3,069 research outputs found
Overview of Neutron-Proton Pairing
The role of neutron-proton pairing correlations on the structure of nuclei
along the line is reviewed. Particular emphasis is placed on the
competition between isovector () and isoscalar ) pair fields. The
expected properties of these systems, in terms of pairing collective motion,
are assessed by different theoretical frameworks including schematic models,
realistic Shell Model and mean field approaches. The results are contrasted
with experimental data with the goal of establishing clear signals for the
existence of neutron-proton () condensates. We will show that there is
clear evidence for an isovector condensate as expected from isospin
invariance. However, and contrary to early expectations, a condensate of
deuteron-like pairs appears quite elusive and pairing collectivity in the
channel may only show in the form of a phonon. Arguments are presented for the
use of direct reactions, adding or removing an pair, as the most promising
tool to provide a definite answer to this intriguing question.Comment: 89 pages, 59 figures. Accepted for publication in Progress in
Particle and Nuclear Physics (ELSEVIER
Partial-wave contributions to pairing in nuclei
We present a detailed study of partial-wave contributions of nuclear forces
to pairing in nuclei. For T=1, J=0 pairing, partial waves beyond the standard
1S0 channel play an interesting role for the pair formation in nuclei. The
additional contributions are dominated by the repulsive 3P1 partial wave. Their
effects, and generally spin-triplet nuclear forces between paired nucleons, are
influenced by the interplay of spin-orbit partners. We explore the impact of
including partial waves beyond the 1S0 channel on neutron-neutron pairing gaps
in semi-magic isotopic chains. In addition, we show that nuclear forces favor
T=1, J=0 over T=0, J=1 pairing, except in low-j orbitals. This is in contrast
to the free-space motivation that suggests the formation of deuteron-like T=0
pairs in N=Z nuclei. The suppression of T=0 pairing is because the 3S1 strength
is distributed on spin-orbit partners and because of the effects of the
repulsive 1P1 channel and of D waves.Comment: 10 pages, 16 figure
Quantum motion of a spinless particle in curved space: A viewpoint of scattering theory
In this work, we study the scattering of a spinless charged particle
constrained to move on a curved surface in the presence of the Aharonov-Bohm
potential. We begin with the equations of motion for the surface and transverse
dynamics previously obtained in the literature (Ferrari G. and Cuoghi G., Phys.
Rev. Lett. \textbf{100}, 230403 (2008)) and describe the surface with
non-trivial curvature in terms of linear defects such as dislocations and
disclinations. Expressions for the modified phase shift, S--matrix and
scattering amplitude are determined by applying a suitable boundary condition
at the origin, which comes from the self-adjoint extension theory. We also
discuss the presence of a bound state obtained from the pole of the S--matrix.
Finally, we claim that the bound state, the additional scattering and the
dependence of the scattering amplitude with energy are solely due to the
curvature effects.Comment: 9 pages, 1 figur
Photon emission as a source of coherent behaviour of polaritons
We show that the combined effect of photon emission and Coulomb interactions
may drive an exciton-polariton system towards a dynamical coherent state, even
without phonon thermalization or any other relaxation mechanism. Exact
diagonalization results for a finite system (a multilevel quantum dot
interacting with the lowest energy photon mode of a microcavity) are presented
in support to this statement
Mach-Zehnder Interferometry at the Heisenberg Limit with coherent and squeezed-vacuum light
We show that the phase sensitivity of a Mach-Zehnder
interferometer fed by a coherent state in one input port and squeezed-vacuum in
the other one is i) independent from the true value of the phase shift and ii)
can reach the Heisenberg limit , where is the
average number of particles of the input states. We also show that the
Cramer-Rao lower bound, , can be saturated for arbitrary values of the squeezing parameter
and the amplitude of the coherent mode by a Bayesian phase
inference protocol.Comment: 4 pages, 4 figure
Loss of correlation between HIV viral load and CD4+ T-cell counts in HIV/HTLV-1 co-infection in treatment naive Mozambican patients
Seven hundred and four HIV-1/2-positive, antiretroviral therapy (ART) naïve patients were screened for HTLV-1 infection. Antibodies to HTLV-1 were found in 32/704 (4.5%) of the patients. Each co-infected individual was matched with two HIV mono-infected patients according to World Health Organization clinical stage, age +/-5 years and gender. Key clinical and laboratory characteristics were compared between the two groups. Mono-infected and co-infected patients displayed similar clinical characteristics. However, co-infected patients had higher absolute CD4+ T-cell counts (P = 0.001), higher percentage CD4+ T-cell counts (P < 0.001) and higher CD4/CD8 ratios (P < 0.001). Although HIV plasma RNA viral loads were inversely correlated with CD4+ T-cell-counts in mono-infected patients (P < 0.0001), a correlation was not found in co-infected individuals (P = 0.11). Patients with untreated HIV and HTLV-1 co-infection show a dissociation between immunological and HIV virological markers. Current recommendations for initiating ART and chemoprophylaxis against opportunistic infections in resource-poor settings rely on more readily available CD4+ T-cell counts without viral load parameters. These guidelines are not appropriate for co-infected individuals in whom high CD4+ T-cell counts persist despite high HIV viral load states. Thus, for co-infected patients, even in resource-poor settings, HIV viral loads are likely to contribute information crucial for the appropriate timing of ART introduction
Polariton Lasing in a Multilevel Quantum Dot Strongly Coupled To a Single Photon Mode
We present an approximate analytic expression for the photoluminescence
spectral function of a model polariton system, which describes a quantum dot,
with a finite number of fermionic levels, strongly interacting with the lowest
photon mode of a pillar microcavity. Energy eigenvalues and wavefunctions of
the electron-hole-photon system are obtained by numerically diagonalizing the
Hamiltonian. Pumping and photon losses through the cavity mirrors are described
with a master equation, which is solved in order to determine the stationary
density matrix. The photon first-order correlation function, from which the
spectral function is found, is computed with the help of the Quantum Regression
Theorem. The spectral function qualitatively describes the polariton lasing
regime in the model, corresponding to pumping rates two orders of magnitude
lower than those needed for ordinary (photon) lasing. The second-order
coherence functions for the photon and the electron-hole subsystems are
computed as functions of the pumping rate.Comment: version accepted in Phys. Rev.
