1,485 research outputs found
Transitions and crossover phenomena in fully frustrated XY systems
We study the two-dimensional fully frustrated XY (FFXY) model and two related
models, a discretization of the Landau-Ginzburg-Wilson Hamiltonian for the
critical modes of the FFXY model and a coupled Ising-XY model, by means of
Monte Carlo simulations on square lattices L x L, L=O(10^3). We show that their
phase diagram is characterized by two very close chiral and spin transitions,
at T_ch > T_sp respectively, of the Ising and Kosterlitz-Thouless type. At T_ch
the Ising regime sets in only after a preasymptotic regime, which appears
universal to some extent. The approach is nonmonotonic for most observables,
with a wide region controlled by an effective exponent nu_eff=0.8.Comment: 9 page
Quantum critical behavior and trap-size scaling of trapped bosons in a one-dimensional optical lattice
We study the quantum (zero-temperature) critical behaviors of confined
particle systems described by the one-dimensional (1D) Bose-Hubbard model in
the presence of a confining potential, at the Mott insulator to superfluid
transitions, and within the gapless superfluid phase. Specifically, we consider
the hard-core limit of the model, which allows us to study the effects of the
confining potential by exact and very accurate numerical results. We analyze
the quantum critical behaviors in the large trap-size limit within the
framework of the trap-size scaling (TSS) theory, which introduces a new trap
exponent theta to describe the dependence on the trap size. This study is
relevant for experiments of confined quasi 1D cold atom systems in optical
lattices. At the low-density Mott transition TSS can be shown analytically
within the spinless fermion representation of the hard-core limit. The
trap-size dependence turns out to be more subtle in the other critical regions,
when the corresponding homogeneous system has a nonzero filling f, showing an
infinite number of level crossings of the lowest states when increasing the
trap size. At the n=1 Mott transition this gives rise to a modulated TSS: the
TSS is still controlled by the trap-size exponent theta, but it gets modulated
by periodic functions of the trap size. Modulations of the asymptotic power-law
behavior is also found in the gapless superfluid region, with additional
multiscaling behaviors.Comment: 26 pages, 34 figure
Health activism and the logic of connective action. A case study of rare disease patient organisations
This exploratory work investigates the role of digital media in expanding health discourse practices in a way to transform traditional structures of agency in public health. By focusing on a sample of rare disease patient organisations as representative of contemporary health activism, this study investigates the role of digital communication in the development of (1) bottom-up sharing and co-production of health knowledge, (2) health public engagement dynamics and (3) health information pathways. Findings show that digital media affordances for patient organisations go beyond the provision of social support for patient communities; they ease one-way, two-way and crowdsourced processes of health knowledge sharing, exchange and co-production, provide personalised routes to health public engagement and bolster the emergence of varied pathways to health information where experiential knowledge and medical authority are equally valued. These forms of organisationally enabled connective action can help the surfacing of personal narratives that strengthen patient communities, the bottom-up production of health knowledge relevant to a wider public and the development of an informational and eventually cultural context that eases patients’ political action
Interplay between temperature and trap effects in one-dimensional lattice systems of bosonic particles
We investigate the interplay of temperature and trap effects in cold particle
systems at their quantum critical regime, such as cold bosonic atoms in optical
lattices at the transitions between Mott-insulator and superfluid phases. The
theoretical framework is provided by the one-dimensional Bose-Hubbard model in
the presence of an external trapping potential, and the trap-size scaling
theory describing the large trap-size behavior at a quantum critical point. We
present numerical results for the low-temperature behavior of the particle
density and the density-density correlation function at the Mott transitions,
and within the gapless superfluid phase.Comment: 9 page
One platform, a thousand worlds: On Twitter irony in the early response to the COVID-19 pandemic in Italy
On 22 February 2020, 11 municipalities in Northern Italy became the first COVID-19 red zone of Europe. Two days later, when it became evident that the virus had been spreading in the country for weeks, Italy entered a “buffer zone,” a temporal zone between normality and pandemic. The buffer zone lasted around 2 weeks and thrived with irony flowing on social media through memes, multimedia remixes, and jokes. As a collective ritual, irony allowed people to temporarily background the mounting feelings of bewilderment and uncertainty by foregrounding the familiar scripts of playful and grassroots expressivity typical of networked publics. While giving the country a way to breathe before grieving, irony delivered both traditional political satire and new symbolic arrangements to frame “us” versus “them”: Northern Italy versus Southern Italy, Italy versus China. We advance initial reflections on irony and its functions during what we call Italy’s COVID-19 buffer zone and argue for the need of more platform research interested in how users appropriate devices and vernaculars in ways that are culturally bound. In other words, can we rethink “The Platform” (e.g., Twitter, Facebook, Instagram) as a constellation of small-world-platforms—sometimes overlapping, other times segregating—each shaped by local hopes and fears, histories and events
Quantum dynamics and entanglement of a 1D Fermi gas released from a trap
We investigate the entanglement properties of the nonequilibrium dynamics of
one-dimensional noninteracting Fermi gases released from a trap. The gas of N
particles is initially in the ground state within hard-wall or harmonic traps,
then it expands after dropping the trap. We compute the time dependence of the
von Neumann and Renyi entanglement entropies and the particle fluctuations of
spatial intervals around the original trap, in the limit of a large number N of
particles. The results for these observables apply to one-dimensional gases of
impenetrable bosons as well.
