and disorder related effects as often proposed.Comment: 4 pages, 3 figure
Field Evidence for Geophysical Detection of Subsurface Zones of Enhanced Microbial Activity
Geochemical data from closely spaced vertical intervals in a hydrocarbon-impacted aquifer were used to assess the relationship between bulk conductivity and zones of enhanced microbial activity. The bulk conductivity was measured using in situ vertical resistivity probes. Microbial activity was verified using terminal electron acceptors (nitrate, sulfate, iron, and manganese), dissolved inorganic carbon (DIC), and major ion chemistry. Peaks in bulk conductivity in the aquifer overlapped with zones where nitrates and sulfates were depleted, total petroleum hydrocarbon, iron, manganese, dissolved ions, and DIC were elevated, suggesting a link between higher electrical conductivity and zones of enhanced microbial activity stimulated by the presence of hydrocarbon. Thus the subsurface expression of microbial activity is apparently recorded in the bulk conductivity measurements. Our results argue for combining geophysics with biogeochemistry studies to delineate subsurface zones of enhanced microbial activity
Self-Structuring of Granular Media under Internal Avalanches
We study the phenomenon of internal avalanching within the context of
recently proposed ``Tetris'' lattice models for granular media. We define a
recycling dynamics under which the system reaches a steady state which is
self-structured, i.e. it shows a complex interplay between textured internal
structures and critical avalanche behavior. Furthermore we develop a general
mean-field theory for this class of systems and discuss possible scenarios for
the breakdown of universality.Comment: 4 pages RevTex, 3 eps figures, revised version to appear in Phys.
Rev. Let
Performance of a megawatt-scale grid-connected solar photovoltaic power plant in Kolar District in Karnataka
A megawatt scale grid-connected photovoltaic power plant was commissioned on 27 December 2009 in
Yalesandra in Kolar district in Karnataka. The Yalesandra plant is one among more than 20 such Megawatt
size solar power plants in India during the past few years. The performance of this plant during its first
year of operation has been discussed. The total electrical energy generated by the Yelasandra plant during
2010 was 3.34 million kWh. Although the performance of photovoltaic modules was good, there were problems
associated with the inverters which led to reduction in energy generation. The impact of temperature
variation of modules on their performance has been highlighted
Eulerian Walkers as a model of Self-Organised Criticality
We propose a new model of self-organized criticality. A particle is dropped
at random on a lattice and moves along directions specified by arrows at each
site. As it moves, it changes the direction of the arrows according to fixed
rules. On closed graphs these walks generate Euler circuits. On open graphs,
the particle eventually leaves the system, and a new particle is then added.
The operators corresponding to particle addition generate an abelian group,
same as the group for the Abelian Sandpile model on the graph. We determine the
critical steady state and some critical exponents exactly, using this
equivalence.Comment: 4 pages, RevTex, 4 figure
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