867 research outputs found

### Universal one-way light speed from a universal light speed over closed paths

This paper gives two complete and elementary proofs that if the speed of
light over closed paths has a universal value $c$, then it is possible to
synchronize clocks in such a way that the one-way speed of light is c. The
first proof is an elementary version of a recent proof. The second provides
high precision experimental evidence that it is possible to synchronize clocks
in such a way that the one-way speed of light has a universal value. We also
discuss an old incomplete proof by Weyl which is important from an historical
perspective.Comment: RevTex4, 6 pages, 2 figures, uses psfrag. New sections added,
discussion expande

### Weak distinction and the optimal definition of causal continuity

Causal continuity is usually defined by imposing the conditions (i)
distinction and (ii) reflectivity. It is proved here that a new causality
property which stays between weak distinction and causality, called feeble
distinction, can actually replace distinction in the definition of causal
continuity. An intermediate proof shows that feeble distinction and future
(past) reflectivity implies past (resp. future) distinction. Some new
characterizations of weak distinction and reflectivity are given.Comment: 9 pages, 2 figures. v2: improved and expanded version. v3: a few
misprints have been corrected and a reference has been update

### Motion of an impurity particle in an ultracold quasi-one-dimensional gas of hard-core bosons

The low-lying eigenstates of a one-dimensional (1D) system of many
impenetrable point bosons and one moving impurity particle with repulsive
zero-range impurity-boson interaction are found for all values of the
impurity-boson mass ratio and coupling constant. The moving entity is a
polaron-like composite object consisting of the impurity clothed by a co-moving
gray soliton. The special case with impurity-boson interaction of point
hard-core form and impurity-boson mass ratio $m_i/m$ unity is first solved
exactly as a special case of a previous Fermi-Bose (FB) mapping treatment of
soluble 1D Bose-Fermi mixture problems. Then a more general treatment is given
using second quantization for the bosons and the second-quantized form of the
FB mapping, eliminating the impurity degrees of freedom by a Lee-Low-Pines
canonical transformation. This yields the exact solution for arbitrary $m_i/m$
and impurity-boson interaction strength.Comment: 4 pp., 2 figures, revtex4; error in Eq.(6) corrected and derivation
simplifie

### Collective excitation frequencies of Bosons in a parabolic potential with interparticle harmonic interactions

The fact that the ground-state first-order density matrix for Bosons in a
parabolic potential with interparticle harmonic interactions is known in exact
form is here exploited to study collective excitations in the weak-coupling
regime. Oscillations about the ground-state density are treated analytically by
a linearized equation of motion which includes a kinetic energy contribution.
We show that the dipole mode has the frequency of the bare trap, in accord with
the Kohn theorem, and derive explicit expressions for the frequencies of the
higher-multipole modes in terms of a frequency renormalized by the
interactions.Comment: 6 pages, no figures, accepted for publication on Physics Letters

### Phase separation in a boson-fermion mixture of Lithium atoms

We use a semiclassical three-fluid model to analyze the conditions for
spatial phase separation in a mixture of fermionic Li-6 and a (stable)
Bose-Einstein condensate of Li-7 atoms under cylindrical harmonic confinement,
both at zero and finite temperature. We show that with the parameters of the
Paris experiment [F. Schrek et al., Phys. Rev. Lett. 87 080403 (2001)] an
increase of the boson-fermion scattering length by a factor five would be
sufficient to enter the phase-separated regime. We give examples of
configurations for the density profiles in phase separation and estimate that
the transition should persist at temperatures typical of current experiments.
For higher values of the boson-fermion coupling we also find a new phase
separation between the fermions and the bosonic thermal cloud at finite
temperature.Comment: 8 pages, 4 figures, new version of Fig. 4 and typos correcte

### Interaction-enhanced flow of a polariton persistent current in a ring

We study the quantum hydrodynamical features of exciton-polaritons flowing
circularly in a ring-shaped geometry. We consider a resonant-excitation scheme
in which the spinor polariton fluid is set into motion in both components by
spin-to-orbital angular momentum conversion. We show that this scheme allows to
control the winding number of the fluid, and to create two circulating states
differing by two units of the angular momentum. We then consider the effect of
a disorder potential, which is always present in realistic nanostructures. We
show that a smooth disorder is efficiently screened by the polariton-polariton
interactions, yielding a signature of polariton superfluidity. This effect is
reminiscent of supercurrent in a superconducting loop.Comment: 7 pages, 4 figure

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