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

氢是宇宙中最丰富的元素，也是被研究得最广泛的元素之一，关于它还有很多有待了解的地方。在其分子形式中，它的凝聚态（即液体和固体）相受到量子力学的极大影响，使其在性质上与普通材料不同。量子力学在宏观尺度上最引人注目的表现之一是超流动性，即物质流动而不耗散的能力。氦是已知的唯一一种在液态状态下会变成超流体的自然元素，它存在于非常低的温度下，仅比绝对零度高4摄氏度；然而，对氢，两种稳定形式的氢分子之一，在45年前就被推测为超流动性的候选者，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），但尽管进行了激烈的研究，但这种相尚未在实验室中观察到。人们普遍认为，观测不到超流体的主要原因是（假定的）超流体的液相接近于晶体，而晶体在低温下是热力学稳定的。目前人们认为，像准氢（甚至氦）这样的晶体不是超流体。因此，任何观察这种迷人的准氢相的尝试都需要避免结晶。***我们小组在过去十年进行的理论计算表明，小的独立簇（30个或更少的分子）在低温下确实应该是液体和超流体，如果簇被限制在特征尺寸为1纳米的腔中，超流动性甚至可能会增强。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表征在现有多孔材料的现实模型中对氢团簇的超流体响应，例如沸石，它由尺寸接近1 nm的互连腔网络组成。如果在这些材料的范围内捕获了小簇的准氢，它们可能是超流体，那么可能存在全局超流体相的条件。提出的机制是量子力学的“隧穿”分子从一个超流体簇到相邻腔中的另一个超流体簇，很像超导（约瑟夫森）阵列。我们的目标是为实验人员提供清晰和定量的预测，使他们能够在实验室中对超流体相进行指导搜索。***我们是加拿大唯一具有执行所描述的第一原理计算的专业知识的小组。由于超流动性的重要性不容置疑，理解氢的相图不仅具有根本的意义，而且很可能与氢的液相用作燃料的可能性有关。

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.