超压缩氦中从激子绝缘体到超导体的相变
Excitonic Insulator to Superconductor Phase Transition in Ultra-Compressed Helium
AI总结：
本文利用密度泛函理论与多体微扰理论预测超压缩固态氦在金属化前形成激子绝缘体，并伴随强电子-声子耦合，在20–100 TPa下呈现约30–100 K超导性。
摘要：
氦是宇宙中含量第二丰富的元素，在低压（≤0.1 GPa）下表现出约20 eV的极大电子带隙。尽管hcp氦的金属化压力已得到准确预测，但迄今为止，对于在太帕斯卡（1 TPa = 1,000 GPa）区间内驱动这种量子晶体带隙闭合和电子性质的具体机制，关注仍然很少。在此，我们采用最先进的密度泛函理论和多体微扰理论计算来填补这一认识空白。研究发现，在达到金属态之前，块体固态氦会成为一种激子绝缘体（EI），这是一种通常在低维系统中观察到的奇异物态，其中静电束缚的电子-空穴对会自发形成。此外，结果表明，在EI到金属的相变过程中，电子-声子耦合（EPC）显著增强，其标志是在特定倒易空间点上出现显著的声子软化以及巨大的EPC强度值（λ约为10–100）。据此，我们预测金属氦是一种超导体，在20 TPa时临界温度约为30 K，在100 TPa时约为100 K。这些此前未预见的现象对金属氦的弹性、热力学和输运性质具有重要影响，因此可能对提升我们对天体的基础认识和建模至关重要。
英文摘要：
Helium, the second most abundant element in the universe, exhibits an extremely large electronic band gap of about $20$ eV at low pressures ($\le 0.1$ GPa). While the metallization pressure of hcp helium has been accurately predicted, thus far little attention has been paid to the specific mechanisms driving the band-gap closure and electronic properties of this quantum crystal in the terapascal regime (1 TPa $= 1,000$ GPa). Here, we employ state-of-the-art density functional theory and many-body perturbation theory calculations to fill up this knowledge gap. It is found that prior to reaching metallicity bulk solid helium becomes an excitonic insulator (EI), an exotic state of matter typically observed in low-dimensional systems in which electrostatically bound electron-hole pairs form spontaneously. Furthermore, it is shown that electron-phonon coupling (EPC) is significantly enhanced across the EI to metal phase transition as signaled by prominent phonon softening and giant EPC strength values ($λ\sim 10-100$) estimated at specific reciprocal space points. Accordingly, we predict metallic helium to be a superconductor with a critical temperature of $\approx 30$ K at $20$ TPa and of $\approx 100$ K at $100$ TPa. These unforeseen phenomena have important consequences on the elastic, thermodynamic and transport properties of metallic helium hence may be critical for improving our fundamental understanding and modelling of celestial bodies.
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