3,831 research outputs found
Dipolar condensates confined in a toroidal trap: ground state and vortices
We study a Bose-Einstein condensate of 52Cr atoms confined in a toroidal trap
with a variable strength of s-wave contact interactions. We analyze the effects
of the anisotropic nature of the dipolar interaction by considering the
magnetization axis to be perpendicular to the trap symmetry axis. In the
absence of a central repulsive barrier, when the trap is purely harmonic, the
effect of reducing the scattering length is a tuning of the geometry of the
system: from a pancake-shaped condensate when it is large, to a cigar-shaped
condensate for small scattering lengths. For a condensate in a toroidal trap,
the interaction in combination with the central repulsive Gaussian barrier
produces an azimuthal dependence of the particle density for a fixed radial
distance. We find that along the magnetization direction the density decreases
as the scattering length is reduced but presents two symmetric density peaks in
the perpendicular axis. For even lower values of the scattering length we
observe that the system undergoes a dipolar-induced symmetry breaking
phenomenon. The whole density becomes concentrated in one of the peaks,
resembling an origin-displaced cigar-shaped condensate. In this context we also
analyze stationary vortex states and their associated velocity field, finding
that this latter also shows a strong azimuthal dependence for small scattering
lengths. The expectation value of the angular momentum along the z direction
provides a qualitative measure of the difference between the velocity in the
different density peaks.Comment: 9 pages, 12 figure
Simulation of Suspensions, Torsion Bars, and Fifth Wheel for Semitrailers Using Finite Elements
The objective of this paper is the simulation of some different types of elements for semitrailers, like the suspension, both mechanical with springs and pneumatic with a spring anddiapresses; other parts like the wheels, the torsion bars, the fifth wheel and the suspension of the tractor unit have also been simulated. Then, the numerical simplified FE model of these elements that allows simulating the real behavior of the suspension to apply adequately the boundary conditions of a heavy vehicle has been obtained for a structural simulation using numerical tools with a good accuracy of the local and global behavior of the vehicle
Three-dimensional flow structure and bed morphology in large elongate meander loops with different outer bank roughness characteristics
© 2016. American Geophysical Union. All Rights Reserved. Few studies have examined the three-dimensional flow structure and bed morphology within elongate loops of large meandering channels. The present study focuses on the spatial patterns of three-dimensional flow structure and bed morphology within two elongate meander loops and examines how differences in outer bank roughness influence near-bank flow characteristics. Three-dimensional velocities were measured during two different events—a near-bankfull flow and an overbank event. Detailed data on channel bathymetry and bed form geometry were obtained during a near-bankfull event. Flow structure within the loops is characterized by strong topographic steering by the point bar, by the development of helical motion associated with flow curvature, and by acceleration of flow where bedrock is exposed along the outer bank. Near-bank velocities during the overbank event are less than those for the near-bankfull flow, highlighting the strong influence of the point bar on redistribution of mass and momentum of the flow at subbankfull stages. Multiple outer bank pools are evident within the elongate meander loop with low outer bank roughness, but are not present in the loop with high outer bank roughness, which may reflect the influence of abundant large woody debris on near-bank velocity characteristics. The positions of pools within both loops can be linked to spatial variations in planform curvature. The findings indicate that flow structure and bed morphology in these large elongate loops is similar to that in small elongate loops, but differs somewhat from flow structure and bed morphology reported for experimental elongate loops
Vortices in dipolar condensates with dominant dipolar interactions
We present full three-dimensional numerical calculations of single vortex
states in rotating dipolar condensates. We consider a Bose-Einstein condensate
of 52Cr atoms with dipole-dipole and s-wave contact interactions confined in an
axially symmetric harmonic trap. We obtain the vortex states by numerically
solving the Gross-Pitaevskii equation in the rotating frame with no further
approximations. We investigate the properties of a single vortex and calculate
the critical angular velocity for different values of the s-wave scattering
length. We show that, whereas the standard variational approach breaks down in
the limit of pure dipolar interactions, exact solutions of the Gross-Pitaevskii
equation can be obtained for values of the s-wave scattering length down to
zero. The energy barrier for the nucleation of a vortex is calculated as a
function of the vortex displacement from the rotation axis for different values
of the angular velocity of the rotating trap.Comment: 10 pages with 9 figure
A Renormalization Group for Hamiltonians: Numerical Results
We describe a renormalization group transformation that is related to the
breakup of golden invariant tori in Hamiltonian systems with two degrees of
freedom. This transformation applies to a large class of Hamiltonians, is
conceptually simple, and allows for accurate numerical computations. In a
numerical implementation, we find a nontrivial fixed point and determine the
corresponding critical index and scaling. Our computed values for various
universal constants are in good agreement with existing data for
area-preserving maps. We also discuss the flow associated with the nontrivial
fixed point.Comment: 11 Pages, 2 Figures. For future updates, check
ftp://ftp.ma.utexas.edu/pub/papers/koch
Phase-slippage and self-trapping in a self-induced bosonic Josephson junction
A dipolar condensate confined in a toroidal trap constitutes a self-induced
Josepshon junction when the dipoles are oriented perpendicularly to the trap
symmetry axis and the s-wave scattering length is small enough. The ring-shaped
double-well potential coming from the anisotropic character of the mean-field
dipolar interaction is robust enough to sustain self-trapping dynamics, which
takes place when the initial population imbalance between the two wells is
large. We show that in this system the self-trapping regime is directly related
to a vortex-induced phase-slip dynamics. A vortex and antivortex are
spontaneously nucleated in the low density regions, before a minimum of the
population imbalance is reached, and then cross the toroidal section in
opposite directions through the junctions.This vortex dynamics yields a phase
slip between the two weakly linked condensates causing an inversion of the
particle flux.Comment: 4 page
Photonic crystal optical waveguides for on-chip Bose-Einstein condensates
We propose an on-chip optical waveguide for Bose-Einstein condensates based
on the evanescent light fields created by surface states of a photonic crystal.
It is shown that the modal properties of these surface states can be tailored
to confine the condensate at distances from the chip surface significantly
longer that those that can be reached by using conventional index-contrast
guidance. We numerically demonstrate that by index-guiding the surface states
through two parallel waveguides, the atomic cloud can be confined in a
two-dimensional trap at about 1m above the structure using a power of
0.1mW.Comment: 5 pages, 4 figure
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