"Novel Ground States of Bose-Condensed Systems: Experiments
with Cold Molecules & Quantized Vortices"
Jamil R. Abo-Shaeer
Massachusetts Institute of Technology
Wednesday, April 7, 2004 -11 AM - 12:00 Noon in 375 LeConte Hall
Abstract
Bose-Einstein condensates (BEC) provide a new tool for the study of
macroscopic quantum phenomena and condensed matter systems. Recently, the field of BEC
has been extended to include cold molecules. Molecular condensates may lead to a host
of new scientific explorations. These included quantum gases with anisotropic dipolar
interactions, test of fundamental symmetries such as the search for a permanent
electric dipole moment, study of rotational and vibrational energy transfer
processes, and coherent chemistry, where reactants and products are in coherent
quantum superposition states.
Ultra-cold sodium molecules were produced were produced from an atomic
Bose-Einstein condensate by ramping an applied magnetic field across a Feshbach
resonance. More than 10 thousand molecules were generated with an apparent
conversion efficiency of ~4%. Using laser light resonant with an atomic
transition, the remaining atoms could be selectively removed, preventing fast collisional
relaxation of the molecules. Time-of-flight analysis of the pure molecular sample
yielded an instantaneous phase-space density greater than 20.
Quantized vortices play a key role in superfluidity and superconductivity.
Magnetic fields can penetrate type-II superconductors only as quantized flux lines.
Vorticity can enter rotating superfluids only in the form of discrete line
defects with quantized circulation. These phenomena are direct consequences of the
existence of a macroscopic wavefunction, the phase of which must change by
integer multiples of 2Pi around magnetic flux or vortex lines. In superconductors,
magnetic flux lines arrange themselves in regular lattices that have been directly
imaged. In superfluid helium-4, direct observation of vortices has been limited to
small arrays.
The formulation of highly-ordered vortex lattices in a Bose-condensed gas
has been observed. These triangular lattices contain more than 150 vortices with lifetimes
of several seconds. The vortices were generated by rotating the condensate with a
scanning blue-detuned laser beam. Depending on the stirrer size, vortices were
either nucleated at discrete surface-mode resonances (large beams) or over a broad
range of stirring frequencies (small beams). Additionally, the dynamics of the
lattices have been studied at finite temperature by varying the condensed fraction
of atoms in the system. The decay of angular momentum is observed to be strongly
temperature-dependant, while the crystallization of the lattice appears to be
insensitive to temperature change.
