Damping of sound waves in superfluid nucleon-hyperon matter of neutron stars

We consider sound waves in superfluid nucleon-hyperon matter of massive neutron-star cores. We calculate and analyze the speeds of sound modes and their damping times due to the shear viscosity and non-equilibrium weak processes of particle transformations. For that, we employ the dissipative relativistic hydrodynamics of a superfluid nucleon-hyperon mixture, formulated recently [M.E. Gusakov and E.M. Kantor, Phys. Rev. D78, 083006 (2008)]. We demonstrate that the damping times of sound modes calculated using this hydrodynamics and the ordinary (nonsuperfluid) one, can differ from each other by several orders of magnitude.Comment: 15 pages, 5 figures, Phys. Rev. D accepte

First and Second Sound Modes of a Bose-Einstein Condensate in a Harmonic Trap

We have calculated the first and second sound modes of a dilute interacting Bose gas in a spherical trap for temperatures ($0.6<T/T_{c}<1.2$) and for systems with $10^4$ to $10^8$ particles. The second sound modes (which exist only below $T_{c}$) generally have a stronger temperature dependence than the first sound modes. The puzzling temperature variations of the sound modes near $T_{c}$ recently observed at JILA in systems with $10^3$ particles match surprisingly well with those of the first and second sound modes of much larger systems.Comment: a shorten version, more discussions are given on the nature of the second sound. A long footnote on the recent work of Zaremba, Griffin, and Nikuni (cond-mat/9705134) is added, the spectrum of the (\ell=1, n_2=0) mode is included in fig.

Transport coefficients from the Boson Uehling-Uhlenbeck Equation

We derive microscopic expressions for the bulk viscosity, shear viscosity and thermal conductivity of a quantum degenerate Bose gas above $T_C$, the critical temperature for Bose-Einstein condensation. The gas interacts via a contact potential and is described by the Uehling-Uhlenbeck equation. To derive the transport coefficients, we use Rayleigh-Schrodinger perturbation theory rather than the Chapman-Enskog approach. This approach illuminates the link between transport coefficients and eigenvalues of the collision operator. We find that a method of summing the second order contributions using the fact that the relaxation rates have a known limit improves the accuracy of the computations. We numerically compute the shear viscosity and thermal conductivity for any boson gas that interacts via a contact potential. We find that the bulk viscosity remains identically zero as it is for the classical case.Comment: 10 pages, 2 figures, submitted to Phys. Rev.

The two-fluid model with superfluid entropy

The two-fluid model of liquid helium is generalized to the case that the superfluid fraction has a small entropy content. We present theoretical arguments in favour of such a small superfluid entropy. In the generalized two-fluid model various sound modes of He$\;$II are investigated. In a superleak carrying a persistent current the superfluid entropy leads to a new sound mode which we call sixth sound. The relation between the sixth sound and the superfluid entropy is discussed in detail.Comment: 22 pages, latex, published in Nuovo Cimento 16 D (1994) 37

Bulk viscosity of superfluid neutron stars

The hydrodynamics, describing dynamical effects in superfluid neutron stars, essentially differs from the standard one-fluid hydrodynamics. In particular, we have four bulk viscosity coefficients in the theory instead of one. In this paper we calculate these coefficients, for the first time, assuming they are due to non-equilibrium beta-processes (such as modified or direct Urca process). The results of our analysis are used to estimate characteristic damping times of sound waves in superfluid neutron stars. It is demonstrated that all four bulk viscosity coefficients lead to comparable dissipation of sound waves and should be considered on the same footing.Comment: 11 pages, 1 figure, this version with some minor stylistic changes is published in Phys. Rev.

Some mechanisms of "spontaneous" polarization of superfluid He-4

Previously, a quantum "tidal" mechanism of polarization of the atoms of He-II was proposed, according to which, as a result of interatomic interaction, each atom of He-II acquires small fluctuating dipole and multipole moments, oriented chaotically on the average. In this work, we show that, in the presence of a temperature or density gradient in He-II, the originally chaotically oriented tidal dipole moments of the atoms become partially ordered, which results in volume polarization of He-II. It is found that the gravitational field of the Earth induces electric induction U =10(-7)V in He-II (for vessel dimensions of the order of 10 cm). We study also the connection of polarization and acceleration, and discuss a possible nature of the electric signal dU = kdT/2e observed by A.S. Rybalko in experiments with second sound.Comment: 13 pages; the calculation is extended and refined; v4: reconstructio

On Reduction of Critical Velocity in a Model of Superfluid Bose-gas with Boundary Interactions

The existence of superfluidity in a 3D Bose-gas can depend on boundary interactions with channel walls. We study a simple model where the dilute moving Bose-gas interacts with the walls via hard-core repulsion. Special boundary excitations are introduced, and their excitation spectrum is calculated within a semiclassical approximation. It turns out that the state of the moving Bose-gas is unstable with respect to the creation of these boundary excitations in the system gas + walls, i.e. the critical velocity vanishes in the semiclassical (Bogoliubov) approximation. We discuss how a condensate wave function, the boundary excitation spectrum and, hence, the value of the critical velocity can change in more realistic models, in which ``smooth'' attractive interaction between the gas and walls is taken into account. Such a surface mode could exist in ``soft matter'' containers with flexible walls.Comment: 9 pages (RevTeX), two figures (.ps) incorporated by epsf. submitted to Phys. Lett.

Mechanisms for Stable Sonoluminescence

A gas bubble trapped in water by an oscillating acoustic field is expected to either shrink or grow on a diffusive timescale, depending on the forcing strength and the bubble size. At high ambient gas concentration this has long been observed in experiments. However, recent sonoluminescence experiments show that in certain circumstances when the ambient gas concentration is low the bubble can be stable for days. This paper presents mechanisms leading to stability which predict parameter dependences in agreement with the sonoluminescence experiments.Comment: 4 pages, 3 figures on request (2 as .ps files

Bulk viscosity of superfluid hyperon stars

We calculated bulk viscosity due to non-equilibrium weak processes in superfluid nucleon-hyperon matter of neutron stars. For that, the dissipative relativistic hydrodynamics, formulated in paper [1] for superfluid mixtures, was extended to the case when both nucleons and hyperons are superfluid. It was demonstrated that in the most general case (when neutrons, protons, Lambda, and Sigma^{-} hyperons are superfluid), non-equilibrium weak processes generate sixteen bulk viscosity coefficients, with only three of them being independent. In addition, we corrected an inaccuracy in a widely used formula for the bulk viscosity of non-superfluid nucleon-hyperon matter.Comment: 22 pages, 2 figure

Topological phases and circulating states of Bose-Einstein condensates

We show that the quantum topological effect predicted by Aharonov and Casher (AC effect) [Phys. Rev. Lett. 53, 319 (1984)] may be used to create circulating states of magnetically trapped atomic Bose-Einstein condensates (BEC). A simple experimental setup is suggested based on a multiply connected geometry such as a toroidal trap or a magnetic trap pinched by a blue-detuned laser. We give numerical estimates of such effects within the current atomic BEC experiments, and point out some interesting properties of the associated quantized circulating states.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev.