高密度液氦的自由能模型
Free-energy model for fluid helium at high density
摘要：
我们提出了一个半解析自由能模型，旨在表征稠密流体氦的热力学性质，从低密度原子相到高密度完全电离制度。该模型是基于自由能最小化方法，包括各种不同的贡献代表原子和离子物种和电子之间的相关性。该模型允许在扩展的密度和温度范围内计算稠密氦的热力学性质，并导致计算稠密流体氦的相图，其各种温度和压力电离轮廓。该模型的预测之一是在$ρ\simgr 10$ g cm $^{-3}$，{ \it，即} $P\simgr 20$ Mbar，从原子氦He到完全电离的氦He $^{2+}$，或者至少到强电离状态，而没有He $^{+}$级，除非在足够高的温度下温度电离成为主导。这些预测和相图为未来的动力学实验或数值第一性原理计算提供了指导，这些实验或数值第一性原理计算旨在研究氦在非常高的密度下的性质，特别是其金属化。事实上，氦相图的特征对冷而致密的天体的热力学、磁性和输运性质具有重要影响，其中包括太阳和最近发现的众多太阳系外巨行星。
英文摘要：
We present a semi-analytical free-energy model aimed at characterizing the thermodynamic properties of dense fluid helium, from the low-density atomic phase to the high-density fully ionized regime. The model is based on a free-energy minimization method and includes various different contributions representative of the correlations between atomic and ionic species and electrons. This model allows the computation of the thermodynamic properties of dense helium over an extended range of density and temperature and leads to the computation of the phase diagram of dense fluid helium, with its various temperature and pressure ionization contours. One of the predictions of the model is that pressure ionization occurs abruptly at $ρ\simgr 10$ g cm$^{-3}$, {\it i.e.} $P\simgr 20$ Mbar, from atomic helium He to fully ionized helium He$^{2+}$, or at least to a strongly ionized state, without He$^{+}$ stage, except at high enough temperature for temperature ionization to become dominant. These predictions and this phase diagram provide a guide for future dynamical experiments or numerical first-principle calculations aimed at studying the properties of helium at very high density, in particular its metallization. Indeed, the characterization of the helium phase diagram bears important consequences for the thermodynamic, magnetic and transport properties of cool and dense astrophysical objects, among which the solar and the numerous recently discovered extrasolar giant planets.
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