Knowledge Bank

The prospect of directly observing superfluidity in para-H$2$ is a tantalizing but elusive goal. Like $4$He, para-H$2$ is a light zero-spin boson. However, H$2$-H$2$ 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-H$2$ in the form of clusters or in nano-confined environments, and superfluid transition temperatures as high as $\sim$6\; K\; have\; been\; predicted.\; \backslash textbf\{15\},\; 343\; (1972).\}\; Spectroscopic\; observations\; of\; (para-H$_2$)$_N$-CO$_2$ 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 H$2$-CO$2$ intermolecular potential surface, submitted.} which explicitly incorporates dependence on the CO$2$ \nub{3} asymmetric stretch, it is now possible to achieve a remarkably consistent picture of (para-H$2$)-CO$2$ clusters in the size range $N$ = 1 $\sim$ 20. By combining the experimental spectroscopic measurements and theoretical simulations, we determine the size evolution of the superfluid response of the CO$2$-doped para-H$2$ clusters, which peaks for the ``magic" number $N$ = 12.