122 research outputs found
Cooperative Robustness to Static Disorder: Superradiance and localization in a nanoscale ring to model natural light-harvesting systems
We analyze a 1-d ring structure composed of many two-level systems, in the
limit where only one excitation is present. The two-level systems are coupled
to a common environment, where the excitation can be lost, which induces super
and subradiant behavior, an example of cooperative quantum coherent effect. We
consider time-independent random fluctuations of the excitation energies. This
static disorder, also called inhomogeneous broadening in literature, induces
Anderson localization and is able to quench Superradiance. We identify two
different regimes: weak opening, in which Superradiance is quenched at the
same critical disorder at which the states of the closed system localize;
strong opening, with a critical disorder strength proportional to both the
system size and the degree of opening, displaying robustness of cooperativity
to disorder. Relevance to photosynthetic complexes is discussed.Comment: 12 pages, 7 figs., Superradiance, Anderson Localization, Cooperative
effects. Accepted for publication in Phys. Rev.
Shielding and localization in presence of long range hopping
We investigate a paradigmatic model for quantum transport with both
nearest-neighbor and infinite range hopping coupling (independent of the
position). Due to long range homogeneous hopping, a gap between the ground
state and the excited states can be induced, which is mathematically equivalent
to the superconducting gap. In the gapped regime, the dynamics within the
excited states subspace is shielded from long range hopping, namely it occurs
as if long range hopping would be absent. This is a cooperative phenomenon
since shielding is effective over a time scale which diverges with the system
size. We named this effect {\it Cooperative Shielding}. We also discuss the
consequences of our findings on Anderson localization. Long range hopping is
usually thought to destroy localization due to the fact that it induces an
infinite number of resonances. Contrary to this common lore we show that the
excited states display strong localized features when shielding is effective
even in the regime of strong long range coupling. A brief discussion on the
extension of our results to generic power-law decaying long range hopping is
also given. Our preliminary results confirms that the effects found for the
infinite range case are generic.Comment: 7 pages, 9 figur
The Topological Non-connectivity Threshold in quantum long-range interacting spin systems
Quantum characteristics of the Topological Non-connectivity Threshold (TNT),
introduced in F.Borgonovi, G.L.Celardo, M.Maianti, E.Pedersoli, J. Stat. Phys.,
116, 516 (2004), have been analyzed in the hard quantum regime. New interesting
perspectives in term of the possibility to study the intriguing
quantum-classical transition through Macroscopic Quantum Tunneling have been
addressed.Comment: contribution to NEXTSIGMAPHI 3r
Broken Ergodicity in classically chaotic spin systems
A one dimensional classically chaotic spin chain with asymmetric coupling and
two different inter-spin interactions, nearest neighbors and all-to-all, has
been considered. Depending on the interaction range, dynamical properties, as
ergodicity and chaoticity are strongly different. Indeed, even in presence of
chaoticity, the model displays a lack of ergodicity only in presence of all to
all interaction and below an energy threshold, that persists in the
thermodynamical limit. Energy threshold can be found analytically and results
can be generalized for a generic XY model with asymmetric coupling.Comment: 6 pages, 3 figure
Transition from isolated to overlapping resonances in the open system of interacting fermions
We study the statistical properties of resonance widths and spacings in an
open system of interacting fermions at the transition between isolated and
overlapping resonances, where a radical change in the width distribution
occurs. Our main interest is to reveal how this transition is influenced by the
onset of chaos in the internal dynamics as the strength of random two-body
interaction between the particles increases. We have found that in the region
of overlapped resonances, the fluctuations of the widths (rather than their
mean values) are strongly affected by the onset of an internal chaos. The
results may be applied to the analysis of neutron cross sections, as well as in
the physics of mesoscopic devices with strongly interacting electrons.Comment: 4 pages, 5 figures, corrected version, figures are replace
Evidence of diffusive fractal aggregation of TiO2 nanoparticles by femtosecond laser ablation at ambient conditions
The specific mechanisms which leads to the formation of fractal
nanostructures by pulsed laser deposition remain elusive despite intense
research efforts, motivated mainly by the technological interest in obtaining
tailored nanostructures with simple and scalable production methods. Here we
focus on fractal nanostructures of titanium dioxide, , a strategic
material for many applications, obtained by femtosecond laser ablation at
ambient conditions. We model the fractal formation through extensive Monte
Carlo simulations based on a set of minimal assumptions: irreversible sticking
and size independent diffusion. Our model is able to reproduce the fractal
dimensions and the area distributions of the nanostructures obtained in the
experiments for different densities of the ablated material. The comparison of
theory and experiment show that such fractal aggregates are formed after
landing of the ablated material on the substrate surface by a diffusive
mechanism. Finally we discuss the role of the thermal conductivity of the
substrate and the laser fluence on the properties of the fractal
nanostructures. Our results represent an advancement towards controlling the
production of fractal nanostructures by pulsed laser deposition.Comment: 21 page
Enhancement of magnetic anisotropy barrier in long range interacting spin systems
Magnetic materials are usually characterized by anisotropy energy barriers
which dictate the time scale of the magnetization decay and consequently the
magnetic stability of the sample. Here we present a unified description, which
includes coherent rotation and nucleation, for the magnetization decay in
generic anisotropic spin systems. In particular, we show that, in presence of
long range exchange interaction, the anisotropy energy barrier grows as the
volume of the particle for on site anisotropy, while it grows even faster than
the volume for exchange anisotropy, with an anisotropy energy barrier
proportional to , where is the particle volume, is the range of interaction and is the embedding dimension. These
results shows a relevant enhancement of the anisotropy energy barrier w.r.t.
the short range case, where the anisotropy energy barrier grows as the particle
cross sectional area for large particle size or large particle aspect ratio.Comment: 7 pages, 6 figures. Theory of Magnetic decay in nanosystem. Non
equilibrium statistical mechanics of many body system
Superradiance Transition in Transport Through Nanosystems
Using an energy-independent non-Hermitian Hamiltonian approach to open
systems, we fully describe transport through a sequence of potential barriers
as external barriers are varied. Analyzing the complex eigenvalues of the
non-Hermitian Hamiltonian model, a transition to a superradiant regime is shown
to occur. Transport properties undergo a strong change at the superradiance
transition, where the transmission is maximized and a drastic change in the
structure of resonances is demonstrated. Finally, we analyze the effect of the
superradiance transition in the Anderson localized regime
Focusing in Multiwell Potentials: Applications to Ion Channels
We investigate out of equilibrium stationary distributions induced by a
stochastic dichotomous noise on double and multi-well models for ion channels.
Ion-channel dynamics is analyzed both through over-damped Langevin equations
and master equations. As a consequence of the external stochastic noise, we
prove a non trivial focusing effect, namely the probability distribution is
concentrated only on one state of the multi-well model. We also show that this
focusing effect, which occurs at physiological conditions, cannot be predicted
by a simple master equation approach.Comment: 8 pages, 7 figure
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