Beyond the Landau Criterion for Superfluidity
According to the Landau criterion for superfluidity, a Bose-Einstein
condensate flowing with a group velocity smaller than the sound velocity is
energetically stable to the presence of perturbing potentials. We found that
this is strictly correct only for vanishingly small perturbations. The
superfluid critical velocity strongly depends on the strength and shape of the
defect. We quantitatively study, both numerically and with an approximate
analytical model, the dynamical response of a one-dimensional condensate
flowing against an istantaneously raised spatially periodic defect. We found
that the critical velocity decreases by incresing the strength of the
defect , up to to a critical value of the defect intensity where the
critical velocity vanishes
Materiais de isolamento térmico de edifícios. Para além da energia operacional
The use of thermal insulation materials for the achievement of energy efficient
buildings intended, in most cases, the fulfilment of the required heating and cooling needs of
the operational phase. The main goal of this paper is â by using exploratory methodology, namely
literature review â identify more sustainable insulating materials and, concomitantly, exposing
the paradoxical effect of other insulation materials with high Global Warming Potential (GWP)
highlighting the role of the Life Cycle Assessment (LCA), Ecodesign and Environmental Product
Declaration (EPD) tools for the framing, comparison and selection of materials. As a main
conclusion, it is noticed the lack of environmental information from the producers which,
together with acquisition prices that do not internalize Life Cycle Costs (LCC), has led to the use
of insulation materials with high carbon footprint and to the "isolation paradox" as well.A utilização de materiais de isolamento térmico para a obtenção de edifícios energeticamente eficientes visa, na generalidade dos casos, a satisfação das respetivas necessidades de aquecimento e arrefecimento da fase operacional. O objetivo principal deste trabalho é o de - mediante a utilização de metodologia exploratória, nomeadamente revisão bibliográfica - identificar materiais de isolamento mais sustentáveis e, concomitantemente, expor o efeito paradoxal de outros com elevado Potencial de Aquecimento Global (GWP), destacando o papel da Avaliação de Ciclo de Vida (ACV), do Ecodesign e da Declaração Ambiental de Produto (DAP) para a conceção, comparação e seleção de materiais. Como principal conclusão, salienta-se a ausência generalizada de informação ambiental por parte dos produtores o que, a par de preços de aquisição que não internalizam Custos de Ciclo de Vida (LCC), tem conduzido à utilização de materiais com elevada pegada de carbono e ao “paradoxo do isolamento”
Discrete and continuous SIS epidemic models: A unifying approach
550030/2010-7.The susceptible-infective-susceptible (SIS) epidemiological scheme is the simplest description of the dynamics of a disease that is contact-transmitted, and that does not lead to immunity. Two by now classical approaches to such a description are: (i) the use of a mass-action compartmental model that leads to a single ordinary differential equation (SIS-ODE); (ii) the use of a discrete-time Markov chain model (SIS-DTMC). While the former can be seen as a mean-field approximation of the latter under certain conditions, it is also known that their dynamics can be significantly different, if the basic reproduction number is greater than one. The goal of this work is to introduce a continuous model, based on a partial differential equation (SIS-PDE), that retains the finite populations effects present in the SIS-DTMC model, and that allows the use of analytical techniques for its study. In particular, it will reduce itself to the SIS-ODE model in many circumstances. This is accomplished by deriving a diffusion-drift approximation to the probability density of the SIS-DTMC model. Such a diffusion is degenerated at the origin, and must conserve probability. These two features then lead to an interesting consequence: the biologically correct solution is a measure solution. We then provide a convenient representation of such a measure solution that allows the use of classical techniques for its computation, and that also provides a tool for obtaining information about several dynamical features of the model. In particular, we show that the SIS-ODE gives the most likely state, conditional on non-absorption. As a further application of such representation, we show how to define the disease-outbreak probability in terms of the SIS-PDE model, and show that this definition can be used both for certain and uncertain initial presence of infected individuals. As a final application, we compute an approximation for the extinction time of the disease. In addition, we present many numerical examples that confirm the good approximation of the SIS-DTMC by the SIS-PDE.preprintpublishe
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