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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, 2007, 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.
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." |
