48,176 research outputs found
Manipulation of the dynamics of many-body systems via quantum control methods
We investigate how dynamical decoupling methods may be used to manipulate the
time evolution of quantum many-body systems. These methods consist of sequences
of external control operations designed to induce a desired dynamics. The
systems considered for the analysis are one-dimensional spin-1/2 models, which,
according to the parameters of the Hamiltonian, may be in the integrable or
non-integrable limits, and in the gapped or gapless phases. We show that an
appropriate control sequence may lead a chaotic chain to evolve as an
integrable chain and a system in the gapless phase to behave as a system in the
gapped phase. A key ingredient for the control schemes developed here is the
possibility to use, in the same sequence, different time intervals between
control operations.Comment: 10 pages, 3 figure
Quantum vacuum effects as generalized f(R) gravity. Application to stars
It is assumed that, for weak spacetime curvature, the main gravitational
effect of the quantum vacuum stress-energy corresponds to adding two terms to
the Einstein-Hilbert action, proportional to the square of the curvature scalar
and to the contraction of two Ricci tensors, respectively. It is shown that
compatibility with terrestrial and solar systems observaction implies that the
square roorts of the coefficients of these terms should be either a few
millimeters or a few hundred meters. It is shown that the vacuum contribution
increase the stability of white dwarfs.Comment: GEneralizes and improves previous versio
How Does a Dipolar Bose-Einstein Condensate Collapse?
We emphasize that the macroscopic collapse of a dipolar Bose-Einstein
condensate in a pancake-shaped trap occurs through local density fluctuations,
rather than through a global collapse to the trap center. This hypothesis is
supported by a recent experiment in a chromium condensate.Comment: Proceedings of 17th International Laser Physics Worksho
Quantizing Majorana Fermions in a Superconductor
A Dirac-type matrix equation governs surface excitations in a topological
insulator in contact with an s-wave superconductor. The order parameter can be
homogenous or vortex valued. In the homogenous case a winding number can be
defined whose non-vanishing value signals topological effects. A vortex leads
to a static, isolated, zero energy solution. Its mode function is real, and has
been called "Majorana." Here we demonstrate that the reality/Majorana feature
is not confined to the zero energy mode, but characterizes the full quantum
field. In a four-component description a change of basis for the relevant
matrices renders the Hamiltonian imaginary and the full, space-time dependent
field is real, as is the case for the relativistic Majorana equation in the
Majorana matrix representation. More broadly, we show that the Majorana
quantization procedure is generic to superconductors, with or without the Dirac
structure, and follows from the constraints of fermionic statistics on the
symmetries of Bogoliubov-de Gennes Hamiltonians. The Hamiltonian can always be
brought to an imaginary form, leading to equations of motion that are real with
quantized real field solutions. Also we examine the Fock space realization of
the zero mode algebra for the Dirac-type systems. We show that a
two-dimensional representation is natural, in which fermion parity is
preserved.Comment: 26 pages, no figure
Condiciones ambientales y diferenciación social en los patrones de movilidad: el caso de las desigualdades de género en el Área Metropolitana de Lisboa
Place determinants and social dimensions interact to produce mobility patterns. The article
seeks to measure to what extent the physical and social conditions of the residency place modulate
the intensity of mobility particularly in what concerns gender inequalities. To do so, we first present
the theoretical framework, which guides our perspective on mobility inequalities and the geographical
focus: Lisbon Metropolitan Area (LMA). Secondly, using logistic regression on representative data
from a survey to LMA inhabitants we elaborate on the concept of environmental motility and its
relationship to various types of inequality. Lastly, we systematize a view on accumulated inequalities
examining how local environments may increase the mobilities gender gap.info:eu-repo/semantics/publishedVersio
X-ray spectra reveal the reawakening of the repeat changing-look AGN NGC 1566
We present simultaneous XMM-Newton and NuSTAR observations of the repeat
changing-look AGN NGC 1566, which dramatically increased in brightness in the
IR to X-ray bands in 2018. The broad-band X-ray spectrum was taken at the peak
of the outburst and is typical of Seyfert 1 AGN. The spectrum shows a soft
excess, Compton hump, warm absorption and reflection, ruling out tidal
disruption as the cause of the outburst and demonstrating that a 'standard'
accretion disk can develop very rapidly. The high resolution grating spectrum
reveals that the outburst has launched a ~ 500 km/s outflow, and shows
photoionised emission lines from rest-frame gas. We discuss possible mechanisms
for the outburst, and conclude that it is most likely caused by a disk
instability.Comment: 5 pages, 2 figures, accepted to MNRAS letter
Controle integrado da broca da haste da mandioca Sternocoelus spp.
Consideradas pragas secundárias, as brocas da haste da mandioca Sternocoelus spp. (Coleoptera: Curculionidae) são problema fitossanitário em algumas áreas produtoras (RODRIGUEZ et al., 2009). O controle dessa praga é difícil, pois o dano ocorre quando ao eclodir e se alimentar no interior da haste as larvas interrompem o fluxo de seiva na planta levando-a à morte (CARVALHO et al. 2009)
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