|
CHAMPAIGN, Ill. -- By using ideas
developed in disparate fields, from earthquake dynamics to
random-field magnets, researchers at the University of Illinois have
constructed a model that describes the avalanchelike, phase-slip
cascades in the superflow of helium.
Just as superconductors have no
electrical resistance, superfluids have no viscosity, and can flow
freely. Like superconductors, which can be used to measure extremely
tiny magnetic fields, superfluids could create a new class of
ultra-sensitive rotation sensors for use in precision guidance
systems and other applications.
But, before new sensors can be built,
scientists and engineers must first acquire a better understanding
of the odd quirks of superfluids arising in these devices.
In the April 23 issue of Physical
Review Letters, U of I physicist Paul Goldbart, graduate student
David Pekker and postdoctoral research associate Roman Barankov
describe a model they developed to explain some of those quirks,
which were found in recent experiments conducted by researchers at
the University of California at Berkeley.
In the Berkeley experiments, physicist
Richard Packard and his students Yuki Sato and Emile Hoskinson
explored the behavior of superfluid helium when forced to flow from
one reservoir to another through an array of several thousand nano-apertures.
Their intent was to amplify the feeble whistling sound of
phase-slips associated with superfluid helium passing through a
single nano-aperture by collecting the sound produced by all of the
apertures acting in concert.
At low temperatures, this amplification turned out, however, to be
surprisingly weak, because of an unanticipated loss of synchronicity
among the apertures. "Our
model reproduces the key physical features of the Berkeley group's
experiments, including a high-temperature synchronous regime, a
low-temperature asynchronous regime and a transition between the
two," said Goldbart, who also is a researcher at the university's
Frederick Seitz Materials
Research Laboratory.
[to top of second column]
 |
 The theoretical model developed by
Pekker, Barankov and Goldbart balances a competition between
interaction and disorder -- two behaviors more commonly associated
with magnetic materials and sliding tectonic plates.
The main components of the
researchers' model are nano-apertures possessing different
temperature-dependent critical flow velocities (the disorder), and
inter-aperture coupling mediated by superflow in the reservoirs (the
interactions).
 For helium,
the superfluid state begins at a temperature of 2.18 kelvins. Very
close to that temperature, inter-pore coupling tends to cause
neighbors of a nano-aperture that already has phase-slipped also to
slip. This process may cascade, creating an avalanche of
synchronously slipping phases that produces a loud whistle.
However, at roughly one-tenth of a
kelvin colder, the differences between the nano-apertures dominate,
and the phase-slips in the nano-apertures are asynchronous, yielding
a non-avalanching regime. The loss of synchronized behavior weakens
the whistle. "In our model,
competition between disorder in critical flow velocities and
effective inter-aperture coupling leads to the emergence of rich
collective dynamics, including a transition between avalanching and
non-avalanching regimes of phase-slips," Goldbart said. "A key
parameter is temperature. Small changes in temperature can lead to
large changes in the number of phase-slipping nano-apertures
involved in an avalanche."
The work was funded by the U.S. Department of Energy and the
National Science Foundation.
[Text copied from
University of Illinois news release]
 |
