Demmel Franz, Hennet Louis, Jakse Noel
ISIS Facility, Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK.
ICMN, CNRS and University of Orleans, 45071, Orléans, France.
Sci Rep. 2021 Jun 3;11(1):11815. doi: 10.1038/s41598-021-91062-0.
The characteristic property of a liquid, discriminating it from a solid, is its fluidity, which can be expressed by a velocity field. The reaction of the velocity field on forces is enshrined in the transport parameter viscosity. In contrast, a solid reacts to forces elastically through a displacement field, the particles are trapped in their potential minimum. The flow in a liquid needs enough thermal energy to overcome the changing potential barriers, which is supported through a continuous rearrangement of surrounding particles. Cooling a liquid will decrease the fluidity of a particle and the mobility of the neighbouring particles, resulting in an increase of the viscosity until the system comes to an arrest. This process with a concomitant slowing down of collective particle rearrangements might already start deep inside the liquid state. The idea of the potential energy landscape provides an attractive picture for these dramatic changes. However, despite the appealing idea there is a scarcity of quantitative assessments, in particular, when it comes to experimental studies. Here we present results on a monatomic liquid metal through a combination of ab initio molecular dynamics, neutron spectroscopy and inelastic x-ray scattering. We investigated the collective dynamics of liquid aluminium to reveal the changes in dynamics when the high temperature liquid is cooled towards solidification. The results demonstrate the main signatures of the energy landscape picture, a reduction in the internal atomic structural energy, a transition to a stretched relaxation process and a deviation from the high-temperature Arrhenius behavior of the relaxation time. All changes occur in the same temperature range at about [Formula: see text], which can be regarded as the temperature when the liquid aluminium enters the landscape influenced phase and enters a more viscous liquid state towards solidification. The similarity in dynamics with other monatomic liquid metals suggests a universal dynamic crossover above the melting point.
液体区别于固体的特性是其流动性，流动性可用速度场来表示。速度场对力的反应体现在输运参数粘度中。相比之下，固体通过位移场对力产生弹性反应，粒子被困在其势能最小值处。液体中的流动需要足够的热能来克服不断变化的势垒，这通过周围粒子的持续重排来实现。冷却液体将降低粒子的流动性以及相邻粒子的迁移率，导致粘度增加，直至系统停止流动。这个伴随着集体粒子重排减缓的过程可能在液态内部深处就已开始。势能景观的概念为这些剧烈变化提供了一个有吸引力的图景。然而，尽管这个想法很吸引人，但定量评估却很匮乏，尤其是在实验研究方面。在此，我们通过结合从头算分子动力学、中子光谱学和非弹性X射线散射，展示了关于单原子液态金属的结果。我们研究了液态铝的集体动力学，以揭示高温液体冷却至凝固时动力学的变化。结果表明了能量景观图景的主要特征，即内部原子结构能量的降低、向拉伸弛豫过程的转变以及弛豫时间偏离高温阿仑尼乌斯行为。所有这些变化都发生在大约[公式：见原文]的相同温度范围内，这个温度可被视为液态铝进入景观影响相并朝着凝固进入更粘稠液态状态的温度。与其他单原子液态金属在动力学上的相似性表明在熔点以上存在普遍的动力学转变。
