1,462 research outputs found

### Induced interactions in dilute atomic gases and liquid helium mixtures

In dilute mixtures of two atomic gases, interactions between two minority
atoms acquire a contribution due to interaction with the majority component.
Using thermodynamic arguments, we derive expressions for this induced
interaction for both fermions and bosons for arbitrary strength of the
interaction between the two components. Implications of the work for the theory
of dilute solutions of $^3$He in liquid $^4$He are discussed.Comment: 7 pages, 1 figure, NORDITA-2012-3

### Instability and control of a periodically-driven Bose-Einstein condensate

We investigate the dynamics of a Bose-Einstein condensate held in an optical
lattice under the influence of a strong periodic driving potential. Studying
the mean-field version of the Bose-Hubbard model reveals that the condensate
becomes highly unstable when the effective intersite tunneling becomes
negative. We further show how controlling the sign of the tunneling can be used
as a powerful tool to manage the dispersion of an atomic wavepacket, and thus
to create a pulsed atomic soliton laser.Comment: 4 pages, 3 eps figure

### Normal mass density of a superfluid Fermi gas at unitarity

We calculate the normal mass density of a paired Fermi gas at unitarity. The
dominant contribution near the superfluid transition is from fermionic
quasiparticle excitations, and is thus sensitive to the pairing gap. A
comparison with the recent experiment of Sidorenkov et al. suggests that the
superfluid gap near the transition temperature is larger than the BCS value,
but the data do not permit a quantitative inference of the gap. Calculations of
the quenched moment of inertia of a BCS superfluid in a harmonic trap are in
reasonable agreement with the earlier experiment of Riedl et al.Comment: 6 pages, 2 figures, published versio

### Dynamics of the inner crust of neutron stars: hydrodynamics, elasticity and collective modes

We present calculations of the hydrodynamics of the inner crust of neutron
stars, where a superfluid neutron liquid coexists with a lattice of
neutron-rich nuclei. The long-wavelength collective oscillations are
combinations of phonons in the lattice and phonons in the superfluid neutrons.
Velocities of collective modes are calculated from information about effective
nucleon-nucleon interactions derived from Lattimer and Swesty's microscopic
calculations based on a compressible liquid drop picture of the atomic nuclei
and the surrounding neutrons.Comment: Preprint NORDITA-2013-1

### Center of mass rotation and vortices in an attractive Bose gas

The rotational properties of an attractively interacting Bose gas are studied
using analytical and numerical methods. We study perturbatively the ground
state phase space for weak interactions, and find that in an anharmonic trap
the rotational ground states are vortex or center of mass rotational states;
the crossover line separating these two phases is calculated. We further show
that the Gross-Pitaevskii equation is a valid description of such a gas in the
rotating frame and calculate numerically the phase space structure using this
equation. It is found that the transition between vortex and center of mass
rotation is gradual; furthermore the perturbative approach is valid only in an
exceedingly small portion of phase space. We also present an intuitive picture
of the physics involved in terms of correlated successive measurements for the
center of mass state.Comment: version2, 17 pages, 5 figures (3 eps and 2 jpg

### Elastic properties of polycrystalline dense matter

Elastic properties of the solid regions of neutron star crusts and white
dwarfs play an important role in theories of stellar oscillations. Matter in
compact stars is presumably polycrystalline and, since the elastic properties
of single crystals of such matter are very anisotropic, it is necessary to
relate elastic properties of the polycrystal to those of a single crystal. We
calculate the effective shear modulus of polycrystalline matter with randomly
oriented crystallites using a self-consistent theory that has been very
successful in applications to terrestrial materials and show that previous
calculations overestimate the shear modulus by approximately 28%.Comment: Preprint NORDITA-2015-1

### Nucleus--nucleus interactions in the inner crust of neutron stars

The interaction between nuclei in the inner crust of neutron stars consists
of two contributions, the so-called "direct" interaction and an "induced" one
due to density changes in the neutron fluid. For large nuclear separations $r$
the contributions from nuclear forces to each of these terms are shown to be
nonzero. In the static limit they are equal in magnitude but have opposite
signs and they cancel exactly. We analyze earlier results on effective
interactions in the light of this finding. We consider the properties of
long-wavelength collective modes and, in particular, calculate the degree of
mixing between the lattice phonons and the phonons in the neutron superfluid.
Using microscopic theory, we calculate the net non-Coulombic contribution to
the nucleus--nucleus interaction and show that, for large $r$, the leading term
is due to exchange of two phonons and varies as $1/r^7$: it is an analog of the
Casimir--Polder interaction between neutral atoms.Comment: 11 pages, 4 figures, 3 table

### Superfluid Density of Neutrons in the Inner Crust of Neutron Stars: New Life for Pulsar Glitch Models

Calculations of the effects of band structure on the neutron superfluid
density in the crust of neutron stars made under the assumption that the
effects of pairing are small [N. Chamel, Phys. Rev. C 85, 035801 (2012)] lead
to moments of inertia of superfluid neutrons so small that the crust alone is
insufficient to account for the magnitude of neutron star glitches. Inspired by
earlier work on ultracold atomic gases in an optical lattice, we investigate
fermions with attractive interactions in a periodic lattice in the mean-field
approximation. The effects of band structure are suppressed when the pairing
gap is of order or greater than the strength of the lattice potential. By
applying the results to the inner crust of neutron stars, we conclude that the
reduction of the neutron superfluid density is considerably less than
previously estimated and, consequently, it is premature to rule out models of
glitches based on neutron superfluidity in the crust.Comment: 5 pages, 3 figure

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