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10/8/2010

The Destruction of Superfluidity by Disorder

The phenomenon of superconductivity portends exciting future technologies, such as magnetically levitated trains for efficient transportation and resistance-free electrical transmission lines. One complication in creating new technologies with superconductors is the presence of random defects in the material. Possibly the most important and interesting consequence of these defects is the destruction of superconductivity in the system.

The complexity of actual materials can make it difficult to quantify the effects of disorder on superconducting systems. Scientists have recently turned to more idealized systems, such as those comprised of ultracold atomic gases, to model real materials. When atoms are cooled to temperatures below 1 millionth of a degree above absolute zero, they form a Bose-Einstein condensate, which is a superfluid. Superfluids flow without impedance, in analogy to the electrons in a superconductor that flow without electrical resistance. But unlike real materials, the ultracold atom are so extremely pure and controllable that they can be used as idealized realizations of theoretical models.

Disorder can be imposed on a Bose-Einstein condensate by using optical speckle. Speckle is produced by passing a laser beam through a diffusive piece of glass, such as one that has been sand-blasted. The variation of the light amplitude across the condensate causes the atoms to experience a random potential, much like the effect of impurities on the electrons in a superconductor. Scientists at Rice University, led by Randy Hulet, have used optical speckle to explore the destruction of superfluidity of a Bose-Einstein condensate of lithium atoms due to the presence of disorder. The condensate is initially trapped in a harmonic potential that allows the condensate to oscillate freely back and forth like a mass on a spring. The researchers found that when optical speckle was superimposed on the harmonic potential, then the motion of the condensate became dissipative. Surprisingly, the damping rate of the motion depends on the velocity of the condensate traveling through the harmonic potential, as shown in the figures below.

 

Figure showing Damped Dipole Oscillation    Figure showing Velocity Dependent Damping

Left: Position of a condensate traveling through a harmonic potential as a function of time. The amplitude of the motion gradually decreases, i.e., the motion is dissipative. The change in rate of the amplitude is maximal at about 3.5 seconds, indicating that the damping depends on the velocity of the condensate in a non-trivial way.

Right: Damping rate extracted from fit to condensate position as a function of the velocity of the condensate. The damping peaks when the velocity of the condensate is equal to the internal speed-of-sound within the condensate.

The damping was found to be caused by the excitation of defects, known as dark solitons, in the otherwise smooth Bose-Einstein condensate. In addition to measuring how superfluid dissipation depends on velocity, the Rice team has also investigated how dissipation depends on the strength of the disorder, interactions between the atoms, and even the effect of confining the atoms in one spatial dimension, rather than in three.


References:

D. Dries, S. E. Pollack, J. M. Hitchcock, and R. G. Hulet, "Dissipative Transport of a Bose-Einstein Condensate", Physical Review A in press (2010). arXiv:1004.1891v2

Yong P. Chen, J. Hitchcock, D. Dries, M. Junker, C. Welford, and R. G. Hulet, "Phase Coherence and Superfluid-Insulator Transition in a Disordered Bose-Einstein Condensate", Physical Review A77, 033632 (2008).

Hulet Lab Website: http://atomcool.rice.edu