Vortices in Bose–Einstein condensates: A review of the experimental results

Abstract

Rotating dilute Bose--Einstein condensates (BEC) of alkali

atoms offer a testing ground for theories of vortices in weakly

interacting superfluids. In a rotating superfluid, quantised

vortices, with a vorticity h/m, form above a critical velocity. Such

vortices have been generated in BEC of alkali atoms by different

techniques such as (a) wave function engineering of a two-component

BEC, (b) decay of solitons, (c) rotation of a thermal cloud before

cooling it below the condensation temperature, (d) stirring with an

`optical' spoon, (e) rotating a deformation in the anisotropic trap

in which the condensate is trapped and (f) by creating Berry phase by

adiabatically reversing the axial magnetic field. Since the core of

a vortex is a fraction of a micrometer in diameter, it cannot be

directly imaged optically. The condensate with vortices is allowed

to ballistically expand till the size increases by one order before

the vortices are imaged. Surface wave spectroscopy and the change in

aspect ratio of a rotating cloud are the other techniques used.

Studies have been made on the creation and dynamics of single vortex

and on systems with more than a hundred vortices. Results have been

obtained on vortex nucleation, stability of vortex structures, nature

of the vortex lattice and defects in such a lattice. Important

results are: (a) evidence exists that vortex nucleation takes place by

a surface mode instability; but this is not the only mechanism; (b)

the vortex lattice is perfectly triangular right up to the edge; (c)

in the initial stages of rotation of the cloud a tangled web of

vortices is seen; it takes a few hundred milliseconds before the

vortices arrange themselves in a lattice; this time appears to be

independent of temperature; (d) the decay of vortices appears to arise

from the transfer of energy to the rotating thermal component and is

dependent on temperature; (e) defects in the lattices such as

dislocations and grain boundaries are seen; (f) transverse

oscillations (Tkachenko modes) of the vortex lattice have been

observed; and (g) giant vortices have been produced. These will be

discussed.