8,045 research outputs found
Emergence of order from turbulence in an isolated planar superfluid
We study the relaxation dynamics of an isolated zero temperature
quasi-two-dimensional superfluid Bose-Einstein condensate (BEC) that is
imprinted with a spatially random distribution of quantum vortices. Following a
period of vortex annihilation, we find that the remaining vortices
self-organise into two macroscopic coherent `Onsager vortex' clusters that are
stable indefinitely. We demonstrate that this occurs due to a novel physical
mechanism --- the evaporative heating of the vortices --- that results in a
negative temperature phase transition in the vortex degrees of freedom. At the
end of our simulations the system is trapped in a non-thermal state. Our
computational results provide a pathway to observing Onsager vortex states in a
superfluid Bose gas.Comment: 10 pages, 7 figure
Gravity on a Little Warped Space
We investigate the consistent inclusion of 4D Einstein gravity on a truncated
slice of AdS_5 whose bulk-gravity and UV scales are much less than the 4D
Planck scale, M_* << M_{Pl}. Such "Little Warped Spaces" have found
phenomenological utility and can be motivated by string realizations of the
Randall-Sundrum framework. Using the interval approach to brane-world gravity,
we show that the inclusion of a large UV-localized Einstein-Hilbert term allows
one to consistently incorporate 4D Einstein gravity into the low-energy theory.
We detail the spectrum of Kaluza-Klein metric fluctuations and, in particular,
examine the coupling of the little radion to matter. Furthermore, we show that
Goldberger-Wise stabilization can be successfully implemented on such spaces.
Our results demonstrate that realistic low-energy effective theories can be
constructed on these spaces, and have relevance for existing models in the
literature.Comment: 1+24 page
Linear density response function in the projector-augmented wave method: Applications to solids, surfaces, and interfaces
We present an implementation of the linear density response function within
the projector-augmented wave (PAW) method with applications to the linear
optical and dielectric properties of both solids, surfaces, and interfaces. The
response function is represented in plane waves while the single-particle
eigenstates can be expanded on a real space grid or in atomic orbital basis for
increased efficiency. The exchange-correlation kernel is treated at the level
of the adiabatic local density approximation (ALDA) and crystal local field
effects are included. The calculated static and dynamical dielectric functions
of Si, C, SiC, AlP and GaAs compare well with previous calculations. While
optical properties of semiconductors, in particular excitonic effects, are
generally not well described by ALDA, we obtain excellent agreement with
experiments for the surface loss function of the Mg(0001) surface with plasmon
energies deviating by less than 0.2 eV. Finally, we apply the method to study
the influence of substrates on the plasmon excitations in graphene. On
SiC(0001), the long wavelength plasmons are significantly damped although
their energies remain almost unaltered. On Al(111) the plasmon is
completely quenched due to the coupling to the metal surface plasmon.Comment: 11 pages, 8 figures, articl
Motion of vortices in inhomogeneous Bose-Einstein condensates
We derive a general and exact equation of motion for a quantised vortex in an
inhomogeneous two-dimensional Bose-Einstein condensate. This equation expresses
the velocity of a vortex as a sum of local ambient density and phase gradients
in the vicinity of the vortex. We perform Gross-Pitaevskii simulations of
single vortex dynamics in both harmonic and hard-walled disk-shaped traps, and
find excellent agreement in both cases with our analytical prediction. The
simulations reveal that, in a harmonic trap, the main contribution to the
vortex velocity is an induced ambient phase gradient, a finding that
contradicts the commonly quoted result that the local density gradient is the
only relevant effect in this scenario. We use our analytical vortex velocity
formula to derive a point-vortex model that accounts for both density and phase
contributions to the vortex velocity, suitable for use in inhomogeneous
condensates. Although good agreement is obtained between Gross-Pitaevskii and
point-vortex simulations for specific few-vortex configurations, the effects of
nonuniform condensate density are in general highly nontrivial, and are thus
difficult to efficiently and accurately model using a simplified point-vortex
description.Comment: 13 pages, 8 figure
Pairwise wave interactions in ideal polytropic gases
We consider the problem of resolving all pairwise interactions of shock
waves, contact waves, and rarefaction waves in 1-dimensional flow of an ideal
polytropic gas. Resolving an interaction means here to determine the types of
the three outgoing (backward, contact, and forward) waves in the Riemann
problem defined by the extreme left and right states of the two incoming waves,
together with possible vacuum formation. This problem has been considered by
several authors and turns out to be surprisingly involved. For each type of
interaction (head-on, involving a contact, or overtaking) the outcome depends
on the strengths of the incoming waves. In the case of overtaking waves the
type of the reflected wave also depends on the value of the adiabatic constant.
Our analysis provides a complete breakdown and gives the exact outcome of each
interaction.Comment: 39 page
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