16,516 research outputs found
Meron-cluster simulation of the quantum antiferromagnetic Heisenberg model in a magnetic field in one- and two-dimensions
Motivated by the numerical simulation of systems which display quantum phase
transitions, we present a novel application of the meron-cluster algorithm to
simulate the quantum antiferromagnetic Heisenberg model coupled to an external
uniform magnetic field both in one and in two dimensions. In the infinite
volume limit and at zero temperature we found numerical evidence that supports
a quantum phase transition very close to the critical values and
for the system in one and two dimensions, respectively. For the one
dimensional system, we have compared the numerical data obtained with
analytical predictions for the magnetization density as a function of the
external field obtained by scaling-behaviour analysis and Bethe Ansatz
techniques. Since there is no analytical solution for the two dimensional case,
we have compared our results with the magnetization density obtained by scaling
relations for small lattice sizes and with the approximated thermodynamical
limit at zero temperature guessed by scaling relations. Moreover, we have
compared the numerical data with other numerical simulations performed by using
different algorithms in one and two dimensions, like the directed loop method.
The numerical data obtained are in perfect agreement with all these previous
results, which confirms that the meron-algorithm is reliable for quantum Monte
Carlo simulations and applicable both in one and two dimensions. Finally, we
have computed the integrated autocorrelation time to measure the efficiency of
the meron algorithm in one dimension.Comment: 18 pages, 11 figure
Influence of a specific aquatic program on social and gross motor skills in adolescents with Autism Spectrum Disorders: Three case reports
Swimming pool activities revealed to be efficacious to train psychomotor skills and increase adaptive behaviors in children with Autism Spectrum Disorders (ASD). Therefore, the purpose of this study was to investigate the efficacy of a specific multi-systemic aquatic therapy (CI-MAT) on gross motor and social skills in three adolescents with Autism Spectrum Disorders (ASD). Methods: three adolescents with ASD of which two boys (M1 with a chronological age of 10.3 years and a mental age of 4.7 years; M2 with a chronological age of 14.6 and a mental age inferior to 4 years) and one girl (chronological age of 14.0 and a mental age inferior to 4 years). The study was divided into three phases: baseline, 12-week CI-MAT program and Post-Test. Participants were administered a battery of tests incorporating anthropometric measurements, gross motor development test and a social skills questionnaire before and after a 12-week MAT-CI program. Results: Subjects improved locomotors and object control skills following the CI-MAT program in a different way. Concerning social behaviors, the higher proportion of gains was observed in the sensitivity of other's presence and eye contact, for the contact domain, and in the comply turn for the relationship domain. Conclusions: The results of this study showed that the CI-MAT program was effective for the development of gross-motor skills and social behaviors in subjects with ASD. Moreover there is an urge to carry out a whole psychological assessment targeting both motor and adaptive development suitable to provide educational and vocational plans of exercises for people with ASD
Structural change in multipartite entanglement sharing: a random matrix approach
We study the typical entanglement properties of a system comprising two
independent qubit environments interacting via a shuttling ancilla. The initial
preparation of the environments is modeled using random-matrix techniques. The
entanglement measure used in our study is then averaged over many histories of
randomly prepared environmental states. Under a Heisenberg interaction model,
the average entanglement between the ancilla and one of the environments
remains constant, regardless of the preparation of the latter and the details
of the interaction. We also show that, upon suitable kinematic and dynamical
changes in the ancilla-environment subsystems, the entanglement-sharing
structure undergoes abrupt modifications associated with a change in the
multipartite entanglement class of the overall system's state. These results
are invariant with respect to the randomized initial state of the environments.Comment: 10 pages, RevTeX4 (Minor typo's corrected. Closer to published
version
Maximum power, ecological function and efficiency of an irreversible Carnot cycle. A cost and effectiveness optimization
In this work we include, for the Carnot cycle, irreversibilities of linear
finite rate of heat transferences between the heat engine and its reservoirs,
heat leak between the reservoirs and internal dissipations of the working
fluid. A first optimization of the power output, the efficiency and ecological
function of an irreversible Carnot cycle, with respect to: internal temperature
ratio, time ratio for the heat exchange and the allocation ratio of the heat
exchangers; is performed. For the second and third optimizations, the optimum
values for the time ratio and internal temperature ratio are substituted into
the equation of power and, then, the optimizations with respect to the cost and
effectiveness ratio of the heat exchangers are performed. Finally, a criterion
of partial optimization for the class of irreversible Carnot engines is herein
presented.Comment: 17 pages, 4 figures. Submitted to Energy Convers. Manag
Physical activity programs for balance and fall prevention in elderly: A systematic review
BACKGROUND:
Due to demographic changes the world's population is progressively ageing. The physiological decay of the elderly adult may lead to a reduction in the ability to balance and an increased risk of falls becoming an important issue among the elderly. In order to counteract the decay in the ability to balance, physical activity has been proven to be effective. The aim of this study is to systematically review the scientific literature in order to identify physical activity programs able to increase balance in the elderly.