We identify different dynamical regimes at small and large times, depending
also on the initial condition, whether it is that of a hard-wall or harmonic
trap. In particular, we analytically show that the expansion from hard-wall
traps is characterized by the asymptotic small-time behavior of the von Neumann entanglement entropy, and the relation
where V is the particle variance, which are analogous to
the equilibrium behaviors whose leading logarithms are essentially determined
by the corresponding conformal field theory with central charge . The time
dependence of the entanglement entropy of extended regions during the expansion
from harmonic traps shows the remarkable property that it can be expressed as a
global time-dependent rescaling of the space dependence of the initial
equilibrium entanglement entropy.Comment: 19 pages, 18 fig
Entanglement and particle correlations of Fermi gases in harmonic traps
We investigate quantum correlations in the ground state of noninteracting
Fermi gases of N particles trapped by an external space-dependent harmonic
potential, in any dimension. For this purpose, we compute one-particle
correlations, particle fluctuations and bipartite entanglement entropies of
extended space regions, and study their large-N scaling behaviors. The
half-space von Neumann entanglement entropy is computed for any dimension,
obtaining S_HS = c_l N^(d-1)/d ln N, analogously to homogenous systems, with
c_l=1/6, 1/(6\sqrt{2}), 1/(6\sqrt{6}) in one, two and three dimensions
respectively. We show that the asymptotic large-N relation S_A\approx \pi^2
V_A/3, between the von Neumann entanglement entropy S_A and particle variance
V_A of an extended space region A, holds for any subsystem A and in any
dimension, analogously to homogeneous noninteracting Fermi gases.Comment: 15 pages, 22 fig
Trap-size scaling in confined particle systems at quantum transitions
We develop a trap-size scaling theory for trapped particle systems at quantum
transitions. As a theoretical laboratory, we consider a quantum XY chain in an
external transverse field acting as a trap for the spinless fermions of its
quadratic Hamiltonian representation. We discuss trap-size scaling at the Mott
insulator to superfluid transition in the Bose-Hubbard model. We present exact
and accurate numerical results for the XY chain and for the low-density Mott
transition in the hard-core limit of the one-dimensional Bose-Hubbard model.
Our results are relevant for systems of cold atomic gases in optical lattices.Comment: Revised and greatly expanded. 15 pages, 7 figure
Are the deficits in navigational abilities present in the Williams syndrome related to deficits in the backward inhibition?
Williams syndrome (WS) is associated with a distinct profile of relatively proficient skills within the verbal domain compared to the severe impairment of visuo-spatial processing. Abnormalities in executive functions and deficits in planning ability and spatial working memory have been described. However, to date little is known about the influence of executive function deficits on navigational abilities in WS. This study aimed at analyzing in WS individuals a specific executive function, the backward inhibition (BI) that allows individuals to flexibly adapt to continuously changing environments. A group of WS individuals and a mental age- and gender-matched group of typically developing children were subjected to three task-switching experiments requiring visuospatial or verbal material to be processed. Results showed that WS individuals exhibited clear BI deficits during visuospatial task-switching paradigms and normal BI effect during verbal task-switching paradigm. Overall, the present results suggest that the BI involvement in updating environment representations during navigation may influence WS navigational abilitie
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