586 research outputs found
Localized to extended states transition for two interacting particles in a two-dimensional random potential
We show by a numerical procedure that a short-range interaction induces
extended two-particle states in a two-dimensional random potential. Our
procedure treats the interaction as a perturbation and solve Dyson's equation
exactly in the subspace of doubly occupied sites. We consider long bars of
several widths and extract the macroscopic localization and correlation lengths
by an scaling analysis of the renormalized decay length of the bars. For ,
the critical disorder found is , and the critical
exponent . For two non-interacting particles we do not find any
transition and the localization length is roughly half the one-particle value,
as expected.Comment: 4 two-column pages, 4 eps figures, Revtex, to be published in
Europhys. Let
Observational Conditioning in Flower Choice Copying by Bumblebees (Bombus terrestris): Influence of Observer Distance and Demonstrator Movement
A. Avargues-Weber was funded by a postdoctoral fellowship from Fyssen fondation: http://www.fondationfyssen.fr/. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
Magnetic Quantum Dot: A Magnetic Transmission Barrier and Resonator
We study the ballistic edge-channel transport in quantum wires with a
magnetic quantum dot, which is formed by two different magnetic fields B^* and
B_0 inside and outside the dot, respectively. We find that the electron states
located near the dot and the scattering of edge channels by the dot strongly
depend on whether B^* is parallel or antiparallel to B_0. For parallel fields,
two-terminal conductance as a function of channel energy is quantized except
for resonances, while, for antiparallel fields, it is not quantized and all
channels can be completely reflected in some energy ranges. All these features
are attributed to the characteristic magnetic confinements caused by nonuniform
fields.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Kolmogorov Spectrum of Quantum Turbulence
There is a growing interest in the relation between classical turbulence and
quantum turbulence. Classical turbulence arises from complicated dynamics of
eddies in a classical fluid. In contrast, quantum turbulence consists of a
tangle of stable topological defects called quantized vortices, and thus
quantum turbulence provides a simpler prototype of turbulence than classical
turbulence. In this paper, we investigate the dynamics and statistics of
quantized vortices in quantum turbulence by numerically solving a modified
Gross-Pitaevskii equation. First, to make decaying turbulence, we introduce a
dissipation term that works only at scales below the healing length. Second, to
obtain steady turbulence through the balance between injection and decay, we
add energy injection at large scales. The energy spectrum is quantitatively
consistent with the Kolmogorov law in both decaying and steady turbulence.
Consequently, this is the first study that confirms the inertial range of
quantum turbulence.Comment: 14pages, 24 figures and 1 table. Appeared in Journal of the Physical
Society of Japan, Vol.74, No.12, p.3248-325
Parasitism perturbs the mucosal microbiome of Atlantic Salmon
Interactions between parasite, host and host-associated microbiota are increasingly understood as important determinants of disease progression and morbidity. Salmon lice, including the parasitic copepod Lepeophtheirus salmonis and related species, are perhaps the most important problem facing Atlantic Salmon aquaculture after feed sustainability. Salmon lice parasitize the surface of the fish, feeding off mucus, scales and underlying tissue. Secondary bacterial infections are a major source of associated morbidity. In this study we tracked the diversity and composition of Salmo salar skin surface microbiota throughout a complete L. salmonis infection cycle among 800 post-smolts as compared to healthy controls. Among infected fish we observed a significant reduction in microbial richness (Chao1, P = 0.0136), raised diversity (Shannon, P < 7.86e-06) as well as highly significant destabilisation of microbial community composition (Pairwise Unifrac, beta-diversity, P < 1.86e-05; P = 0.0132) by comparison to controls. While undetectable on an individual level, network analysis of microbial taxa on infected fish revealed the association of multiple pathogenic genera (Vibrio, Flavobacterium, Tenacibaculum, Pseudomonas) with high louse burdens. We discuss our findings in the context of ecological theory and colonisation resistance, in addition to the role microbiota in driving primary and secondary pathology in the host
Spectroscopic variability of two Oe stars
The Oe stars HD45314 and HD60848 have recently been found to exhibit very
different X-ray properties: whilst HD60848 has an X-ray spectrum and emission
