<b>Complex Systems</b><br><i>Quantum Turbulence</i>

Speaker: Dan Lathrop (University of Maryland)

Long range quantum order underlies a number of related physical phenomena including superfluidity, superconductivity, the Higgs mechanism, Bose-Einstein condensates, and spin systems. While superfluidity in Helium-4 was one of the earliest discovered of these, it is not the best understood, owing to the strong interactions (making theoretical progress difficult) and the lack of local experimental probes. Approximately three years ago, our group discovered that micron-sized hydrogen particles may be used to label quantized vortices in flows of superfluid helium. Particles not on vortices trace the motion of the normal component of the superfluid. This ability has given a new perspective on an old subject.

By directly observing and tracking these particles, we have directly confirmed the two-fluid model, observed vortex rings and reconnection, characterized thermal counterflows, and taken local observations of the very peculiar nature of quantum turbulence. One of many surprising observations is the existence of power law tails in the probability distribution of velocity for these flows. That is easily understood as stemming from the reconnection on quantized vortices. Our summary conclusions are that quantum turbulence is dominated by reconnection and ring vortex collapse, making turbulence in a quantum liquid distinct from classical turbulence of a Newtonian fluid.