4,301 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
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 eventsâa 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
A dipolar self-induced bosonic Josephson junction
We propose a new scheme for observing Josephson oscillations and macroscopic
quantum self-trapping phenomena in a toroidally confined Bose-Einstein
condensate: a dipolar self-induced Josephson junction. Polarizing the atoms
perpendicularly to the trap symmetry axis, an effective ring-shaped,
double-well potential is achieved which is induced by the dipolar interaction.
By numerically solving the three-dimensional time-dependent Gross-Pitaevskii
equation we show that coherent tunneling phenomena such as Josephson
oscillations and quantum self-trapping can take place. The dynamics in the
self-induced junction can be qualitatively described by a two-mode model taking
into account both s-wave and dipolar interactions.Comment: Major changes. Accepted for publication in EP
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
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
Resonant transmission of light through finite chains of subwavelength holes
In this paper we show that the extraordinary optical transmission phenomenon
found before in 2D hole arrays is already present in a linear chain of
subwavelength holes, which can be considered as the basic geometrical unit
showing this property. In order to study this problem we have developed a new
theoretical framework, able to analyze the optical properties of finite
collections of subwavelength apertures and/or dimples (of any shape and placed
in arbitrary positions) drilled in a metallic film.Comment: Accepted for publication in Phys. Rev. Let
The Ramsey property for operator spaces and noncommutative Choquet simplices
The noncommutative Gurarij space NG, initially defined by Oikhberg, is a canonical object in the theory of operator spaces. As the Fraisse limit of the class of finite-dimensional nuclear operator spaces, it can be seen as the noncommutative analogue of the classical Gurarij Banach space. In this paper, we prove that the automorphism group of NG is extremely amenable, i.e. any of its actions on compact spaces has a fixed point. The proof relies on the Dual Ramsey Theorem, and a version of the Kechris-Pestov-Todorcevic correspondence in the setting of operator spaces. Recent work of Davidson and Kennedy, building on previous work of Arveson, Effros, Farenick, Webster, and Winkler, among others, shows that nuclear operator systems can be seen as the noncommutative analogue of Choquet simplices. The analogue of the Poulsen simplex in this context is the matrix state space NP of the Fraisse limit A(NP) of the class of finite-dimensional nuclear operator systems. We show that the canonical action of the automorphism group of NP on the compact set NP1 of unital linear functionals on A(NP) is minimal and it factors onto any minimal action, whence providing a description of the universal minimal flow ofAut(NP). (C) 2021 Elsevier Inc. All rights reserved
Integrability of a t-J model with impurities
A t-J model for correlated electrons with impurities is proposed. The
impurities are introduced in such a way that integrability of the model in one
dimension is not violated. The algebraic Bethe ansatz solution of the model is
also given and it is shown that the Bethe states are highest weight states with
respect to the supersymmetry algebra gl(2/1)Comment: 14 page
Integrable su(3) spin chain combining different representations
The general expression for the local matrix of a quantum chain
with the site space in any representation of su(3) is obtained. This is made by
generalizing from the fundamental representation and imposing the
fulfillment of the Yang-Baxter equation. Then, a non-homogeneous spin chain
combining different representations of su(3) is solved by developing a method
inspired in the nested Bethe ansatz. The solution for the eigenvalues of the
trace of the monodromy matrix is given as two coupled Bethe equations. A
conjecture about the solution of a chain with the site states in different
representations of su(n) is presented. The thermodynamic limit of the ground
state is calculated.Comment: PlainTex harvmac, 30 pages, 7 figures, to appear in Journal of
Physics
- âŠ