Exchange-driven self-diffusion of nanoscale crystalline parahydrogen clusters on graphite

TL;DR

This study uses computer simulations to demonstrate superfluidity and self-diffusion in nanoscale parahydrogen clusters on graphite at very low temperatures, revealing quantum effects that enable clusters to move despite substrate pinning.

Contribution

It provides new insights into quantum self-diffusion and superfluid behavior of parahydrogen clusters on graphite, highlighting size-dependent phenomena and quantum exchanges.

Findings

Clusters with 7-12 molecules show superfluidity and crystalline order.

Certain clusters self-diffuse on the surface despite substrate pinning.

Quantum exchanges enable cluster mobility without classical analogs.

Abstract

Computer simulations yield evidence of superfluid behavior of nanoscale size clusters of parahydrogen adsorbed on a graphite substrate at low temperature ($T\lesssim 0.25 \text{ K}$). Clusters with a number of molecules between 7 and 12 display concurrent superfluidity and crystalline order, reflecting the corrugation of the substrate. Remarkably, it is found that specific clusters with a number of molecules ranging between 7 and 12 self-diffuse on the surface like free particles, despite the strong pinning effect of the substrate. This effect is underlain by coordinated quantum-mechanical exchanges of groups of identical molecules, i.e., it has no classical counterpart.