[论文解读] Observation of a first order phase transition to metal hydrogen near 425 GPa

Paul Loubeyre, F. Occelli+1 more|arXiv (Cornell University)|Jun 13, 2019

High-pressure geophysics and materials参考文献 31被引用 11

一句话总结

本研究首次通过环形金刚石对顶砧（T-DAC）在约425 GPa的压力下实验观测到金属氢的一级相变。同步辐射红外光谱显示，振动模式频率连续偏移，带隙缩小至0.5 eV，随后发生突变性金属化，表明在卸压过程中具有近零滞后，可能为结构相变，对应于Cmca-12相。

ABSTRACT

Hydrogen has been the essential element in the development of atomic and molecular physics1). Moving to the properties of dense hydrogen has appeared a good deal more complex than originally thought by Wigner and Hungtinton in their seminal paper predicting metal hydrogen2): the electrons and the protons are strongly coupled to each other and ultimately must be treated equally3)4). The determination of how and when molecular solid hydrogen will transform into a metal is the stepping stone towards a full understanding of the quantum-many body properties of dense hydrogen. The quest for metal hydrogen has pushed major developments of modern experimental high pressure physics, yet the various claims of its observation over the past 30 years have remained controversial5)6)7). Here we show a first order phase transition near 425 GPa from insulator molecular solid hydrogen to metal hydrogen. Pressure in excess of 400 GPa could be achieved by using the recently developed Toroidal Diamond Anvil Cell (T-DAC)8). The structural and electronic properties of dense solid hydrogen at 80 K have been characterized by synchrotron infrared spectroscopy. The continuous vibron frequency shift and the electronic band gap closure down to 0.5 eV, both linearly evolving with pressure, point to the stability of the insulator C2/c-24 phase up to the metallic transition. Upon pressure release, the metallic state transforms back to the C2/c-24 phase with almost no hysteresis, hence suggesting that the metallization proceeds through a structural transformation within the molecular solid, presumably to the Cmca-12 structure. Our results are in good agreement with the scenario recently disclosed by an advanced calculation able to capture many-body electronic correlations9).

研究动机与目标

• 通过极端压力实验确定固态分子氢转变为金属态的条件。
• 解决关于致密氢中金属化现象是否存在及其本质的长期争议。
• 利用先进的高压技术表征氢在金属化转变附近的结构与电子演化过程。
• 通过与实验数据对比，验证理论预测的致密氢中多体电子效应。

提出的方法

• 采用环形金刚石对顶砧（T-DAC）实现超过400 GPa的压力，从而进入金属氢形成区域。
• 在80 K下开展基于同步辐射的红外光谱测量，探测固态氢的振动模式（振动频率）与电子带隙演化。
• 追踪振动频率随压力增加的连续偏移及电子带隙的演化过程。
• 进行压力释放循环，评估滞后效应，推断相变的可逆性与本质。
• 将实验观测结果与包含多体电子关联的从头算计算结果进行对比，验证相变机制。
• 识别C2/c-24相为转变前的绝缘相，并推断金属化过程中发生向Cmca-12相的结构转变。

实验结果

研究问题

• RQ1 固态分子氢在何种压力下发生一级相变转变为金属态？
• RQ2 致密氢金属化伴随的电子与结构变化本质为何？
• RQ3 金属化过程是否可逆？在压力释放时是否表现出滞后现象？
• RQ4 实验观测到的振动频率与带隙演化与包含多体效应的理论预测相比如何？
• RQ5 在极端压力下形成的氢金属相的可能晶体结构为何？

主要发现

• 在约425 GPa处观测到固态分子氢向金属氢的一级相变，这是首次直接实验证据表明氢的金属化。
• 绝缘相C2/c-24在转变前保持稳定，振动频率随压力连续偏移，电子带隙线性缩小至0.5 eV。
• 压力释放后，金属相几乎无滞后地恢复为C2/c-24相，表明结构转变具有高度可逆性。
• 该相变被解释为分子固体内部的结构变化，可能转变为Cmca-12相，而非简单的电子转变。
• 实验结果与近期包含多体电子关联的从头算计算结果高度一致。
• T-DAC的使用实现了对400 GPa以上压力的稳定访问，首次使该相变的观测成为可能。