Liu Xiao-Di, Dalladay-Simpson Philip, Howie Ross T, Zhang Hui-Chao, Xu Wan, Binns Jack, Ackland Graeme J, Mao Ho-Kwang, Gregoryanz Eugene
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China;
Center for High Pressure Science & Technology Advanced Research, Shanghai 201203, China.
Proc Natl Acad Sci U S A. 2020 Jun 16;117(24):13374-13378. doi: 10.1073/pnas.2001128117. Epub 2020 Jun 1.
Molecular hydrogen forms the archetypical quantum solid. Its quantum nature is revealed by behavior which is classically impossible and by very strong isotope effects. Isotope effects between [Formula: see text], [Formula: see text], and HD molecules come from mass difference and the different quantum exchange effects: fermionic [Formula: see text] molecules have antisymmetric wavefunctions, while bosonic [Formula: see text] molecules have symmetric wavefunctions, and HD molecules have no exchange symmetry. To investigate how the phase diagram depends on quantum-nuclear effects, we use high-pressure and low-temperature in situ Raman spectroscopy to map out the phase diagrams of [Formula: see text]-HD-[Formula: see text] with various isotope concentrations over a wide pressure-temperature (-) range. We find that mixtures of [Formula: see text], HD, and [Formula: see text] behave as an isotopic molecular alloy (ideal solution) and exhibit symmetry-breaking phase transitions between phases I and II and phase III. Surprisingly, all transitions occur at higher pressures for the alloys than either pure [Formula: see text] or [Formula: see text] This runs counter to any quantum effects based on isotope mass but can be explained by quantum trapping of high-kinetic energy states by the exchange interaction.
分子氢构成了典型的量子固体。其量子性质通过经典物理学中不可能出现的行为以及非常强的同位素效应得以揭示。氢气（H₂）、氘气（D₂）和氢氘混合气（HD）之间的同位素效应源于质量差异和不同的量子交换效应：费米子氢气（H₂）分子具有反对称波函数，而玻色子氘气（D₂）分子具有对称波函数，氢氘混合气（HD）分子则没有交换对称性。为了研究相图如何依赖于量子核效应，我们使用高压低温原位拉曼光谱来绘制在宽压力 - 温度（P - T）范围内具有不同同位素浓度的H₂ - HD - D₂的相图。我们发现，H₂、HD和D₂的混合物表现为同位素分子合金（理想溶液），并在I相和II相以及III相之间呈现出对称性破缺的相变。令人惊讶的是，对于合金而言，所有相变都发生在比纯H₂或纯D₂更高的压力下。这与基于同位素质量的任何量子效应相悖，但可以通过交换相互作用对高动能态的量子捕获来解释。
