Colloquium Series: Dr. Robert Brady, University of Cambridge
Tuesday, October 4 @ 4:00 pm - 5:00 pm
Join us in Chapman Hall 125 for our Colloquium Series. Robert Brady from the University of Cambridge will be joining us to present “The physics of a ball-pit: Correction to the phase diagram of helium”
Dr. Robert Brady
Research Synopsis: Helium is the only atom that is spherical; all the others are aspherical due to their electron orbits or have covalent bonds. Ed Samulski and Robert Brady have been doing extensive research using a kids’ ball pit. Our findings overturn the 60-year-old phase diagram of helium.
Like the balls in a ball-pit, low temperature helium goes from a liquid-like phase to a solid when it is pressurized. There are two solid phases: hexagonal close-packed (hcp) to body-centered cubic (bcc). But the volume change at the transition between them is 19 times smaller than expected from the packing efficiencies. This is very strong counterevidence to the current phase diagram. It is amply documented but has never been acknowledged, much less explained, in the literature.
Here we correct the phase diagram of helium by showing that the high pressure has forced approximately 8% interstitial atoms into the canonical bcc structure. The new atomic arrangement has lower Gibbs free energy than simple bcc, quantitatively accounting for the volume change and for further anomalies including the phase’s giant entropy, specific heat anomaly, very large crystals, degraded x-ray diffraction pattern, solid flow, melting transition at low temperature and its footprint in the phase diagram, which is much larger in 3He than 4He. Our findings challenge the assumption that the phase is a quantum solid—all the foregoing phenomena follow from ordinary laws of thermodynamics.