Superfluidity in D_2(H_2)_NClusters

摘要

The low-temperature properties of small para-112 clusters doped by a single ortho-D_2 moleculeare computed using the path-integral Monte Carlo method. The clusters of D_2(H_2)_NwithN =13and 18 are found to show significant superfluid fractions below 2 K, and the effects due to thepresence of a D_2impurity molecule are quite different between these two cluster sizes. TheN=13cluster has superfluid fractions virtually identical to those of the impurity-free (H_2)_(13)cluster atall temperatures considered here, but the superfluidity for N = 18 is shown to be considerablysuppressed by addition of the impurity molecule. The heavier D_2molecule is also shown to belocated around the cluster center with negligible thermal effects on its density distribution below2 K, which can be understood with the occurrence of Bose condensation manifested by largesuperfluid fractions. While the density distribution of D_2for the case ofN=13 is found to bepeaked at the cluster center, the D_2density peak forN=18 is located slightly off the centerbelow the onset temperature of superfluidity. We conclude that the difference in the effects of theimpurity molecule on the superfluidity between the N 13 and the N = 18 clusters results fromthe recently-discovered magic number behavior of hydrogen clusters