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Results of a comprehensive theoretical investigation of the low temperature (T ) properties of clusters of para-hydrogen, both pristine as well as doped with isotopic impurities (i.e., ortho-deuterium), will be presented. We study clusters comprising up to N =40 molecules, by means of quantum simulations based on the continuous-space Worm Algorithm. There are strong numerical indications to the effect that pristine clusters are liquid-like and superfluid in the T → 0 limit. The superfluid signal is uniform throughout these clusters; it is underlain by long cycles of permutation of molecules. Clusters with more than 22 molecules display solid-like, essentially classical behavior at temperatures down to T ∼ 1 K; some of them are seen to turn liquid-like at sufficiently low T (quantum melting). The introduction of even a single impurity can have dramatic effects, stabilizing a solid-like configuration, which emphasizes the role of long exchanges in these systems. |
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