液态相变材料在熔化温度以上Stokes-Einstein关系的破坏
Breakdown of the Stokes-Einstein Relation Above the Melting Temperature in a Liquid Phase-Change Material
摘要：
液态相变材料（PCM）的动力学性质，如粘度$η$和原子自扩散系数D，在新型非挥发性相变存储器应用的超快相变行为中起着至关重要的作用，因为它们与结晶动力学和相稳定性密切相关。为了将$η$与D联系起来，通常假定斯托克斯-爱因斯坦关系（SER）在接近或高于熔化温度$T_{m}$的高温下有效，并且经常用于评估技术上重要的PCM组合物的液体脆性（或晶体生长速度）。然而，使用准弹性中子散射（QENS），我们在这里给出了实验证据的SER击穿，即使在温度高于$T_{m}$在高原子迁移率状态的一个典型的PCM，锗$_{1}$锑$_{2}$碲$_{4}$，密度关联函数的衰减仍然保持指数。因此，击穿的起源不太可能是动力学不均匀性的结果，就像通常对粘性液体所假设的那样。相反，我们讨论它可能的连接到一个金属半导体和脆弱的强过渡隐藏在$T_{m}$。
英文摘要：
The dynamic properties of liquid phase-change materials (PCMs), such as viscosity $η$ and atomic self-diffusion coefficients D, play an essential role in ultrafast phase switching behavior of novel non-volatile phase-change memory applications, as they are intimately related to crystallization kinetics and phase stabilities. To connect $η$ to D, the Stokes-Einstein relation (SER) is commonly assumed to be valid at high temperatures near or above the melting temperature $T_{m}$ and is frequently employed for assessing liquid fragility (or crystal growth velocity) of technologically important PCM compositions. However, using quasi-elastic neutron scattering (QENS), we give here experimental evidence for a breakdown of the SER even at temperatures above $T_{m}$ in the high-atomic-mobility state of a typical PCM, Ge$_{1}$Sb$_{2}$Te$_{4}$, where the decay of density correlation functions still remains exponential. The origin of the breakdown is thus unlikely the result of dynamical heterogeneities, as is usually postulated for viscous liquids. Rather, we discuss its possible connections to a metal-semiconductor and fragile-strong transition hidden below $T_{m}$.
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