CPM Seminar

The physics of hydrogen clusters: superfluidity and quantum melting

Massimo Boninsegni

Department of Physics
University of Alberta

Molecular hydrogen has long been speculated to be a potential “second superfluid”, owing to the light mass of its constituents. Unfortunately, unlike helium bulk hydrogen crystallizes at low temperature, due to the strength of the intermolecular potential. Small clusters of para-Hydrogen, on the other hand, have been speculated to remain liquid-like at low temperature, possibly enabling one to observe superfluid behavior using recently developed spectroscopic techniques. Using Path Integral Monte Carlo simulations based on a recently developed “Worm” Algorithm, we have investigated superfluid properties of para-Hydrogen clusters of size up to 40 molecules at temperatures down to 0.5 K. Superfluidity is observed at low temperature in clusters of up to 27 molecules, with exchanges playing an important role even in the largest clusters. Superfluidity is strongly dependent on the structure of the clusters. We have also observed a behavior referred to as “quantum melting”, whereby clusters become increasingly liquid- like at low temperature, due to zero-point motion, and freeze at higher temperature. The effect of isotopic impurities has also been studied.