METHODS:
This review is based on the data from Medline-NLM, Pubmed, ScienceDirect, and SPORTDiscuss and includes randomized control trials that have analyzed balance and physical activity in healthy elderly over 65 years of age during the last decade. A final number of 8 manuscripts were included in the qualitative synthesis, which comprised 200 elderly with a mean age of 75.1 ± 4.4 years. The sample size of the studies varied from 9 to 61 and the intervention periods from 8 to 32 weeks.
RESULTS:
Eight articles were considered eligible and included in the quantitative synthesis. The articles investigated the effects of resistance and aerobic exercise, balance training, T-bow© and wobble board training, aerobic step and stability ball training, adapted physical activity and Wii Fit training on balance outcomes. Balance measures of the studies showed improvements between 16% and 42% compared to baseline assessments.
CONCLUSIONS:
Balance is a multifactorial quality that can be effectively increased by different exercise training means. It is fundamental to promote physical activity in the aging adult, being that a negative effect on balance performance has been seen in the no-intervention control groups
Physical activity and sports practice in children and adolescents at the time of the COVID-19 pandemic: Hypothetical future scenarios and preventive practical applications
As a logical and direct consequence of the closure of schools and sports facilities, an expected reduction in the practice of physical activity (PA) and sports has been detected in both children and adolescents all over the world. Hence, we analysed the short-term and long-term consequences of the lack/low level of PA and sports activity in this population, which we referred to as primary and secondary risks, respectively, the latter of which have to be considered in hypothetical future scenarios
Collective decoherence of cold atoms coupled to a Bose-Einstein condensate
We examine the time evolution of cold atoms (impurities) interacting with an
environment consisting of a degenerate bosonic quantum gas. The impurity atoms
differ from the environment atoms, being of a different species. This allows
one to superimpose two independent trapping potentials, each being effective
only on one atomic kind, while transparent to the other. When the environment
is homogeneous and the impurities are confined in a potential consisting of a
set of double wells, the system can be described in terms of an effective
spin-boson model, where the occupation of the left or right well of each site
represents the two (pseudo)-spin states. The irreversible dynamics of such
system is here studied exactly, i.e., not in terms of a Markovian master
equation. The dynamics of one and two impurities is remarkably different in
respect of the standard decoherence of the spin - boson system. In particular
we show: i) the appearance of coherence oscillations, i) the presence of super
and sub decoherent states which differ from the standard ones of the spin boson
model, and iii) the persistence of coherence in the system at long times. We
show that this behaviour is due to the fact that the pseudospins have an
internal spatial structure. We argue that collective decoherence also prompts
information about the correlation length of the environment. In a one
dimensional configuration one can change even stronger the qualitative
behaviour of the dephasing just by tuning the interaction of the bath.Comment: 18 pages, 6 figures, two references adde
Developments of the pinned photodiode terahertz rectifier
This paper presents we presents a development of the structure of the pinned photodiode terahertz rectifier, in which the metal whisker of the antenna is separated from the semiconductor by a silane oxide layer, in order to reduce the surface defectiveness. The rectifies is the basic component of an image detection system based on the structure of actual CMOS image detectors. The structure combines a nano-antenna, fabricated on the top of a standard image sensor, the rectifier, and the readout electronics. The rectifier device proposed has vertical extension of some tenths of nanometers, can be created at the foot of the nano-whisker at the end of the terahertz antenna, above the storage well
Observable geometric phase induced by a cyclically evolving dissipative process
In a prevous paper (Phys. Rev. Lett. 96, 150403 (2006)) we have proposed a
new way to generate an observable geometric phase on a quantum system by means
of a completely incoherent phenomenon. The basic idea was to force the ground
state of the system to evolve ciclically by "adiabatically" manipulating the
environment with which it interacts. The specific scheme we have previously
analyzed, consisting of a multilevel atom interacting with a broad-band
squeezed vacuum bosonic bath whose squeezing parameters are smoothly changed in
time along a closed loop, is here solved in a more direct way. This new
solution emphasizes how the geometric phase on the ground state of the system
is indeed due to a purely incoherent dynamicsComment: 6 pages, 1 figur
- …