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 $N$ 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 $S_A$ consists of a {\it comb} of $L$ spins, spaced $p$ sites apart. Our results are thus not restricted to a simple `area law', but contain non-local information, parameterized by the spacing $p$. For the XX model we calculate the von-Neumann entropy analytically when $N\to \infty$ and investigate its dependence on $L$ and $p$. 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 $x=0$. 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 $y<0$ and a virtual one for $y>0$, 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 $s-$Emptiness Formation Probability ($s-$EFP). This is a natural generalization of the Emptiness Formation Probability (EFP), which is the probability that the first $n$ spins of the chain are all aligned downwards. In the $s-$EFP we let the spins in question be separated by $s$ sites. The usual EFP corresponds to the special case when $s=1$, and taking $s>1$ allows us to quantify non-local correlations. We express the $s-$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