Search for superfluidity in solid hydrogen

Saga of Superfluid Solids

The article presents the state of the art and reviews the literature on the long-standing problem of the possibility for a sample to be at the same time solid and superfluid. Theoretical models, numerical simulations, and experimental results are discussed.

Producing and quantifying enriched para-H2

The production of enriched para-H(2) is useful for many scientific applications, but the technology for producing and measuring para-H(2) is not yet widespread. In this note and in the accompanying auxiliary material, we describe the design, construction, and use of a versatile standalone converter that is capable of producing para-H(2) enrichments of up to > or = 99.99% at continuous flow rates of up to 0.4 SLM. We also discuss para-H(2) storage and back conversion rates, and improvements to three techniques (thermal conductance, NMR, and solid hydrogen impurity spectroscopy) used to quantify the para-H(2) enrichment.

Hydrogen clusters that remain fluid at low temperature

Superfluidity of hydrogen, predicted three decades ago, continues to elude experimental observation, due to the high freezing temperature of H2 at T=13.8 K. Here, large para-H2 clusters are obtained in a cryogenic pulsed free jet expansion and are studied via nonlinear Raman spectroscopy. Clusters formed from neat pH_{2} are solid as evidenced by the characteristic splitting of the rotational S0(0) line. However, clusters formed of highly diluted pH_{2} (< or = 1%) in He have a single S0(0) line and remain liquid at the estimated superfluid transition temperature of T=1-2 K.

Superfluidity of grain boundaries in solid 4He

By large-scale quantum Monte Carlo simulations we show that grain boundaries in 4He crystals are generically superfluid at low temperature, with a transition temperature of the order of approximately 0.5 K at the melting pressure; nonsuperfluid grain boundaries are found only for special orientations of the grains. We also find that close vicinity to the melting line is not a necessary condition for superfluid grain boundaries, and a grain boundary in direct contact with the superfluid liquid at the melting curve is found to be mechanically stable and the grain-boundary superfluidity observed by Sasaki et al. [Science 313, 1098 (2006)10.1126/science.1130879] is not just a crack filled with superfluid.