1,188 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
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
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
Multicritical behavior of two-dimensional anisotropic antiferromagnets in a magnetic field
We study the phase diagram and multicritical behavior of anisotropic
Heisenberg antiferromagnets on a square lattice in the presence of a magnetic
field along the easy axis. We argue that, beside the Ising and XY critical
lines, the phase diagram presents a first-order spin-flop line starting from
T=0, as in the three-dimensional case. By using field theory we show that the
multicritical point where these transition lines meet cannot be O(3) symmetric
and occurs at finite temperature. We also predict how the critical temperature
of the transition lines varies with the magnetic field and the uniaxial
anisotropy in the limit of weak anisotropy.Comment: 21 pages, 8 fig
A neurodevelopment approach for a transitional model of early onset schizophrenia
In the last decades, the conceptualization of schizophrenia has dramatically changed, moving from a neurodegenerative process occurring in early adult life to a neurodevelopmental disorder starting be-fore birth, showing a variety of premorbid and prodromal symptoms and, in relatively few cases, evolving in the full-blown psychotic syndrome. High rates of co-occurring different neurodevelopmental disorders such as Autism spectrum disorder and ADHD, predating the onset of SCZ, and neurobio-logical underpinning with significant similarities, support the notion of a pan-developmental disturbance consisting of impairments in neuromotor, receptive language, social and cognitive development. Con-sidering that many SCZ risk factors may be similar to symptoms of other neurodevelopmental psychi-atric disorders, transition processes from child & adolescent to adult systems of care should include both high risk people as well as subject with other neurodevelopmental psychiatric disorders with different levels of severity. This descriptive mini-review discuss the need of innovative clinical approaches, re-considering specific diagnostic categories, stimulating a careful analysis of risk factors and promoting the appropriate use of new and safer medications
High-precision estimate of g4 in the 2D Ising model
We compute the renormalized four-point coupling in the 2d Ising model using
transfer-matrix techniques. We greatly reduce the systematic uncertainties
which usually affect this type of calculations by using the exact knowledge of
several terms in the scaling function of the free energy. Our final result is
g4=14.69735(3).Comment: 17 pages, revised version with minor changes, accepted for
publication in Journal of Physics
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