arXiv subjects

A. Sedrakian

Publications and source records attributed to A. Sedrakian.

16 recordsLinked to original sources

arXiv2013

Pathway to the Square Kilometre Array - The German White Paper -

The Square Kilometre Array (SKA) is the most ambitious radio telescope ever planned. With a collecting area of about a square kilometre, the SKA will be far superior in sensitivity and observing speed to all current radio facilities. The scientific capability promised by the SKA and its technological challenges provide an ideal base for interdisciplinary research, technology transfer, and collaboration between universities, research centres and industry. The SKA in the radio regime and the European Extreme Large Telescope (E-ELT) in the optical band are on the roadmap of the European Strategy Forum for Research Infrastructures (ESFRI) and have been recognised as the essential facilities for European research in astronomy. This "White Paper" outlines the German science and R&D interests in the SKA project and will provide the basis for future funding applications to secure German involvement in the Square Kilometre Array.

astro-ph.IM↗

arXiv2004

Spontaneous breaking of rotational symmetry in superconductors

We show that homogeneous superconductors with broken spin/isospin symmetry lower their energy via a transition to a novel superconducting state where the Fermi-surfaces are deformed to a quasi-ellipsoidal form at zero total momentum of Cooper pairs. In this state, the gain in the condensation energy of the pairs dominates over the loss in the kinetic energy caused by the lowest order (quadrupole) deformation of Fermi-surfaces from the spherically symmetric form. There are two energy minima in general, corresponding to the deformations of the Fermi-spheres into either prolate or oblate forms. The phase transition from spherically symmetric state to the superconducting state with broken rotational symmetry is of the first order.

cond-mat.supr-con↗

arXiv2003

Breaking rotational symmetry in two-flavor color superconductors

The color superconductivity under flavor asymmetric conditions relevant to the compact star phenomenology is studied within the Nambu-Jona-Lasinio model. We focus on the effect of the deformation of the Fermi surfaces on the pairing properties and the energy budget of the superconducting state. We find that at finite flavor asymmetries the color superconducting BCS state is unstable towards spontaneous quadrupole deformation of the Fermi surfaces of the $d$ and $u$ quarks into ellipsoidal form. The ground state of the phase with deformed Fermi surfaces corresponds to a superposition of prolate and oblate deformed Fermi ellipsoids of $d$ and $u$ quarks.

hep-ph↗

arXiv2002

The structure of nuclear systems derived from low momentum nucleon-nucleon potentials

Various nuclear structure observables are evaluated employing low-momentum nucleon-nucleon (NN) potentials $V_{\rm low-k}$ derived from the CD-Bonn and Nijmegen NN interactions $V_{NN}$. By construction, the high momentum modes of the original $V_{NN}$ are integrated out in $V_{\rm low-k}$, with the requirement that the deuteron binding energy and low energy phase shifts of $V_{NN}$ are exactly reproduced. Using this interaction, we evaluate the bulk properties (binding energy and saturation density) of nuclear matter and finite nuclei, in particular their dependence on the cut-off parameter. We also study the pairing gap and the residual interaction in nuclear matter in terms of the Landau parametrization. At low and medium densities, the HF and BHF binding energies for nuclear matter calculated with the $V_{\rm low-k}$'s derived from the CD-Bonn and Nijmegen potentials are nearly identical. The pairing gaps and Landau parameters derived from $V_{\rm low-k}$ are remarkably close to those given by the full-space $V_{NN}$. The $V_{\rm low-k}$ interactions, however, fail to reproduce the saturation property of nuclear matter at higher densities if the cut-off for the high momentum modes is assumed density independent.

nucl-th↗

arXiv2002

Interaction induced deformation in momentum distribution of spin polarized nuclear matter

Effects of spin polarization on the structure of symmetric nuclear matter and pure neutron matter are investigated. We show that the spin polarization induces a deformation of the Fermi spheres for nucleons with spin parallel and opposite to the polarization axes. This feature can be related to the structure of the one pion exchange contribution to a realistic nucleon-nucleon interaction. While the anisotropies in the momentum distribution lower the energy of the system by small amount, the associated variations of the single particle energies with the angle between the polarization axis and the particle momentum are significant.

nucl-th↗

arXiv2001

Transition from BCS pairing to Bose-Einstein condensation in low-density asymmetric nuclear matter

We study the isospin-singlet neutron-proton pairing in bulk nuclear matter as a function of density and isospin asymmetry within the BCS formalism. In the high-density, weak-coupling regime the neutron-proton paired state is strongly suppressed by a minor neutron excess. As the system is diluted, the BCS state with large, overlapping Cooper pairs evolves smoothly into a Bose-Einstein condensate of tightly bound neutron-proton pairs (deuterons). In the resulting low-density system a neutron excess is ineffective in quenching the pair correlations because of the large spatial separation of the deuterons and neutrons. As a result, the Bose-Einstein condensation of deuterons is weakly affected by an additional gas of free neutrons even at very large asymmetries.

