7,766 research outputs found
Real trajectories in the semiclassical coherent state propagator
The semiclassical approximation to the coherent state propagator requires
complex classical trajectories in order to satisfy the associated boundary
conditions, but finding these trajectories in practice is a difficult task that
may compromise the applicability of the approximation. In this work several
approximations to the coherent state propagator are derived that make use only
of real trajectories, which are easier to handle and have a more direct
physical interpretation. It is verified in a particular example that these real
trajectories approximations may have excellent accuracy.Comment: 19 pages, 6 figure
Comb entanglement in quantum spin chains
Bipartite entanglement in the ground state of a chain of quantum spins
can be quantified either by computing pairwise concurrence or by dividing the
chain into two complementary subsystems. In the latter case the smaller
subsystem is usually a single spin or a block of adjacent spins and the
entanglement differentiates between critical and non-critical regimes. Here we
extend this approach by considering a more general setting: our smaller
subsystem consists of a {\it comb} of spins, spaced sites apart.
Our results are thus not restricted to a simple `area law', but contain
non-local information, parameterized by the spacing . For the XX model we
calculate the von-Neumann entropy analytically when and
investigate its dependence on and . We find that an external magnetic
field induces an unexpected length scale for entanglement in this case.Comment: 6 pages, 4 figure
Simultaneous occurrence of sliding and crossing limit cycles in piecewise linear planar vector fields
In the present study we consider planar piecewise linear vector fields with
two zones separated by the straight line . Our goal is to study the
existence of simultaneous crossing and sliding limit cycles for such a class of
vector fields. First, we provide a canonical form for these systems assuming
that each linear system has center, a real one for and a virtual one for
, and such that the real center is a global center. Then, working with a
first order piecewise linear perturbation we obtain piecewise linear
differential systems with three crossing limit cycles. Second, we see that a
sliding cycle can be detected after a second order piecewise linear
perturbation. Finally, imposing the existence of a sliding limit cycle we prove
that only one adittional crossing limit cycle can appear. Furthermore, we also
characterize the stability of the higher amplitude limit cycle and of the
infinity. The main techniques used in our proofs are the Melnikov method, the
Extended Chebyshev systems with positive accuracy, and the Bendixson
transformation.Comment: 24 pages, 7 figure
Quantum chaotic resonances from short periodic orbits
We present an approach to calculating the quantum resonances and resonance
wave functions of chaotic scattering systems, based on the construction of
states localized on classical periodic orbits and adapted to the dynamics.
Typically only a few of such states are necessary for constructing a resonance.
Using only short orbits (with periods up to the Ehrenfest time), we obtain
approximations to the longest living states, avoiding computation of the
background of short living states. This makes our approach considerably more
efficient than previous ones. The number of long lived states produced within
our formulation is in agreement with the fractal Weyl law conjectured recently
in this setting. We confirm the accuracy of the approximations using the open
quantum baker map as an example.Comment: 4 pages, 4 figure
A new correlator in quantum spin chains
We propose a new correlator in one-dimensional quantum spin chains, the
Emptiness Formation Probability (EFP). This is a natural generalization
of the Emptiness Formation Probability (EFP), which is the probability that the
first spins of the chain are all aligned downwards. In the EFP we let
the spins in question be separated by sites. The usual EFP corresponds to
the special case when , and taking allows us to quantify non-local
correlations. We express the EFP for the anisotropic XY model in a
transverse magnetic field, a system with both critical and non-critical
regimes, in terms of a Toeplitz determinant. For the isotropic XY model we find
that the magnetic field induces an interesting length scale.Comment: 6 pages, 1 figur
Supersymmetric Higgs pair discovery prospects at hadron colliders
We study the potential of hadron colliders in the search for the pair
production of neutral Higgs bosons in the framework of the Minimal
Supersymmetric Standard Model. Using analytical expressions for the relevant
amplitudes, we perform a detailed signal and background analysis, working out
efficient kinematical cuts for the extraction of the signal. The important role
of squark loop contributions to the signal is emphasised. If the signal is
sufficiently enhanced by these contributions, it could even be observable at
the next run of the upgraded Tevatron collider in the near future. At the LHC
the pair production of light and heavy Higgs bosons might be detectable
simultaneously.Comment: 5 pages, hep99, 6 figures; Presented at the International Europhysics
Conference on High Energy Physics, Tampere, Finland, 15-21 July 199
Neurocognitive assessment in alcohol abuse and dependence: implications for treatment
The aim of the Neuropsychology applied to the Alcohol Dependence field is the comprehension of the effects of brain dysfunction on cognition and human behavior. It investigates neurocognitive impairments and associates them to structural and functional neuroimaging findings (CT, MRI, PET and SPECT). Acute use of alcohol impairs attention, memory, executive functions and visuospatial skills, while chronic abuse causes neurocognitive deficits in memory, learning, visuospatial functions, psychomotor speed processing, executive functions and decision-making, and may lead to persistent amnesic disorder and alcoholic dementia. Executive Dysfunction related to frontal lobe has direct implications on treatment, by the choosing of strategies and for prognostic evaluation. It is presented an easy tool to screen cognitive impairments, the Frontal Assessment Battery - FAB.15 Neuropsychological Assessment is useful for early detection of impairments and evaluation of their evolution and Cognitive Rehabilitation has a significant role on deficits recovery and psychosocial adjustment of these patients.A Neuropsicologia, aplicada ao abuso e dependência do álcool, busca a compreensão da relação entre danos cerebrais e seus efeitos na cognição e no comportamento do indivíduo. Estuda, ainda, os comprometimentos neurocognitivos dos pacientes, relacionando-os a achados estruturais e funcionais de neuroimagem (TC, RM, PET e SPECT). No uso agudo, o álcool tende a comprometer a atenção, memória, funções executivas e viso-espaciais, enquanto no uso crônico altera a memória, aprendizagem, análise e síntese viso-espacial, velocidade psicomotora, funções executivas e tomada de decisões, podendo chegar a transtornos persistentes de memória e demência alcoólica. Os déficits cognitivos encontrados nos dependentes de álcool, principalmente das funções executivas (frontais), têm implicação direta no tratamento, tanto para a escolha de estratégias a serem adotadas como para a análise do prognóstico. Ao final do artigo, é apresentado um instrumento útil e breve para rastreio de alterações cognitivas, a Bateria de Avaliação Frontal - FAB.15 Acredita-se que a Avaliação Neuropsicológica pode ser muito importante para a detecção e avaliação da progressão destas alterações e que a reabilitação cognitiva tem papel relevante na recuperação dos déficits e reinserção psicossocial destes pacientes.USP Faculdade de Medicina Instituto e Departamento de PsiquiatriaEscola Paulista de Medicina Unidade de Pesquisa em Álcool e DrogasUNIFESP, EPM, Unidade de Pesquisa em Álcool e DrogasSciEL
Oxygen clamps in gold nanowires
We investigate how the insertion of an oxygen atom in an atomically thin gold
nanowire can affect its rupture. We find, using ab initio total energy density
functional theory calculations, that O atoms when inserted in gold nanowires
form not only stable but also very strong bonds, in such a way that they can
extract atoms from a stable tip, serving in this way as a clamp that could be
used to pull a string of gold atoms.Comment: 4 pages; 4 figure
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