Faraday Waves in a BEC

Observation of Farraday Waves in a Bose-Einstein Condensate

P. Engels, C. Atherton, and M.A. Hoefer

PRL 98, 095301 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e095301

In 1831, Farraday observed the formation of patterns on thin layers of liquid on top of an oscillating piston. (There's an excellent video of this on YouTube, created by Robert Deegan, Florian Merkt, and Harry Swinney of the Center for Nonlinear Dynamics at the University of Texas.) According to the authors, this is one of the first scientific investigations of dynamically generated pattern formation.

One interesting aspect of Faraday waves is their nonlinear origin. The cylinder is being driven up and down, yet a pattern of waves (that oscillates at half the driving frequency) forms on the surface. These waves are not fundamental modes of a vibrating cylindrical membrane, but arise from nonlinear terms describing the fluid motion.

The authors do something similar with a Bose-Einstein condensate. They create a cigar-shaped BEC. The authors force the radial potential confining the atoms to oscillate periodically, then observe periodic variations in the density along the long axis of the cigar. They also investigate the regime of very strong driving by exciting a radial breathing mode of the condensate and driving it near its resonant frequency.

This didn't sound too surprising until I considered the symmetry of the system. Imagine taking an infinite cylinder (or, like the authors, a 250 micron BEC cigar), and periodically varying the radius. The system still has translational symmetry on the long axis, yet waves form. The continuous translation symmetry is broken into a discrete symmetry by driving radial fluctuations.

As with many good experimental papers, the graphs tell the story remarkably well. No wonder many people lament the decline in popularity of printed journals. You don't get to browse through all the figures in an online version. What's to catch your eye and tempt you to read an article outside your area of specialization? Very few titles in PRL are "catchy" ...