81 research outputs found
R-mode oscillations and rocket effect in rotating superfluid neutron stars. I. Formalism
We derive the hydrodynamical equations of r-mode oscillations in neutron
stars in presence of a novel damping mechanism related to particle number
changing processes. The change in the number densities of the various species
leads to new dissipative terms in the equations which are responsible of the
{\it rocket effect}. We employ a two-fluid model, with one fluid consisting of
the charged components, while the second fluid consists of superfluid neutrons.
We consider two different kind of r-mode oscillations, one associated with
comoving displacements, and the second one associated with countermoving, out
of phase, displacements.Comment: 10 page
Diluted neural networks with adapting and correlated synapses
We consider the dynamics of diluted neural networks with clipped and adapting
synapses. Unlike previous studies, the learning rate is kept constant as the
connectivity tends to infinity: the synapses evolve on a time scale
intermediate between the quenched and annealing limits and all orders of
synaptic correlations must be taken into account. The dynamics is solved by
mean-field theory, the order parameter for synapses being a function. We
describe the effects, in the double dynamics, due to synaptic correlations.Comment: 6 pages, 3 figures. Accepted for publication in PR
The rigidity of crystalline color superconducting quark matter
We calculate the shear modulus of crystalline color superconducting quark
matter, showing that this phase of dense, but not asymptotically dense,
three-flavor quark matter responds to shear stress like a very rigid solid. To
evaluate the shear modulus, we derive the low energy effective Lagrangian that
describes the phonons that originate from the spontaneous breaking of
translation invariance by the spatial modulation of the gap parameter .
These massless bosons describe space- and time-dependent fluctuations of the
crystal structure and are analogous to the phonons in ordinary crystals. The
coefficients of the spatial derivative terms of the phonon effective Lagrangian
are related to the elastic moduli of the crystal; the coefficients that encode
the linear response of the crystal to a shearing stress define the shear
modulus. We analyze the two particular crystal structures which are
energetically favored over a wide range of densities, in each case evaluating
the phonon effective action and the shear modulus up to order in a
Ginzburg-Landau expansion, finding shear moduli which are 20 to 1000 times
larger than those of neutron star crusts. The crystalline color superconducting
phase has long been known to be a superfluid -- by picking a phase its order
parameter breaks the quark-number symmetry spontaneously. Our results
demonstrate that this superfluid phase of matter is at the same time a rigid
solid. We close with a rough estimate of the pinning force on the rotational
vortices which would be formed embedded within this rigid superfluid upon
rotation. Our results raise the possibility that (some) pulsar glitches could
originate within a quark matter core deep within a neutron star.Comment: 38 pages, 5 figures. v3. Two new paragraphs in Section V
(Conclusion); some additional small changes. A paragraph discussing
supercurrents added in Section I (Introduction). Version to appear in Phys.
Rev.
Gapless phases of color-superconducting matter
We discuss gapless color superconductivity for neutral quark matter in beta
equilibrium at zero as well as at nonzero temperature. Basic properties of
gapless superconductors are reviewed. The current progress and the remaining
problems in the understanding of the phase diagram of strange quark matter are
discussed.Comment: 8 pages, 2 figures. Plenary talk at Strangeness in Quark Matter 2004
(SQM2004), Cape Town, South Africa, 15-20 September 2004. Minor correction
Bulk viscosity in 2SC quark matter
The bulk viscosity of three-flavor color-superconducting quark matter
originating from the nonleptonic process u+s u+d is computed. It is assumed
that up and down quarks form Cooper pairs while the strange quark remains
unpaired (2SC phase). A general derivation of the rate of strangeness
production is presented, involving contributions from a multitude of different
subprocesses, including subprocesses that involve different numbers of gapped
quarks as well as creation and annihilation of particles in the condensate. The
rate is then used to compute the bulk viscosity as a function of the
temperature, for an external oscillation frequency typical of a compact star
r-mode. We find that, for temperatures far below the critical temperature T_c
for 2SC pairing, the bulk viscosity of color-superconducting quark matter is
suppressed relative to that of unpaired quark matter, but for T >~ 10^(-3) T_c
the color-superconducting quark matter has a higher bulk viscosity. This is
potentially relevant for the suppression of r-mode instabilities early in the
life of a compact star.Comment: 18 pages + appendices (28 pages total), 8 figures; v3: corrected
numerical error in the plots; 2SC bulk viscosity is now larger than unpaired
bulk viscosity in a wider temperature rang
Transport in holographic superfluids
We construct a slowly varying space-time dependent holographic superfluid and
compute its transport coefficients. Our solution is presented as a series
expansion in inverse powers of the charge of the order parameter. We find that
the shear viscosity associated with the motion of the condensate vanishes. The
diffusion coefficient of the superfluid is continuous across the phase
transition while its third bulk viscosity is found to diverge at the critical
temperature. As was previously shown, the ratio of the shear viscosity of the
normal component to the entropy density is 1/(4 pi). As a consequence of our
analysis we obtain an analytic expression for the backreacted metric near the
phase transition for a particular type of holographic superfluid.Comment: 45 pages + appendice
Gravitational waves from single neutron stars: an advanced detector era survey
With the doors beginning to swing open on the new gravitational wave
astronomy, this review provides an up-to-date survey of the most important
physical mechanisms that could lead to emission of potentially detectable
gravitational radiation from isolated and accreting neutron stars. In
particular we discuss the gravitational wave-driven instability and
asteroseismology formalism of the f- and r-modes, the different ways that a
neutron star could form and sustain a non-axisymmetric quadrupolar "mountain"
deformation, the excitation of oscillations during magnetar flares and the
possible gravitational wave signature of pulsar glitches. We focus on progress
made in the recent years in each topic, make a fresh assessment of the
gravitational wave detectability of each mechanism and, finally, highlight key
problems and desiderata for future work.Comment: 39 pages, 12 figures, 2 tables. Chapter of the book "Physics and
Astrophysics of Neutron Stars", NewCompStar COST Action 1304. Minor
corrections to match published versio
Strongly Correlated Quantum Fluids: Ultracold Quantum Gases, Quantum Chromodynamic Plasmas, and Holographic Duality
Strongly correlated quantum fluids are phases of matter that are
intrinsically quantum mechanical, and that do not have a simple description in
terms of weakly interacting quasi-particles. Two systems that have recently
attracted a great deal of interest are the quark-gluon plasma, a plasma of
strongly interacting quarks and gluons produced in relativistic heavy ion
collisions, and ultracold atomic Fermi gases, very dilute clouds of atomic
gases confined in optical or magnetic traps. These systems differ by more than
20 orders of magnitude in temperature, but they were shown to exhibit very
similar hydrodynamic flow. In particular, both fluids exhibit a robustly low
shear viscosity to entropy density ratio which is characteristic of quantum
fluids described by holographic duality, a mapping from strongly correlated
quantum field theories to weakly curved higher dimensional classical gravity.
This review explores the connection between these fields, and it also serves as
an introduction to the Focus Issue of New Journal of Physics on Strongly
Correlated Quantum Fluids: from Ultracold Quantum Gases to QCD Plasmas. The
presentation is made accessible to the general physics reader and includes
discussions of the latest research developments in all three areas.Comment: 138 pages, 25 figures, review associated with New Journal of Physics
special issue "Focus on Strongly Correlated Quantum Fluids: from Ultracold
Quantum Gases to QCD Plasmas"
(http://iopscience.iop.org/1367-2630/focus/Focus%20on%20Strongly%20Correlated%20Quantum%20Fluids%20-%20from%20Ultracold%20Quantum%20Gases%20to%20QCD%20Plasmas
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