8 research outputs found
Current-Phase Relation of a Bose-Einstein Condensate Flowing Through a Weak Link
We study the current-phase relation of a Bose-Einstein condensate flowing
through a repulsive square barrier by solving analytically the one dimensional
Gross-Pitaevskii equation. The barrier height and width fix the current-phase
relation , which tends to for weak
barriers and to the Josephson sinusoidal relation for
strong barriers. Between these two limits, the current-phase relation depends
on the barrier width. In particular, for wide enough barriers, we observe two
families of multivalued current-phase relations. Diagrams belonging to the
first family, already known in the literature, can have two different positive
values of the current at the same phase difference. The second family, new to
our knowledge, can instead allow for three different positive currents still
corresponding to the same phase difference. Finally, we show that the
multivalued behavior arises from the competition between hydrodynamic and
nonlinear-dispersive components of the flow, the latter due to the presence of
a soliton inside the barrier region.Comment: 6 pages, 5 figure
Critical velocity of superfluid flow through single barrier and periodic potentials
We investigate the problem of an ultracold atomic gas in the superfluid phase
flowing in the presence of a potential barrier or a periodic potential. We use
a hydrodynamic scheme in the local density approximation (LDA) to obtain an
analytic expression for the critical current as a function of the barrier
height or the lattice intensity, which applies to both Bose and Fermi
superfluids. In this scheme, the stationary flow becomes energetically unstable
when the local superfluid velocity is equal to the local sound velocity at the
point where the external potential is maximum. We compare this prediction with
the results of the numerical solutions of the Gross-Pitaevskii and
Bogoliubov-de Gennes equations. We discuss the role of long wavelength
excitations in determining the critical velocity. Our results allow one to
identify the different regimes of superfluid flow, namely, the LDA hydrodynamic
regime, the regime of quantum effects beyond LDA for weak barriers and the
regime of tunneling between weakly coupled superfluids for strong barriers. We
finally discuss the relevance of these results in the context of current
experiments with ultracold gases.Comment: 10 pages, 6 figures; appendix extended, to appear in Phys. Rev.
Effective Potential for Scalar Field in Three Dimensions: Ising Model in the Ferromagnetic Phase
We compute the effective potential for one-component real
scalar field in three Euclidean dimensions (3D) in the case of
spontaneously broken symmetry, from the Monte Carlo simulation of the 3D Ising
model in external field at temperatures approaching the phase transition from
below. We study probability distributions of the order parameter on the
lattices from to , at . We find that, in close
analogy with the symmetric case, plays an important role: is very well approximated by the sum of , and
terms. An unexpected feature is the negative sign of the
term. As close to the continuum limit as we can get (), we
obtain
We also compute several universal coupling constants and ratios, including
the combination of critical amplitudes .Comment: 13 pages, 5 Postscript figures, uses epsf.st
Suppression of Superfluidity of He in a Nanoporous Glass by Preplating a Kr Layer
Helium in nanoporous media has attracted much interest as a model Bose system
with disorder and confinement. Here we have examined how a change in porous
structure by preplating a monolayer of krypton affects the superfluid
properties of He adsorbed or confined in a nanoporous Gelsil glass, which
has a three-dimensional interconnected network of nanopores of 5.8 nm in
diameter. Isotherms of adsorption and desorption of nitrogen show that
monolayer preplating of Kr decreases the effective pore diameter to 4.7 nm and
broadens the pore size distribution by about eight times from the sharp
distribution of the bare Gelsil sample. The superfluid properties were studied
by a torsional oscillator for adsorbed film states and pressurized liquid
states, both before and after the monolayer Kr preplating. In the film states,
both the superfluid transition temperature and the superfluid
density decrease about 10 percent by Kr preplating. The suppression of film
superfluidity is attributed to the quantum localization of He atoms by the
randomness in the substrate potential, which is caused by the
preplating--induced broadening of the pore size distribution. In the
pressurized liquid states, the superfluid density is found
to increase by 10 percent by Kr preplating, whereas is
decreased by 2 percent at all pressures. The unexpected enhancement of
might indicate the existence of an unknown disorder effect
for confined He.Comment: 27 pages, 8 figures, submitted to J. Phys. Soc. Jp