Superfluidity and Bose-Einstein condensation in free and confined parahydrogen clusters

点击翻译按钮获取中文摘要

Hydrogen is the most abundant element of the Universe, one of the most extensively investigated, about which a lot remains to be understood. In its molecular form, its condensed (i.e., liquid and solid) phases are greatly affected by quantum mechanics, rendering it qualitatively different from ordinary materials.One of the most spectacular manifestations of quantum mechanics on a macroscopic scale is superfluidity, namely the ability of a substance to flow without dissipation. Helium is the only known naturally occurring element to turn superfluid in its liquid phase, which exists at very low temperature, only 4 degrees K above absolute zero; however, parahydrogen, one of the two stable forms of molecular hydrogen, was speculated 45 years ago to be a candidate for superfluidity, as it features many of the same qualities of helium (mainly very light elementary constituents, i.e., molecules) but such a phase, despite intense investigation, has not yet been observed in the laboratory. The main reason for the failure to observe superfluidity is accepted to be the close proximity of the (putative) superfluid liquid phase to a crystalline one, which is thermodynamically stable at low temperature. It is presently believed that crystals like parahydrogen (or even helium) are not superfluid. Thus, any attempt to observe such a fascinating phase of parahydrogen requires that crystallization be eluded.Theoretical calculations carried out in our group over the past decade have shown that small free-standing clusters (of thirty molecules or less) should indeed be liquid like and superfluid at low temperature, and that superfluidity may be even enhanced, if clusters are confined in cavities of characteristic size of 1 nanometer. These predictions are an important first step, but based on fairly simplistic models.Our current research effort attempts to characterize the superfluid response of parahydrogen clusters confined in realistic models of existing porous materials, e.g., zeolites, which consists of network of interconnected cavities of size close to 1 nm. If small clusters of parahydrogen are captured in the confines of these materials, in which they may be superfluid, the conditions for a possible global superfluid phase may exist. The proposed mechanism is quantum-mechanical "tunnelling" of molecules from one superfluid cluster to one in an adjacent cavity, much like in superconducting (Josephson) arrays. Our aim is providing experimenters with as clear and quantitative predictions, enabling a guided search in the laboratory for the superfluid phase.Ours is the only group in Canada with the expertise to carry out the first principles calculations described. Understanding the phase diagram of hydrogen is not only of fundamental interest, due to the unquestionable importance of superfluidity, but may well have relevance to the possible use of the liquid phase of hydrogen for fueling purposes.