nucl-th↗

arXiv2001

Oscillations of rotating trapped Bose-Einstein condensates

The tensor-virial method is applied for a study of oscillation modes of uniformly rotating Bose-Einstein condensed gases, whose rigid body rotation is supported by an vortex array. The second order virial equations are derived in the hydrodynamic regime for an arbitrary external harmonic trapping potential assuming that the condensate is a superfluid at zero temperature. The axisymmetric equilibrium shape of the condensate is determined as a function of the deformation of the trap; its domain of stability is bounded by the constraint $Ω<1$ on the rotation rate (measured in units of the trap frequency $ω_0$.) The oscillations of the axisymmetric condensate are stable with respect to the transverse-shear, toroidal and quasi-radial modes of oscillations, corresponding to the $l= 2$, $| m| = 0,1,2 $ surface deformations. In non-axisymmetric traps, the equilibrium constrains the (dimensionless) deformation in the plane orthogonal to the rotation to the domain $A_2 > Ω^2$ with $Ω< 1$. The second harmonic oscillation modes in non-axisymmetric traps separate into two classes which have even or odd parity with respect to the direction of the rotation axis. Numerical solutions show that these modes are stable in the parameter domain where equilibrium figures exist.

cond-mat↗

arXiv2001

The tensor virial method and its applications to self-gravitating superfluids

This review starts with a discussion of the hierarchy of scales, relevant to the description of superfluids in neutron stars, which motivates a subsequent elementary exposition of the Newtonian superfluid hydrodynamics. Starting from the Euler equations for a superfluid and a normal fluid we apply the tensor virial method to obtain the virial equations of the first, second, and third order and to compute their Eulerian perturbations. Special emphasis is put on the computation of perturbations of the new terms due to mutual gravitational attraction and mutual friction between the two fluids. The oscillation modes of superfluid Maclaurin spheroids are derived from the first and second order perturbed virial equations. We discuss two generic classes of oscillation modes which correspond to the co-moving and relative oscillations of two fluids. These modes decouple if the normal fluid is inviscid. We also discuss the mixing of these modes (when the normal fluid is viscous) and its effect on the dynamical and secular instabilities of the co-moving modes and their damping.

astro-ph↗

arXiv2001

Spatially inhomogeneous condensate in asymmetric nuclear matter

We study the isospin singlet pairing in asymmetric nuclear matter with nonzero total momentum of the condensate Cooper pairs. The quasiparticle excitation spectrum is fourfold split compared to the usual BCS spectrum of the symmetric, homogeneous matter. A twofold splitting of the spectrum into separate branches is due to the finite momentum of the condensate, the isospin asymmetry, or the finite quasiparticle lifetime. The coupling of the isospin singlet and triplet paired states leads to further twofold splitting of each of these branches. We solve the gap equation numerically in the isospin singlet channel in the case where the pairing in the isospin triplet channel is neglected and find nontrivial solutions with finite total momentum of the pairs. The corresponding phase assumes a periodic spatial structure which carries a isospin density wave at constant total number of particles. The phase transition from the BCS to the inhomogeneous superconducting phase is found to be first order and occurs when the density asymmetry is increased above 0.25. The transition from the inhomogeneous superconducting to the unpaired normal state is second order. The maximal values of the critical total momentum (in units of the Fermi momentum) and the critical density asymmetry at which condensate disappears are $P_c/p_F = 0.3$ and $α_c = 0.41$. The possible spatial forms of the ground state of the inhomogeneous superconducting phase are briefly discussed.

nucl-th↗

arXiv2000

Perturbations of self-gravitating, ellipsoidal superfluid-normal fluid mixtures

We study the perturbation modes of rotating superfluid ellipsoidal figures of equilibrium in the framework of the two-fluid superfluid hydrodynamics and Newtonian gravity. Our calculations focus on linear perturbations of background equilibria in which the two fluids move together, the total density is uniform, and the densities of the two components are proportional to one another, with ratios that are independent of position. The motions of the two fluids are coupled by mutual friction, as formulated by Khalatnikov. We show that there are two general classes of modes for small perturbations: one class in which the two fluids move together and the other in which there is relative motion between them. The former are identical to the modes found for a single fluid, except that the rate of viscous dissipation, when computed in the secular (or ``low Reynolds number'') approximation under the assumption of a constant kinematic viscosity, is diminished by a factor $f_N$, the fraction of the total mass in the normal fluid. The relative modes are completely new, and are studied in detail for a range of values for the phenomenological mutual friction coefficients, relative densities of the superfluid and normal components, and, for Roche ellipsoids, binary mass ratios. We find that there are no new secular instabilities connected with the relative motions of the two fluid components. Moreover, although the new modes are subject to viscous dissipation (a consequence of viscosity of the normal matter), they do not emit gravitational radiation at all.

