3,665 research outputs found
Effect of noise for two interacting particles in a random potential
We investigated the effect of noise on propagation of two interacting
particles pairs in a quasi one--dimensional random potential. It is shown that
pair diffusion is strongly enhanced by short range interaction comparing with
the non--interacting case.Comment: 8 Latex pages + 3 postscript figures uu- compressed submitted to
Europhysics Letter
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
Quantum Chaos and Thermalization in Isolated Systems of Interacting Particles
This review is devoted to the problem of thermalization in a small isolated
conglomerate of interacting constituents. A variety of physically important
systems of intensive current interest belong to this category: complex atoms,
molecules (including biological molecules), nuclei, small devices of condensed
matter and quantum optics on nano- and micro-scale, cold atoms in optical
lattices, ion traps. Physical implementations of quantum computers, where there
are many interacting qubits, also fall into this group. Statistical
regularities come into play through inter-particle interactions, which have two
fundamental components: mean field, that along with external conditions, forms
the regular component of the dynamics, and residual interactions responsible
for the complex structure of the actual stationary states. At sufficiently high
level density, the stationary states become exceedingly complicated
superpositions of simple quasiparticle excitations. At this stage, regularities
typical of quantum chaos emerge and bring in signatures of thermalization. We
describe all the stages and the results of the processes leading to
thermalization, using analytical and massive numerical examples for realistic
atomic, nuclear, and spin systems, as well as for models with random
parameters. The structure of stationary states, strength functions of simple
configurations, and concepts of entropy and temperature in application to
isolated mesoscopic systems are discussed in detail. We conclude with a
schematic discussion of the time evolution of such systems to equilibrium.Comment: 69 pages, 31 figure
A semiquantal approach to finite systems of interacting particles
A novel approach is suggested for the statistical description of quantum
systems of interacting particles. The key point of this approach is that a
typical eigenstate in the energy representation (shape of eigenstates, SE) has
a well defined classical analog which can be easily obtained from the classical
equations of motion. Therefore, the occupation numbers for single-particle
states can be represented as a convolution of the classical SE with the quantum
occupation number operator for non-interacting particles. The latter takes into
account the wavefunctions symmetry and depends on the unperturbed energy
spectrum only. As a result, the distribution of occupation numbers can be
numerically found for a very large number of interacting particles. Using the
model of interacting spins we demonstrate that this approach gives a correct
description of even in a deep quantum region with few single-particle
orbitals.Comment: 4 pages, 2 figure
Cooperative shielding in many-body systems with long-range interaction
In recent experiments with ion traps, long-range interactions were associated
with the exceptionally fast propagation of perturbation, while in some
theoretical works they have also been related with the suppression of
propagation. Here, we show that such apparently contradictory behavior is
caused by a general property of long-range interacting systems, which we name
"Cooperative Shielding". It refers to shielded subspaces that emerge as the
system size increases and inside of which the evolution is unaffected by
long-range interactions for a long time. As a result, the dynamics strongly
depends on the initial state: if it belongs to a shielded subspace, the
spreading of perturbation satisfies the Lieb-Robinson bound and may even be
suppressed, while for initial states with components in various subspaces, the
propagation may be quasi-instantaneous. We establish an analogy between the
shielding effect and the onset of quantum Zeno subspaces. The derived effective
Zeno Hamiltonian successfully describes the short-ranged dynamics inside the
subspaces up to a time scale that increases with system size. Cooperative
Shielding can be tested in current experiments with trapped ions.Comment: 9 pages, 5 figures (accepted at Phys. Rev. Lett.
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.
Is the literacy achievement of teenage boys poorer than that of teenage girls, or do estimates of gender gaps depend on the test? A comparison of PISA and PIAAC
Data from international large-scale assessments (ILSAs) of schooled populations indicate that boys have considerably poorer literacy skills than girls. New evidence from a household-based ILSA—Organisation for Economic Cooperation and Development Survey of Adult Skills (PIAAC)—indicates that the gender gap in literacy is negligible, even though its assessment framework is similar to that of one of the most widely used school-based assessments, the Program for International Student Assessment (PISA). Individual-level data from 15-, 16-, and 17-year-old teens in countries that administered both assessments were used to estimate and compare literacy gender gaps in the two assessments, after accounting for differences in target population, response rates, scoring scheme, test length, mode of delivery, prevalence of items involving different stimuli in the two assessments (e.g., types of texts), and cognitive processes test-takers need to engage in to solve assessment items (e.g., accessing and retrieving information or reflecting and evaluating information presented in the text). These differences explain only part of the differences across the two studies in estimated literacy gender gaps: Even when these factors are considered, gender gaps remain large in PISA and small (though imprecisely estimated) in PIAAC. The potential roles of test-taking motivation and administration conditions in explaining differences across the studies and implications for research and policy are discussed
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