It is shown by computer simulations that superfluid { \it para}-hydrogen clusters of more than 22 molecules can be turned insulating and "solidlike" by the replacement of as few as one or two molecules, with ones of the heavier { \it ortho}-deuterium isotope. A much smaller effect is observed with substitutional { \it ortho}-hydrogen. Substitutional { \it ortho}-deuterium molecules prevalently sit in the inner part of the cluster, whereas { \it ortho}-hydrogen impurities reside primarily in the outer shell, near the surface. Implications on the superfluidity of pure { \it para}-hydrogen clusters are discussed.
Categories: cond-mat.stat-mech