O. M. Marago

Vortex Nucleation in Bose-Einstein Condensates in an Oblate, Purely Magnetic Potential

We have investigated the formation of vortices by rotating the purely magnetic potential confining a Bose-Einstein condensate. We modified the bias field of an axially symmetric TOP trap to create an elliptical potential that rotates in the radial plane. This enabled us to study the conditions for vortex nucleation over a wide range of eccentricities and rotation rates.

Observation of the scissors mode and evidence for superfluidity of a trapped bose-einstein condensed Gas

We report the observation of the scissors mode of a Bose-Einstein condensed gas of 87Rb atoms in a magnetic trap, which gives direct evidence of superfluidity in this system. The scissors mode of oscillation is excited by a sudden rotation of the anisotropic trapping potential. For a gas above T(c) (normal fluid) we detect the occurrence of oscillations at two frequencies, with the lower frequency corresponding to the rigid body value of the moment of inertia. Well below T(c) the condensate oscillates at a single frequency, without damping, as expected for a superfluid.

A pyramidal magneto-optical trap as a source of slow atoms

Abstract We have constructed and characterised a novel source of slow atoms based on a pyramidal magneto optical trap with a small hole at its vertex. Atoms are first captured in the trap and then pushed through the hole by a laser beam. The size and velocity of the resulting pulses of atoms were measured. The flux of cold atoms was 1.1×10 9 atoms/s and the apparatus is readily scaleable to obtain higher fluxes.

Experimental Observation of Beliaev Coupling in a Bose-Einstein Condensate

We report the first experimental observation of Beliaev coupling between collective excitations of a Bose-Einstein condensed gas. Beliaev coupling is not predicted by the Gross-Pitaevskii equation and so this experiment tests condensate theory beyond the mean field approximation. Measurements of the amplitude of a high frequency scissors mode show that the Beliaev process transfers energy to a lower-lying mode and then back and forth between these modes, unlike Landau processes which lead to a monotonic decrease in amplitude. To enhance the Beliaev process we adjusted the geometry of the magnetic trapping potential to give a frequency ratio of 2 to 1 between the two scissors modes.

Observation of harmonic generation and nonlinear coupling in the collective dynamics of a bose-einstein condensate

We report the observation of harmonic generation and strong nonlinear coupling of two collective modes of a condensed gas of rubidium atoms. Using a modified time averaged orbiting potential trap we changed the trap anisotropy to a value where the frequency of the m = 0 high-lying mode corresponds to twice the frequency of the m = 0 low-lying mode, thus leading to strong nonlinear coupling between these modes. By changing the anisotropy of the trap and exciting the low-lying mode we observed significant frequency shifts of this fundamental mode and also the generation of its second harmonic.

Direct observation of irrotational flow and evidence of superfluidity in a rotating Bose-Einstein condensate.

We have observed the expansion of vortex-free, rotating Bose condensates after their sudden release from a slowly rotating anisotropic trap. Conservation of angular momentum, combined with the constraint of irrotational flow, cause the rotating condensate to expand in a distinctively different way to one released from a static (nonrotating) trap. This difference provides clear experimental evidence of the purely irrotational velocity field associated with a superfluid. We observed this behavior in absorption images taken along the rotation axis.

Calculation of mode coupling for quadrupole excitations in a Bose-Einstein condenstate

In this paper, we give a theoretical description of resonant coupling between two collective excitations of a Bose-condensed gas on, or close to, a second-harmonic resonance. Using analytic expressions for the quasiparticle wave functions, we show that the coupling between quadrupole modes is strong, leading to a coupling time of a few milliseconds (for a TOP trap with radial frequency ∼ 100 Hz and ∼10 4 atoms). Using the hydrodynamic approximation, we derive an analytic expression for the coupling matrix element. These can be used with an effective Hamiltonian (that we also derive) to describe the dynamics of the coupling process and the associated squeezing effects.

The moment of inertia and the scissors mode of a Bose-condensed gas

We relate the frequency of the scissors mode to the moment of inertia of a trapped Bose gas at finite temperature in a semi-classical approximation. We apply these theoretical results to the data obtained in our previous study of the properties of the scissors mode of a trapped Bose-Einstein condensate of 87Rb atoms as a function of the temperature. The frequency shifts that we measured show quenching of the moment of inertia of the Bose gas at temperatures below the transition temperature - the system has a lower moment of inertia than that of a rigid body with the same mass distribution, because of superfluidity.

Bose-Einstein condensation in a rotating anisotropic TOP trap

We describe the construction and operation of a time-orbiting potential trap that has different oscillation frequencies along its three principal axes. These axes can be rotated and we have observed Bose-Einstein condensates of 87Rb with a rotating ellipsoidal shape. Under these conditions it has been predicted that quantized vortices form and are stable.We describe the construction and operation of a time-orbiting potential trap (TOP trap) that has different oscillation frequencies along its three principal axes. These axes can be rotated and we have observed Bose-Einstein condensates of 87^Rb with a rotating ellipsoidal shape. Under these conditions it has been predicted that quantized vortices form and are stable.

Bose-Einstein condensation in a stiff TOP trap with adjustable geometry

We report on the realization of a stiff magnetic trap with independently adjustable trap frequencies, ωz and ωr, in the axial and radial directions, respectively. This has been achieved by applying an axial modulation to a time-averaged orbiting potential (TOP) trap. The frequency ratio of the trap, ωz/ωr, can be decreased continuously from the original TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein condensate (BEC) into this trap and obtained very good agreement between its observed anisotropic expansion and the hydrodynamic predictions. Our method can be extended to obtain a spherical trapping potential, which has a geometry of particular theoretical interest.