366 research outputs found
Slowly Rotating Anisotropic Neutron Stars in General Relativity and Scalar-Tensor Theory
Some models (such as the Skyrme model, a low-energy effective field theory
for QCD) suggest that the high-density matter prevailing in neutron star
interiors may be significantly anisotropic. Anisotropy is known to affect the
bulk properties of nonrotating neutron stars in General Relativity. In this
paper we study the effects of anisotropy on slowly rotating stars in General
Relativity. We also consider one of the most popular extensions of Einstein's
theory, namely scalar-tensor theories allowing for spontaneous scalarization (a
phase transition similar to spontaneous magnetization in ferromagnetic
materials). Anisotropy affects the moment of inertia of neutron stars (a
quantity that could potentially be measured in binary pulsar systems) in both
theories. We find that the effects of scalarization increase (decrease) when
the tangential pressure is bigger (smaller) than the radial pressure, and we
present a simple criterion to determine the onset of scalarization by
linearizing the scalar-field equation. Our calculations suggest that binary
pulsar observations may constrain the degree of anisotropy or even, more
optimistically, provide evidence for anisotropy in neutron star cores.Comment: 19 pages, 7 figures, 1 table. Matches version in press in CQG. Fixed
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On the classical-quantum correspondence for the scattering dwell time
Using results from the theory of dynamical systems, we derive a general
expression for the classical average scattering dwell time, tau_av. Remarkably,
tau_av depends only on a ratio of phase space volumes. We further show that,
for a wide class of systems, the average classical dwell time is not in
correspondence with the energy average of the quantum Wigner time delay.Comment: 5 pages, 1 figur
Electronic nature of the aromatic adamantanediyl ions and its analogues
The relative stability of the 1,3-dehydro-5,7-adamantanediyl dication is ascribed to its tridimensional aromaticity. However, its electronic nature is not well known. In order to improve its understanding, dicationic and monocationic adamantanedyil species and some key analogues were studied by atoms in molecules (AIM) theory. They were compared to non-aromatic adamantane analogues. AIM results indicate that the density in center of the cage structure and the average of all delocalization indexes involving its bridged atoms are higher in aromatic than in non-aromatic compounds. Degeneracy in energy of the bridged atoms, uniformity and magnitude of their shared charge distinguish the dications 1,3-adamantyl and the 1,3-dehydro-5,7-adamantanediyl. However, both are aromatic as well as the 1,3-dehydro-5,7-diboroadamantane. The 1,3-dehydro-7-adamantyl cation has a characteristic planar homoaromaticity
Identification of carbonium and carbenium ions by QTAIM
The Gassman-Fentiman tool of increasing electron demand was used to identify carbonium and carbenium ions. Nonetheless, due to its ambiguous understanding, it was pivot of a historical dispute. We applied the Quantum Theory of Atoms in Molecules (QTAIM) metodology to characterize the carbonium and carbenium ions on an easier and more effective way. By comparing selected topological information of reference carbenium ions the QTAIM metodology can be used to evaluate whether a carbocation is classical or not. In addition, it is possible to rank a set of carbonium ions in order of their corresponding σ or π delocalization. There are few differences between our QTAIM-based model and Gassman-Fentiman tool. Unlike Gassman-Fentiman tool results, 7-anisyl-7-norbornenyl and 2-anisyl-2-norbornyl cations are non-classical, although they are the least nonclassical ions in their corresponding set of studied cations
Quantum walks in two dimensions: controlling directional spreading with entangling coins and tunable disordered step operator
We study a 2-D disordered time-discrete quantum walk based on 1-D
`generalized elephant quantum walk' where an entangling coin operator is
assumed and which paves the way to a new set of properties. We show that
considering a given disorder in one direction, it is possible to control the
degree of spreading and entanglement in the other direction. This observation
helps assert that the random quantum walks of this ilk serve as a controllable
decoherence channel with the degree of randomness being the tunable parameter
and highlight the role of dimensionality in quantum systems regarding
information and transport.Comment: 16 pages, 19 figure
On the semiclassical theory for universal transmission fluctuations in chaotic systems: the importance of unitarity
The standard semiclassical calculation of transmission correlation functions
for chaotic systems is severely influenced by unitarity problems. We show that
unitarity alone imposes a set of relationships between cross sections
correlation functions which go beyond the diagonal approximation. When these
relationships are properly used to supplement the semiclassical scheme we
obtain transmission correlation functions in full agreement with the exact
statistical theory and the experiment. Our approach also provides a novel
prediction for the transmission correlations in the case where time reversal
symmetry is present
Quantum walks with spatiotemporal fractal disorder
We investigate the transport and entanglement properties exhibited by quantum
walks with coin operators concatenated in a space-time fractal structure.
Inspired by recent developments in photonics, we choose the paradigmatic
Sierpinski gasket. The 0-1 pattern of the fractal is mapped into an alternation
of the generalized Hadamard-Fourier operators. In fulfilling the blank space on
the analysis of the impact of disorder in quantum walk properties --
specifically, fractal deterministic disorder --, our results show a robust
effect of entanglement enhancement as well as an interesting novel road to
superdiffusive spreading with a tunable scaling exponent attaining effective
ballistic diffusion. Namely, with this fractal approach it is possible to
obtain an increase in quantum entanglement without jeopardizing spreading.
Alongside those features, we analyze further properties such as the degree of
interference and visibility. The present model corresponds to a new application
of fractals in an experimentally feasible setting, namely the building block
for the construction of photonic patterned structures.Comment: 16 pages, 9 figure
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