astro-ph↗

arXiv2000

Neutrino-pair emission in a strong magnetic field

We study the neutrino emissivity of strongly magnetized neutron stars due to the charged and neutral current couplings of neutrinos to baryons in strong magnetic fields. The leading order neutral current process is the one-body neutrino-pair bremsstrahlung, which does not have an analogue in the zero field limit. The leading order charged current reaction is the known generalization of the direct Urca processes to strong magnetic fields. While for superstrong magnetic fields in excess of $10^{18}$ G the direct Urca process dominates the one-body bremsstrahlung, we find that for fields on the order $10^{16}-10^{17}$ G and temperatures a few times $10^9$ K the one-body bremsstrahlung is the dominant process. Numerical computation of the resulting emissivity, based on a simple parametrization of the equation of state of the $npe$-matter in a strong magnetic field, shows that the emissivity of this reaction is of the same order of magnitude as that of the modified Urca process in the zero field limit.

nucl-th↗

arXiv2000

Coherent neutrino radiation in supernovae at two loops

We develop a neutrino transport theory, in terms of the real-time non-equilibrium Green's functions, which is applicable to physical conditions arbitrary far from thermal equilibrium. We compute the coherent neutrino radiation in cores of supernovae by evaluating the two-particle-two-hole (2p-2h) polarization function with dressed propagators. The propagator dressing is carried out in the particle-particle channel to all orders in the interaction. We show that at two loops there are two distinct sources of coherence effects in the bremsstrahlung. One is the generically off-shell intermediate state propagation, which leads to the Landau-Pomeranchuk-Migdal type suppression of radiation. We extend previous perturbative results, obtained in the leading order in quasiparticle width, by deriving the exact non-perturbative expression. A new contribution due to off-shell finial/initial baryon states is treated in the leading order in the quasiparticle width. The latter contribution corresponds to processes of higher order than second order in the virial expansion in the number of quasiparticles. At 2p-2h level, the time component of the polarization tensor for the vector transitions vanishes identically in the soft neutrino limit. Vector current thereby is conserved. The contraction of the neutral axial vector current with tensor interaction among the baryons leads to a non-vanishing contribution to the bremsstrahlung rate. These rates are evaluated numerically for finite temperature pure neutron matter at and above the nuclear saturation density.

astro-ph↗

arXiv1998

A quantum kinetic equation for Fermi-systems including three-body correlations

A single-time quantum transport equation, which includes effects beyond the quasiparticle approximation, is derived for Fermi-systems in the framework of non-equilibrium real-time Green's functions theory. Ternary correlations are incorporated in the kinetic description via a cluster expansion for the self-energies (e.g., the transport vertex and the width) truncated at the level of three-body scattering amplitudes. A finite temperature/density formulation of the three-body problem is given. Corresponding three-body equations reduce to the well-known Faddeev equations in the vacuum limit. In equilibrium the equation of state contains virial corrections proportional to the third quantum virial coefficient.

nucl-th↗

arXiv1997

Type I Superconductivity in Neutron Stars

The magnetic structure of neutron vortices in the superfluid cores of neutron stars is determined assuming that the proton condensate forms a type I superconductor. It is shown that the entrainment currents induced by the neutron vortex circulation cause the proton superconductor to break into successive domains of normal and superconducting regions. The Gibbs free-energy is found in the case in which the normal domains form cylindrical tubes coaxial with the neutron vortex. The minimum of the energy functional corresponds to a tube radius $a\sim 0.1-0.5 ~b$, where $b$ is the outer radius of the neutron vortex. The magnetic field within the tube is of the order of $ 5 \times 10^{14}$ G.

astro-ph↗

arXiv1997

Generation of Pulsar Glitches: A Superfluid Core Model

We show that the neutron star's crust-core interface acts as a potential barrier on the peripheral neutron vortices approaching this interface and thus prevents their continuous decay on it in the course of equilibrium state deceleration. The barrier arises due to the interaction of vortex magnetic flux with the Meissner currents set up by the crustal magnetic field at the interface. When the non-balanced part of the Magnus force reaches the value at which the vortices are able to annihilate at the interface, the rapid transfer of angular momentum from the superfluid spins-up the observable crust on short dynamical coupling times.

astro-ph↗

arXiv1996

Medium modifications of the nucleon--deuteron break--up cross section in the Faddeev approach

The three--nucleon scattering problem in a nuclear medium is considered within the Faddeev technique. In particular the deuteron break--up cross section that governs the formation and the break--up reactions of deuterons (NNN <-> Nd) in a nuclear environment is calculated at finite temperatures and densities. A significant enhancement of the in--medium break--up cross section with increasing density has been found.

nucl-th↗