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

氢是宇宙中最丰富的元素，也是研究最广泛的元素之一，关于它还有很多东西有待了解。在其分子形式中，其浓缩的（即，液体和固体）相受到量子力学的极大影响，使其与普通材料有质的不同。量子力学在宏观尺度上最引人注目的表现之一是超流性，即物质流动而不耗散的能力。氦是唯一已知的在液相中变成超流的自然存在的元素，液相存在于非常低的温度下，仅比绝对零度高4 K;然而，仲氢，分子氢的两种稳定形式之一，在45年前被推测为超流的候选者，因为它具有许多与氦相同的性质（主要是非常轻的基本成分，即，分子），但这样的阶段，尽管深入研究，尚未在实验室中观察到。 未能观察到超流性的主要原因被认为是（假定的）超流液相与结晶相非常接近，结晶相在低温下是稳定的。目前人们认为仲氢（甚至氦）等晶体不是超流体。因此，任何试图观察仲氢如此迷人的相的尝试都需要避开结晶，我们小组在过去十年中进行的理论计算表明，小的独立团簇（30个分子或更少）在低温下确实应该是液体状和超流体，如果团簇被限制在1纳米的特征尺寸的空腔中，超流性甚至可能增强。这些预测是重要的第一步，但基于相当简单的模型。我们目前的研究工作试图描述限制在现有多孔材料的现实模型中的仲氢团簇的超流响应，例如，沸石，其由尺寸接近1 nm的互连空腔的网络组成。如果在这些材料的范围内捕获了小的仲氢团簇，它们可能是超流体，那么可能存在全球超流体相的条件。所提出的机制是量子力学的“隧道效应”，分子从一个超流团到相邻腔中的一个，很像超导（约瑟夫森）阵列。我们的目标是为实验者提供清晰和定量的预测，使他们能够在实验室中对超流相进行指导性搜索。我们是加拿大唯一一个拥有进行上述第一原理计算的专业知识的团队。理解氢的相图不仅是基本的兴趣，因为超流性的重要性是毋庸置疑的，而且可能与氢的液相用于燃料目的的可能性有关。

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.