34,867 research outputs found
Quantum anomalies and linear response theory
The analysis of diffusive energy spreading in quantized chaotic driven
systems, leads to a universal paradigm for the emergence of a quantum anomaly.
In the classical approximation a driven chaotic system exhibits stochastic-like
diffusion in energy space with a coefficient that is proportional to the
intensity of the driving. In the corresponding quantized problem
the coherent transitions are characterized by a generalized Wigner time
, and a self-generated (intrinsic) dephasing process leads to
non-linear dependence of on .Comment: 8 pages, 2 figures, textual improvements (as in published version
Non-equilibrium steady state of sparse systems
A resistor-network picture of transitions is appropriate for the study of
energy absorption by weakly chaotic or weakly interacting driven systems. Such
"sparse" systems reach a novel non-equilibrium steady state (NESS) once coupled
to a bath. In the stochastic case there is an analogy to the physics of
percolating glassy systems, and an extension of the fluctuation-dissipation
phenomenology is proposed. In the mesoscopic case the quantum NESS might differ
enormously from the stochastic NESS, with saturation temperature determined by
the sparsity. A toy model where the sparsity of the system is modeled using a
log-normal random ensemble is analyzed.Comment: 6 pages, 6 figures, EPL accepted versio
Energy absorption by "sparse" systems: beyond linear response theory
The analysis of the response to driving in the case of weakly chaotic or
weakly interacting systems should go beyond linear response theory. Due to the
"sparsity" of the perturbation matrix, a resistor network picture of
transitions between energy levels is essential. The Kubo formula is modified,
replacing the "algebraic" average over the squared matrix elements by a
"resistor network" average. Consequently the response becomes semi-linear
rather than linear. Some novel results have been obtained in the context of two
prototype problems: the heating rate of particles in Billiards with vibrating
walls; and the Ohmic Joule conductance of mesoscopic rings driven by
electromotive force. Respectively, the obtained results are contrasted with the
"Wall formula" and the "Drude formula".Comment: 8 pages, 7 figures, short pedagogical review. Proceedings of FQMT
conference (Prague, 2011). Ref correcte
Absorption of Energy at a Metallic Surface due to a Normal Electric Field
The effect of an oscillating electric field normal to a metallic surface may
be described by an effective potential. This induced potential is calculated
using semiclassical variants of the random phase approximation (RPA). Results
are obtained for both ballistic and diffusive electron motion, and for two and
three dimensional systems. The potential induced within the surface causes
absorption of energy. The results are applied to the absorption of radiation by
small metal spheres and discs. They improve upon an earlier treatment which
used the Thomas-Fermi approximation for the effective potential.Comment: 19 pages (Plain TeX), 2 figures, 1 table (Postscript
Quantum response of weakly chaotic systems
Chaotic systems, that have a small Lyapunov exponent, do not obey the common
random matrix theory predictions within a wide "weak quantum chaos" regime.
This leads to a novel prediction for the rate of heating for cold atoms in
optical billiards with vibrating walls. The Hamiltonian matrix of the driven
system does not look like one from a Gaussian ensemble, but rather it is very
sparse. This sparsity can be characterized by parameters and that
reflect the percentage of large elements, and their connectivity respectively.
For we use a resistor network calculation that has direct relation to the
semi-linear response characteristics of the system.Comment: 7 pages, 5 figures, expanded improved versio
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Evaluating the transition of e-Government: A review of local authorities in England
The goal of e-Government is seen as a panacea for governmental authorities. The emerging needs of citizens, their inclusion and engagement in policy development, political and participatory processes have meant new perspectives on e-Government are required. This paper seeks to identify and evaluate the preparedness of 10 UK-based local authorities to transition from basic e-Government to a more sophisticated and integrated e-Government. A categorical assessment of e-Government characteristics is made and these authorities are ranked accordingly. Our findings reveal the majority of local authorities sampled had reached a high percentage of informational and transactional e-Government but few had reached the interactional level and none had achieved assimilation. This suggests that local authorities seem to have focused on basic e-Government services. There is a need now to forge ahead to integration and assimilation of e-Government in order to address the critical objectives of citizen inclusion and engagement, and alignment of institutional processes to provide an infrastructure for the transition to e-governance and e-knowledge
Rhythms of social interaction: messaging within a massive online network
We have analyzed the fully-anonymized headers of 362 million messages
exchanged by 4.2 million users of Facebook, an online social network of college
students, during a 26 month interval. The data reveal a number of strong daily
and weekly regularities which provide insights into the time use of college
students and their social lives, including seasonal variations. We also
examined how factors such as school affiliation and informal online friend
lists affect the observed behavior and temporal patterns. Finally, we show that
Facebook users appear to be clustered by school with respect to their temporal
messaging patterns
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