148 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.
Optimal efficiency of quantum transport in a disordered trimer
Disordered quantum networks, as those describing light-harvesting complexes,
are often characterized by the presence of peripheral ring-like structures,
where the excitation is initialized, and inner reaction centers (RC), where the
excitation is trapped. The peripheral rings display coherent features: their
eigenstates can be separated in the two classes of superradiant and subradiant
states. Both are important to optimize transfer efficiency. In the absence of
disorder, superradiant states have an enhanced coupling strength to the RC,
while the subradiant ones are basically decoupled from it. Static on-site
disorder induces a coupling between subradiant and superradiant states,
creating an indirect coupling to the RC. The problem of finding the optimal
transfer conditions, as a function of both the RC energy and the disorder
strength, is very complex even in the simplest network, namely a three-level
system. In this paper we analyze such trimeric structure choosing as initial
condition a subradiant state, rather than the more common choice of an
excitation localized on a site. We show that, while the optimal disorder is of
the order of the superradiant coupling, the optimal detuning between the
initial state and the RC energy strongly depends on system parameters: when the
superradiant coupling is much larger than the energy gap between the
superradiant and the subradiant levels, optimal transfer occurs if the RC
energy is at resonance with the subradiant initial state, whereas we find an
optimal RC energy at resonance with a virtual dressed state when the
superradiant coupling is smaller than or comparable with the gap. The presence
of dynamical noise, which induces dephasing and decoherence, affects the
resonance structure of energy transfer producing an additional 'incoherent'
resonance peak, which corresponds to the RC energy being equal to the energy of
the superradiant state.Comment: This article shares part of the introduction and most of Section II
with arXiv:1508.01613, the remaining parts of the two articles treat
different problem
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
Open system of interacting fermions: Statistical properties of cross sections and fluctuations
Statistical properties of cross sections are studied for an open system of
interacting fermions. The description is based on the effective non-Hermitian
Hamiltonian that accounts for the existence of open decay channels preserving
the unitarity of the scattering matrix. The intrinsic interaction is modelled
by the two-body random ensemble of variable strength. In particular, the
crossover region from isolated to overlapping resonances accompanied by the
effect of the width redistribution creating super-radiant and trapped states is
studied in detail. The important observables, such as average cross section,
its fluctuations, autocorrelation functions of the cross section and scattering
matrix, are very sensitive to the coupling of the intrinsic states to the
continuum around the crossover. A detailed comparison is made of our results
with standard predictions of statistical theory of cross sections, such as the
Hauser-Feshbach formula for the average cross section and Ericson theory of
fluctuations and correlations of cross sections. Strong deviations are found in
the crossover region, along with the dependence on intrinsic interactions and
degree of chaos inside the system.Comment: 13 pages, 11 figure
Optimal Dephasing for Ballistic Energy Transfer in Disordered Linear Chains
We study the interplay between dephasing, disorder, and openness on transport
efficiency in a one-dimensional chain of finite length , and in particular
the beneficial or detrimental effect of dephasing on transport. The excitation
moves along the chain by coherent nearest-neighbor hopping , under the
action of static disorder and dephasing . The system is open due to
the coupling of the last site with an external acceptor system (sink), where
the excitation can be trapped with a rate , which determines
the opening strength. While it is known that dephasing can help transport in
the localized regime, here we show that dephasing can enhance energy transfer
even in the ballistic regime. Specifically, in the localized regime we recover
previous results, where the optimal dephasing is independent of the chain
length and proportional to or . In the ballistic regime, the
optimal dephasing decreases as or respectively for weak and
moderate static disorder. When focusing on the excitation starting at the
beginning of the chain, dephasing can help excitation transfer only above a
critical value of disorder , which strongly depends on the opening
strength . Analytic solutions are obtained for short chains.Comment: 16 pages, inlcuding 9 figure
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
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