30,280 research outputs found
The conductance of a multi-mode ballistic ring: beyond Landauer and Kubo
The Landauer conductance of a two terminal device equals to the number of
open modes in the weak scattering limit. What is the corresponding result if we
close the system into a ring? Is it still bounded by the number of open modes?
Or is it unbounded as in the semi-classical (Drude) analysis? It turns out that
the calculation of the mesoscopic conductance is similar to solving a
percolation problem. The "percolation" is in energy space rather than in real
space. The non-universal structures and the sparsity of the perturbation matrix
cannot be ignored.Comment: 7 pages, 8 figures, with the correct version of Figs.6-
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
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
Rate of energy absorption by a closed ballistic ring
We make a distinction between the spectroscopic and the mesoscopic
conductance of closed systems. We show that the latter is not simply related to
the Landauer conductance of the corresponding open system. A new ingredient in
the theory is related to the non-universal structure of the perturbation matrix
which is generic for quantum chaotic systems. These structures may created
bottlenecks that suppress the diffusion in energy space, and hence the rate of
energy absorption. The resulting effect is not merely quantitative: For a
ring-dot system we find that a smaller Landauer conductance implies a smaller
spectroscopic conductance, while the mesoscopic conductance increases. Our
considerations open the way towards a realistic theory of dissipation in closed
mesoscopic ballistic devices.Comment: 18 pages, 5 figures, published version with updated ref
The Quantum-Classical Crossover in the Adiabatic Response of Chaotic Systems
The autocorrelation function of the force acting on a slow classical system,
resulting from interaction with a fast quantum system is calculated following
Berry-Robbins and Jarzynski within the leading order correction to the
adiabatic approximation. The time integral of the autocorrelation function is
proportional to the rate of dissipation. The fast quantum system is assumed to
be chaotic in the classical limit for each configuration of the slow system. An
analytic formula is obtained for the finite time integral of the correlation
function, in the framework of random matrix theory (RMT), for a specific
dependence on the adiabatically varying parameter. Extension to a wider class
of RMT models is discussed. For the Gaussian unitary and symplectic ensembles
for long times the time integral of the correlation function vanishes or falls
off as a Gaussian with a characteristic time that is proportional to the
Heisenberg time, depending on the details of the model. The fall off is
inversely proportional to time for the Gaussian orthogonal ensemble. The
correlation function is found to be dominated by the nearest neighbor level
spacings. It was calculated for a variety of nearest neighbor level spacing
distributions, including ones that do not originate from RMT ensembles. The
various approximate formulas obtained are tested numerically in RMT. The
results shed light on the quantum to classical crossover for chaotic systems.
The implications on the possibility to experimentally observe deterministic
friction are discussed.Comment: 26 pages, including 6 figure
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
Bridging the gap between social tagging and semantic annotation: E.D. the Entity Describer
Semantic annotation enables the development of efficient computational methods for analyzing and interacting with information, thus maximizing its value. With the already substantial and constantly expanding data generation capacity of the life sciences as well as the concomitant increase in the knowledge distributed in scientific articles, new ways to produce semantic annotations of this information are crucial. While automated techniques certainly facilitate the process, manual annotation remains the gold standard in most domains. In this manuscript, we describe a prototype mass-collaborative semantic annotation system that, by distributing the annotation workload across the broad community of biomedical researchers, may help to produce the volume of meaningful annotations needed by modern biomedical science. We present E.D., the Entity Describer, a mashup of the Connotea social tagging system, an index of semantic web-accessible controlled vocabularies, and a new public RDF database for storing social semantic annotations
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
The effect of stellar-mass black holes on the structural evolution of massive star clusters
We present the results of realistic N-body modelling of massive star clusters
in the Magellanic Clouds, aimed at investigating a dynamical origin for the
radius-age trend observed in these systems. We find that stellar-mass black
holes, formed in the supernova explosions of the most massive cluster stars,
can constitute a dynamically important population. If a significant number of
black holes are retained (here we assume complete retention), these objects
rapidly form a dense core where interactions are common, resulting in the
scattering of black holes into the cluster halo, and the ejection of black
holes from the cluster. These two processes heat the stellar component,
resulting in prolonged core expansion of a magnitude matching the observations.
Significant core evolution is also observed in Magellanic Cloud clusters at
early times. We find that this does not result from the action of black holes,
but can be reproduced by the effects of mass-loss due to rapid stellar
evolution in a primordially mass segregated cluster.Comment: Accepted for publication in MNRAS Letters; 2 figures, 1 tabl
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