Low Temperature Physics

Professor Emeritus Gary Williams

When liquid helium is cooled to temperatures less than a few degrees above absolute zero (0 K, or -460 F), it undergoes a phase transition to a new quantum state, where the atoms all flow together coherently.  In this superfluid state the liquid loses all viscosity, and can flow in persistent currents that can last as long as the age of the universe.

In our lab we have studied the superfluidity of very thin films of liquid helium, less than a few atomic layers thick.  Graduate student Emin Menachekanian (now a Professor at Santa Monica College) carried out experiments at temperatures of 0.1 K and above on thin films adsorbed on carbon nanotubes using sound wave propagation in the films, known as third sound.  He was able to observe the Kosterlitz-Thouless transition with increasing temperature, where the superfluidity disappears. These measurements are now being extended with a new torsion oscillator apparatus, which  will allow direct measurements of the superfluidity for film thicknesses less than a single atomic layer. 

In conjunction with our measurements on these two-dimensional superfluid films, we have carried out theoretical studies to better understand the Kosterlitz-Thouless superfluid phase transition that involves the creation of quantized vortices in the films. This work won Professors Kosterlitz and Thouless the 2017 Nobel Prize.  With former graduate students Andrew Forrester and Han-Ching Chu Czarnecka we have studied the dynamics of the vortices, and with this have been able to formulate the first analytic theory of superfluid quantum turbulence.    

With former undergrad intern Mingyu Fan (now a UCSB PhD) we have formulated a study of the statistical fluctuations of those quantized vortices using Monte Carlo simulations, and found that they can be characterized by sum rules first formulated for two-dimensional Coulomb gases.  A highlight of the work was that when extending it to infinite temperature, we were able to exactly confirm a prediction in an obscure paper from 40 years ago by Indian Professor Deepak Dhar, who had just won the 2022 Boltzmann Medal for his other work in statistical mechanics.  Our paper is submitted to Physical Review E, and a preprint can be found here.

Speed of third sound thickness waves in very thin films of superfluid helium adsorbed on carbon nanotubes.