Hysteresis in a Quantized BEC Superfluid
February 14, 2014
In hysteretic systems, changes in state are not symmetrically reversible. That is, one pathway is required to place the system in a new state, but simply reversing that pathway will not return the system to its original state.
Instead, a different pathway is required. A graph of both pathways forms a shape called a hysteresis loop. Thus the state of the system at any given instant depends critically on its history.
To observe this phenomenon, the scientists created a toroidal BEC containing approximately 400,000 ultracold sodium atoms. The atoms were "stirred" into superfluid rotation by penetrating the BEC vertically with a green laser beam about 8 micrometers wide, which produces a low-density barrier. The beam was moved radially around the ring to induce flow. The beam could be adjusted for rotation rate and intensity.
Flow velocity in the BEC ring is quantized, and the researchers determined the conditions required to place the atoms in the first stable excited state. This occurs as an abrupt transition when the stir rate reaches a critical value.
They then found, as anticipated by theory, that merely slowing the laser beam rotation did not alter the superfluid's fluid velocity. In fact, under some circumstances, the researchers had to reverse the direction of the stirring beam in order to bring the atoms back to their initial condition. The results suggest that the hysteresis effect is likely caused by vortices that develop in the BEC during rotation, and which take time to dissipate. By tuning various aspects of the experiment, the researchers were able to control the size of the hysteresis loop.
The project, led by Gretchen Campbell of the Quantum Measurement Division's Laser Cooling and Trapping Group, had earlier observed quantized rotation in a toroidal BEC, and identified ways to place the system in a desired quantum state. The lastest work builds on that accomplishment.