734 research outputs found
Exact static solutions in four dimensional Einstein-Maxwell-Dilaton gravity
Classes of exact static solutions in four-dimensional
Einstein-Maxwell-Dilaton gravity are found. Besides of the well-known solutions
previously found in the literature, new solutions are presented.It's shown that
spherically symmetric solutions, except the case of charged dilaton black hole,
represent globally naked strong curvature singularities.Comment: 8 pages, late
Variability of signal to noise ratio and the network analysis of gravitational wave burst signals
The detection and estimation of gravitational wave burst signals, with {\em a
priori} unknown polarization waveforms, requires the use of data from a network
of detectors. For determining how the data from such a network should be
combined, approaches based on the maximum likelihood principle have proven to
be useful. The most straightforward among these uses the global maximum of the
likelihood over the space of all waveforms as both the detection statistic and
signal estimator. However, in the case of burst signals, a physically
counterintuitive situation results: for two aligned detectors the statistic
includes the cross-correlation of the detector outputs, as expected, but this
term disappears even for an infinitesimal misalignment. This {\em two detector
paradox} arises from the inclusion of improbable waveforms in the solution
space of maximization. Such waveforms produce widely different responses in
detectors that are closely aligned. We show that by penalizing waveforms that
exhibit large signal-to-noise ratio (snr) variability, as the corresponding
source is moved on the sky, a physically motivated restriction is obtained that
(i) resolves the two detector paradox and (ii) leads to a better performing
statistic than the global maximum of the likelihood. Waveforms with high snr
variability turn out to be precisely the ones that are improbable in the sense
mentioned above. The coherent network analysis method thus obtained can be
applied to any network, irrespective of the number or the mutual alignment of
detectors.Comment: 13 pages, 6 figure
Instabilities of the AA-stacked graphene bilayer
Tight-binding calculations predict that the AA-stacked graphene bilayer has
one electron and one hole conducting bands, and that the Fermi surfaces of
these bands coincide. We demonstrate that as a result of this degeneracy, the
bilayer becomes unstable with respect to a set of spontaneous symmetry
violations. Which of the symmetries is broken depends on the microscopic
details of the system. We find that antiferromagnetism is the more stable order
parameter. This order is stabilized by the strong on-site Coulomb repulsion.
For an on-site repulsion energy typical for graphene systems, the
antiferromagnetic gap can exist up to room temperatures.Comment: 4 pages, 2 eps figure, submitted to Phys. Rev. Let
Majorana fermions in pinned vortices
Exploiting the peculiar properties of proximity-induced superconductivity on
the surface of a topological insulator, we propose a device which allows the
creation of a Majorana fermion inside the core of a pinned Abrikosov vortex.
The relevant Bogolyubov-de Gennes equations are studied analytically. We
demonstrate that in this system the zero-energy Majorana fermion state is
separated by a large energy gap, of the order of the zero-temperature
superconducting gap , from a band of single-particle non-topological
excitations. In other words, the Majorana fermion remains robust against
thermal fluctuations, as long as the temperature remains substantially lower
than the critical superconducting temperature. Experimentally, the Majorana
state may be detected by measuring the tunneling differential conductance at
the center of the Abrikosov vortex. In such an experiment, the Majorana state
manifests itself as a zero-bias anomaly separated by a gap, of the order of
, from the contributions of the nontopological excitations.Comment: 9 pages, 2 eps figures, new references are added, several typos are
correcte
Two-qubit parametric amplifier: large amplification of weak signals
Using numerical simulations, we show that two coupled qubits can amplify a
weak signal about hundredfold. This can be achieved if the two qubits are
biased simultaneously by this weak signal and a strong pump signal, both of
which having frequencies close to the inter-level transitions in the system.
The weak signal strongly affects the spectrum generated by the strong pumping
drive by producing and controlling mixed harmonics with amplitudes of the order
of the main harmonic of the strong drive. We show that the amplification is
robust with respect to noise, with an intensity of the order of the weak
signal. When deviating from the optimal regime (corresponding to strong qubit
coupling and a weak-signal frequency equal to the inter-level transition
frequency) the proposed amplifier becomes less efficient, but it can still
considerably enhance a weak signal (by several tens). We therefore propose to
use coupled qubits as a combined parametric amplifier and frequency shifter.Comment: 6 figure
Unified description of neutron superfluidity in the neutron-star crust with analogy to anisotropic multi-band BCS superconductors
The neutron superfluidity in the inner crust of a neutron star has been
traditionally studied considering either homogeneous neutron matter or only a
small number of nucleons confined inside the spherical Wigner-Seitz cell.
Drawing analogies with the recently discovered multi-band superconductors, we
have solved the anisotropic multi-band BCS gap equations with Bloch boundary
conditions, thus providing a unified description taking consistently into
account both the free neutrons and the nuclear clusters. Calculations have been
carried out using the effective interaction underlying our recent
Hartree-Fock-Bogoliubov nuclear mass model HFB-16. We have found that even
though the presence of inhomogeneities lowers the neutron pairing gaps, the
reduction is much less than that predicted by previous calculations using the
Wigner-Seitz approximation. We have studied the disappearance of superfluidity
with increasing temperature. As an application we have calculated the neutron
specific heat, which is an important ingredient for modeling the thermal
evolution of newly-born neutron stars. This work provides a new scheme for
realistic calculations of superfluidity in neutron-star crusts.Comment: 15 pages, 31 figures, accepted for publication in Physical Review
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