Current Projects

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 sound resonance.

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 the refrigerator.

Vortex Pinning

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.

Aerogel Mechanics

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.

Past Projects

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 on this...

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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