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HUI LI, ROBERT J. LE ROY, PIERRE-NICHOLAS ROY, Department of Chemistry, University of Waterloo, Waterloo, ON N2L 3G1, Canada; A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, ON K1A 0R6, Canada.
The prospect of directly observing superfluidity in para-H2 is a tantalizing but elusive goal. Like 4He, para-H2 is a light zero-spin boson. However, H2-H2 intermolecular interactions, though weak, are stronger than He-He interactions, and hydrogen is a solid below about 14 K. This makes detection of superfluidity in bulk hydrogen problematical, to say the least. But there are still possibilities for para-H2 in the form of clusters or in nano-confined environments, and superfluid transition temperatures as high as
6 K have been predicted.
Spectroscopic observations of (para-H2)N-CO2 clusters were at first very difficult to interpret for N > 5. However, with the help of path integral Monte Carlo simulations and an accurate new H2-CO2 intermolecular potential surface which explicitly incorporates dependence on the CO2
3 asymmetric stretch, it is now possible to achieve a remarkably consistent picture of (para-H2)N-CO2 clusters in the size range N = 1
20. By combining the experimental spectroscopic measurements and theoretical simulations, we determine the size evolution of the superfluid response of the CO2-doped para-H2 clusters, which peaks for the ``magic" number N = 12.