Percolation in aggregates of nanoclusters immersed in superfluid helium

Abstract

Impurity-helium condensates created by injection of hydrogen (deuterium) atoms and molecules as well as rare gas (RG) atoms (Ne and Kr) into superfluid $^{4}\mathrm{He}$ have been studied via electron spin resonance (ESR) techniques. Measurements of the ground-state spectroscopic parameters of hydrogen and deuterium atoms show that the nanoclusters have a shell structure. H and D atoms reside in solid molecular layers of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$, respectively. These layers form on the surfaces of RG (Ne or Kr) nanoclusters. By monitoring the recombination of H atoms in the collection of hydrogen-neon nanoclusters, we show that nanoclusters form a gel-like porous structure which enables the H atoms to be transported through the structure via percolation. Observation of percolation in the collection of nanoclusters containing stabilized hydrogen atoms may open possibilities for a search for macroscopic collective quantum phenomena at ultralow temperatures accessible by a dilution refrigerator.