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

自由和受限仲氢簇中的超流性和玻色-爱因斯坦凝聚

• 批准号: RGPIN-2018-04227
• 负责人: Boninsegni, Massimo
• 金额: $ 2.04万
• 依托单位: University of Alberta
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=682827
• 关键词: Superfluidity; Bose; Einstein; condensation; free

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.

氢是宇宙中最丰富的元素，也是研究最广泛的元素之一，但仍有很多问题有待了解。在其分子形式下，其凝聚相（即液体和固体）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壮观的表现之一是超流性，即物质流动而不耗散的能力。氦是唯一已知的能够在液相中转变为超流体的天然元素，其存在温度非常低，仅比绝对零度高 4 度 K；然而，仲氢是分子氢的两种稳定形式之一，45 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（主要是非常轻的元素成分，即分子），但这种尽管进行了深入的研究，但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）超流体液相非常接近结晶液相，而结晶液相在低温下是热力学稳定的。目前人们认为仲氢（甚至氦）等晶体不是超流体。因此，任何观察如此令人着迷的仲氢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，小的独立簇（三十个分子或更少）确实应该是低温下类似液体和超流体，如果簇被限制在特征尺寸为 1 纳米的空腔中，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。***我们当前的研究工作试图表征限制在现有多孔材料（例如沸石）的现实模型中的仲氢簇的超流体响应，该多孔材料由互连的网络组成空腔尺寸接近1 nm。如果在这些材料的范围内捕获小簇仲氢，它们可能是超流体，那么可能存在整体超流体相的条件。所提出的机制是分子从一个超流体簇到相邻空腔中的一个超流体簇的量子力学“隧道”，就像超导（约瑟夫森）阵列一样。我们的目标是为实验者提供清晰且定量的预测，从而能够在实验室中引导搜索超流体相。***我们是加拿大唯一拥有执行所述第一原理计算专业知识的小组。由于超流动性的重要性毋庸置疑，了解氢的相图不仅具有根本意义，而且很可能与氢液相用于燃料用途的可能相关。