Third Sound Amplification by Stimulated Condensation
The superfluid state is essentially an example of a matter "laser":
Condensation of normal vapor atoms into the superfluid can be thought of
as a slight population inversion (out of equilibrium vapor phase) undertaking
a transition to a lower state (the liquid), joining coherently into a macroscopically
occupied Bose state (the superfluid) in the process.
Our experiments take this analogy one step closer to a laser.
Through a continuous reflux of vapor atoms onto a third sound resonator,
the macroscopic quantum acoustic state (third sound) will be amplified
as a consequence of the particles condensing coherently into the mode.
The acoustic energy is preserved in the original mode if the particles
are removed via film flow involving an orthoganal acousticstate.
Self sustained third sound will result if roughly all of the particles
in the resonating film are replaced within the free decay time of the third
We are currently de-bugging a third sound resonator apparatus specifically
designed to test this effect. Here's a photo of our latest attempt
at the resonator and vapor
source which bolt together onto the mixing chamber at the bottom of
We have shown that vortices are quite strongly pinned at low temperatures,
with critical velocities on the order of one or two meters per second.
Experimental evidence, as well as simple models of pinning to idealized
surface defects lead us to believe that pinned vortices will be an innate
feature of any film.
We are performing measurements of the adsorption and diffusion of water
in organic aerogels as well as the complex elastic shear moduli under different
hydration conditions. The results are being used to develop a universal
model for reversible crispy to soggy transformations.
Third Sound Attenuation
This is still a can of worms. There are lots of competing mechanisms
for dissipation, with pinned vortices right in the fray. We are working
Nonlinear Mode Coupling
As in classical waves, third sound has nonlinear terms in the equations
of motion that are emphasized when the dispersion relation is nearly linear.
This makes terms such as the Bernoulli pressure couple temporally and spatially,
i.e. w1 + w2
= w3 AND k1 + k2
= k3. Third was thought to have a highly linear dispersion
relation, yet there has been little evidence of strong nonlinearities.
We have verified what nonlinear terms are active and identified several
dispersive effects that lead to much larger quadratic terms in the third
sound dispersion relation.
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