level typical of most OB stars, HD45314 features a much harder and brighter
X-ray emission, making it a so-called gamma Cas analogue. Monitoring the
optical spectra could provide hints towards the origin of these very different
behaviours. We analyse a large set of spectroscopic observations of HD45314 and
HD60848, extending over 20 years. We further attempt to fit the H-alpha line
profiles of both stars with a simple model of emission line formation in a
Keplerian disk. Strong variations in the strengths of the H-alpha, H-beta, and
He I 5876 emission lines are observed for both stars. In the case of HD60848,
we find a time lag between the variations in the equivalent widths of these
lines. The emission lines are double peaked with nearly identical strengths of
the violet and red peaks. The H-alpha profile of this star can be successfully
reproduced by our model of a disk seen under an inclination of 30 degrees. In
the case of HD45314, the emission lines are highly asymmetric and display
strong line profile variations. We find a major change in behaviour between the
2002 outburst and the one observed in 2013. This concerns both the relationship
between the equivalent widths of the various lines and their morphologies at
maximum strength (double-peaked in 2002 versus single-peaked in 2013). Our
simple disk model fails to reproduce the observed H-alpha line profiles of
HD45314. Our results further support the interpretation that Oe stars do have
decretion disks similar to those of Be stars. Whilst the emission lines of
HD60848 are explained by a disk with a Keplerian velocity field, the disk of
HD45314 seems to have a significantly more complex velocity field that could be
related to the phenomenon that produces its peculiar X-ray emission.Comment: Accepted for Publication in A&
No measure for culture? Value in the new economy
This paper explores articulations of the value of investment in culture and the arts through a critical discourse analysis of policy documents, reports and academic commentary since 1997. It argues that in this period, discourses around the value of culture have moved from a focus on the direct economic contributions of the culture industries to their indirect economic benefits. These indirect benefits are discussed here under three main headings: creativity and innovation, employability, and social inclusion. These are in turn analysed in terms of three forms of capital: human, social and cultural. The paper concludes with an analysis of this discursive shift through the lens of autonomist Marxist concerns with the labour of social reproduction. It is our argument that, in contemporary policy discourses on culture and the arts, the government in the UK is increasingly concerned with the use of culture to form the social in the image of capital. As such, we must turn our attention beyond the walls of the factory in order to understand the contemporary capitalist production of value and resistance to it. </jats:p
Symmetry Dependence of Localization in Quasi- 1- dimensional Disordered Wires
The crossover in energy level statistics of a quasi-1-dimensional disordered
wire as a function of its length L is used, in order to derive its averaged
localization length, without magnetic field, in a magnetic field and for
moderate spin orbit scattering strength. An analytical function of the magnetic
field for the local level spacing is obtained, and found to be in excellent
agreement with the magnetic field dependent activation energy, recently
measured in low-mobility quasi-one-dimensional wires\cite{khavin}. This formula
can be used to extract directly and accurately the localization length from
magnetoresistance experiments. In general, the local level spacing is shown to
be proportional to the excitation gap of a virtual particle, moving on a
compact symmetric space.Comment: 4 pages, 2 Eqs. added, Eperimental Data included in Fig.
Living on thin abstractions: more power/economic knowledge
Debates over the role of knowledge and know-how as key economic assets in the contemporary economy, although far from new, are now increasingly couched in terms of a new-found economic immateriality which allows for their costless reproduction and widespread geographical dissemination. In the rush to tie down and reproduce economic know-how in abstract codifiable form, it has become almost baffling to argue that our stock of economic knowledge may rest upon affects as much as analysis, expressive symbolism as much as abstract symbolism. This paper is an attempt to think through how such 'elusive' economic knowledges may be grasped, yet neither formalized nor codified in abstract terms. It is also a plea to consider the geography of economic knowledge outside of the tacit - explicit distinction
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