2,261 research outputs found
Theory of severe slowdown in the relaxation of rings and clusters with antiferromagnetic interactions
We show that in the severe slowing down temperature regime the relaxation of
antiferromagnetic rings and similar magnetic nanoclusters is governed by the
quasi-continuum portion of their quadrupolar fluctuation spectrum and not by
the lowest excitation lines. This is at the heart of the intriguing
near-universal power-law temperature dependence of the electronic correlation
frequency with an exponent close to 4. The onset of this behavior is
defined by an energy scale which is fixed by the lowest spin gap .
This explains why experimental curves of for different cluster sizes
and spins nearly coincide when is rescaled by .Comment: new slightly extended version (6 pages, 1 fig. added
Non-ergodic dynamics of the extended anisotropic Heisenberg chain
The issue of ergodicity is often underestimated. The presence of
zero-frequency excitations in bosonic Green's functions determine the
appearance of zero-frequency momentum-dependent quantities in correlation
functions. The implicit dependence of matrix elements make such quantities also
relevant in the computation of susceptibilities. Consequently, the correct
determination of these quantities is of great relevance and the
well-established practice of fixing them by assuming the ergodicity of the
dynamics is quite questionable as it is not justifiable a priori by no means.
In this manuscript, we have investigated the ergodicity of the dynamics of the
-component of the spin in the 1D Heisenberg model with anisotropic
nearest-neighbor and isotropic next-nearest-neighbor interactions. We have
obtained the zero-temperature phase diagram in the thermodynamic limit by
extrapolating Exact and Lanczos diagonalization results computed on chains with
sizes . Two distinct non-ergodic regions have been found: one
for and and another for
and . On the contrary,
finite-size scaling of results, obtained by means of Exact
diagonalization on chains with sizes , indicates an ergodic
behavior of dynamics in the whole range of parameters.Comment: 6 pages, 7 figure
Deficit of temporal dynamics of detection of a moving object during egomotion in a stroke patient: a psychophysical and MEG study
To investigate the temporal dynamics underlying object motion detection during egomotion, we used psychophysics and MEG with a motion discrimination task. The display contained nine spheres moving for 1 second, eight moved consistent with forward observer translation, and one (the target) with independent motion within the scene (approaching or receding). Observers's task was to detect the target. Seven healthy subjects (7HS) and patient PF with an infarct involving the left occipital-temporal cortex participated in both the psychophysical and MEG study. Psychophysical results showed that PF was severely impaired on this task. He was also impaired on the discrimination of radial motion (with even poorer performance on contraction) and 2D direction as well as on detecting motion discontinuity. We used anatomically constrained MEG and dynamic Granger causality to investigate the direction and dynamics of connectivity between the functional areas involved in the object-motion task and compared the results of 7HS and PF. The dynamics of the causal connections among the motion responsive cortical areas (MT, STS, IPS) during the first 200 ms of the stimulus was similar in all subjects. However, in the later part of the stimulus (>200 ms) PF did not show significant causal connections among these areas. Also the 7HS had a strong, probably attention modulatory connection, between MPFC and MT, which was completely absent in PF. In PF and the 7HS, analysis of onset latencies revealed two stages of activations: early after motion onset (200–400 ms) bilateral activations in MT, IPS, and STS, followed (>500 ms) by activity in the postcentral sulcus and middle prefrontal cortex (MPFC). We suggest that the interaction of these early and late onset areas is critical to object motion detection during self-motion, and disrupted connections among late onset areas may have contributed to the perceptual deficits of patient PF.Published versio
Online assessment of negotiation skills through 3D role play simulation
The lack of standardised technological tools to assess psychological characteristics allows traditional tests to be still widely used, though these tests require double processing and expensive procedures. ENACT is both a serious game for a standardised assessment of the user negotiation skills and an Intelligent Tutoring System, which makes use of the data collected during the interaction in order to generate a tailored environment for the user to experience and improve their skills and be guided through learning
New perspectives in ecosystem services science as instruments to understand environmental securities
As societal demand for food, water and other life-sustaining resources grows, the science of ecosystem services (ES) is seen as a promising tool to improve our understanding, and ultimately the management, of increasingly uncertain supplies of critical goods provided or supported by natural ecosystems. This promise, however, is tempered by a relatively primitive understanding of the complex systems supporting ES, which as a result are often quantified as static resources rather than as the dynamic expression of human-natural systems. This article attempts to pinpoint the minimum level of detail that ES science needs to achieve in order to usefully inform the debate on environmental securities, and discusses both the state of the art and recent methodological developments in ES in this light. We briefly review the field of ES accounting methods and list some desiderata that we deem necessary, reachable and relevant to address environmental securities through an improved science of ES. We then discuss a methodological innovation that, while only addressing these needs partially, can improve our understanding of ES dynamics in data-scarce situations. The methodology is illustrated and discussed through an application related to water security in the semi-arid landscape of the Great Ruaha river of Tanzania. © 2014 The Author(s) Published by the Royal Society. All rights reserved
Mesoscopic continuous and discrete channels for quantum information transfer
We study the possibility of realizing perfect quantum state transfer in
mesoscopic devices. We discuss the case of the Fano-Anderson model extended to
two impurities. For a channel with an infinite number of degrees of freedom, we
obtain coherent behavior in the case of strong coupling or in weak coupling
off-resonance. For a finite number of degrees of freedom, coherent behavior is
associated to weak coupling and resonance conditions
Metal abundances in extremely distant Galactic old open clusters. II. Berkeley 22 and Berkeley 66
We report on high resolution spectroscopy of four giant stars in the Galactic
old open clusters Berkeley~22 and Berkeley~66 obtained with HIRES at the Keck
telescope. We find that and for
Berkeley~22 and Berkeley~66, respectively. Based on these data, we first revise
the fundamental parameters of the clusters, and then discuss them in the
context of the Galactic disk radial abundance gradient. We found that both
clusters nicely obey the most updated estimate of the slope of the gradient
from \citet{fri02} and are genuine Galactic disk objects.Comment: 20 pages, 6 eps figures, accepted for publication in the Astronomical
Journa
Double dot chain as a macroscopic quantum bit
We consider an array of N quantum dot pairs interacting via Coulomb
interaction between adjacent dots and hopping inside each pair. We show that at
the first order in the ratio of hopping and interaction amplitudes, the array
maps in an effective two level system with energy separation becoming
exponentially small in the macroscopic (large N) limit. Decoherence at zero
temperature is studied in the limit of weak coupling with phonons. In this case
the macroscopic limit is robust with respect to decoherence. Some possible
applications in quantum information processing are discussed.Comment: Phys. Rev. A (in press
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