263 research outputs found
Strongly correlated hopping and many-body bound states
We study a system in which the quantum dynamics of electrons depend on the
particle density in their neighborhood. For any on-site repulsive interaction,
we show that the exact two-body and three-body ground states are bound states.
We also discuss the finite density case in a mean-field framework and we show
that the system can undergo an unusual transition from an effective attractive
interaction to a repulsive one, when varying the electron density.Comment: 6 pages, 6 EPS figures, minor modifications and references adde
Corpuscular Event-by-Event Simulation of Quantum Optics Experiments: Application to a Quantum-Controlled Delayed-Choice Experiment
A corpuscular simulation model of optical phenomena that does not require the
knowledge of the solution of a wave equation of the whole system and reproduces
the results of Maxwell's theory by generating detection events one-by-one is
discussed. The event-based corpuscular model gives a unified description of
multiple-beam fringes of a plane parallel plate and single-photon Mach-Zehnder
interferometer, Wheeler's delayed choice, photon tunneling, quantum eraser,
two-beam interference, Einstein-Podolsky-Rosen-Bohm and Hanbury Brown-Twiss
experiments. The approach is illustrated by application to a recent proposal
for a quantum-controlled delayed choice experiment, demonstrating that also
this thought experiment can be understood in terms of particle processes only.Comment: Invited paper presented at FQMT11. Accepted for publication in
Physica Scripta 27 June 201
Equilibrium and nonequilibrium contributions to x-ray scattering from supercooled water
The relation between stellar populations, structure and environment for dwarf elliptical galaxies from the MAGPOP-ITP
Dwarf galaxies, as the most numerous type of galaxy, offer the potential to
study galaxy formation and evolution in detail in the nearby Universe. Although
they seem to be simple systems at first view, they remain poorly understood. In
an attempt to alleviate this situation, the MAGPOP EU Research and Training
Network embarked on a study of dwarf galaxies named MAGPOP-ITP (Peletier et
al., 2007). In this paper, we present the analysis of a sample of 24 dwarf
elliptical galaxies (dEs) in the Virgo Cluster and in the field, using optical
long-slit spectroscopy. We examine their stellar populations in combination
with their light distribution and environment. We confirm and strengthen
previous results that dEs are, on average, younger and more metal-poor than
normal elliptical galaxies, and that their [alpha/Fe] abundance ratios scatter
around solar. This is in accordance with the downsizing picture of galaxy
formation where mass is the main driver for the star formation history. We also
find new correlations between the luminosity-weighted mean age, the large-scale
asymmetry, and the projected Virgocentric distance. We find that environment
plays an important role in the termination of the star formation activity by
ram pressure stripping of the gas in short timescales, and in the
transformation of disky dwarfs to more spheroidal objects by harassment over
longer timescales. This points towards a continuing infall scenario for the
evolution of dEs.Comment: Accepted for publication in MNRAS: 22 pages, 13 figures and 9 table
Phase Diagram of the Extended Hubbard Model with Correlated Hopping Interaction
A one-dimensional model of interacting electrons with on-site ,
nearest-neighbor , and correlated-hopping interaction is studied
at half-filling using the continuum-limit field theory approach. The ground
state phase diagram is obtained for a wide range of coupling constants. In
addition to the insulating spin- and charge-density wave phases for large
and , respectively, we identify bond-located ordered phases corresponding to
an enhanced Peierls instability in the system for ,
and to a staggered magnetization located on bonds between
sites for , . The general ground state phase
diagram including insulating, metallic, and superconducting phases is
discussed.Comment: 8 pages, 4 eps-figure
Superconductivity from correlated hopping
We consider a chain described by a next-nearest-neighbor hopping combined
with a nearest-neighbor spin flip. In two dimensions this three-body term
arises from a mapping of the three-band Hubbard model for CuO planes to a
generalized model and for large O-O hopping favors resonance-valence-bond
superconductivity of predominantly -wave symmetry. Solving the ground state
and low-energy excitations by analytical and numerical methods we find that the
chain is a Luther-Emery liquid with correlation exponent , where is the particle density.Comment: 10 pages, RevTeX 3.0 + 2 PostScript figs. Accepted for publication in
Phys.Rev.
Formation and evolution of dwarf early-type galaxies in the Virgo cluster I. Internal kinematics
We present new medium resolution kinematic data for a sample of 21 dwarf
early-type galaxies (dEs) mainly in the Virgo cluster, obtained with the WHT
and INT telescopes at the Roque de los Muchachos Observatory (La Palma, Spain).
These data are used to study the origin of the dwarf elliptical galaxy
population inhabiting clusters. We confirm that dEs are not dark matter
dominated galaxies, at least up to the half-light radius. We also find that the
observed galaxies in the outer parts of the cluster are mostly rotationally
supported systems with disky morphological shapes. Rotationally supported dEs
have rotation curves similar to those of star forming galaxies of similar
luminosity and follow the Tully-Fisher relation. This is expected if dE
galaxies are the descendant of low luminosity star forming systems which
recently entered the cluster environment and lost their gas due to a ram
pressure stripping event, quenching their star formation activity and
transforming into quiescent systems, but conserving their angular momentum.Comment: 24 pages, 15 figures and 7 tables. Replaced to match the journal
versio
The need to promote behaviour change at the cultural level: one factor explaining the limited impact of the MEMA kwa Vijana adolescent sexual health intervention in rural Tanzania. A process evaluation
Background - Few of the many behavioral sexual health interventions in Africa have been rigorously evaluated. Where biological outcomes have been measured, improvements have rarely been found. One of the most rigorous trials was of the multi-component MEMA kwa Vijana adolescent sexual health programme, which showed improvements in knowledge and reported attitudes and behaviour, but none in biological outcomes. This paper attempts to explain these outcomes by reviewing the process evaluation findings, particularly in terms of contextual factors.
Methods - A large-scale, primarily qualitative process evaluation based mainly on participant observation identified the principal contextual barriers and facilitators of behavioural change.
Results - The contextual barriers involved four interrelated socio-structural factors: culture (i.e. shared practices and systems of belief), economic circumstances, social status, and gender. At an individual level they appeared to operate through the constructs of the theories underlying MEMA kwa Vijana - Social Cognitive Theory and the Theory of Reasoned Action – but the intervention was unable to substantially modify these individual-level constructs, apart from knowledge.
Conclusion - The process evaluation suggests that one important reason for this failure is that the intervention did not operate sufficiently at a structural level, particularly in regard to culture. Recently most structural interventions have focused on gender or/and economics. Complementing these with a cultural approach could address the belief systems that justify and perpetuate gender and economic inequalities, as well as other barriers to behaviour change
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