Exploring the origin of the breakdown of the Stokes-Einstein Relation above the melting temperature in metalloid glassforming systems

所提出的研究的中心目标是深入了解过冷和平衡的液态行为的非金属玻璃形成系统，并了解的起源的斯托克斯-爱因斯坦关系（SER）以上的熔化温度Tm的故障。此外，我们将探讨这些合金中可能的液-液转变（LLT），并更好地理解液态行为之间的关系（可能包括LLT），结晶动力学和玻璃化转变。我们建议使用准弹性中子散射（QENS）实验来确定平均自扩散系数和α-液体非金属玻璃形成系统中的弛豫时间。这些系统包括那些技术上重要的相变材料（PCM），如GeTe，Ge 2Sb 2 Te 5，Ge 15 Sb 85，Sb 2 Te 3和AIST，以及非PCM类金属系统，如GeSe，As 2 Te 3和As 2Se 3。通过测量扩散系数和α-弛豫时间，可以在Tm附近和Tm以上实验研究SER的有效性。这些结果将使我们能够确定SER在相变材料和非相变材料中的有效性，并在不同的微观尺度上深入了解液体动力学。在SER故障的情况下，将确定用于描述实验数据的修改的“分数”SER模型的参数。 一个异常故障的SER附近的Tm将提供支持的证据的假设存在一个LLT低于Tm隐藏在PCM结晶。此外，通过对同时存在SER击穿和LLT的非易失性存储器系统的研究，我们旨在建立SER击穿和LLT之间的定量关系。这有助于理解LLT低于Tm被结晶所隐藏的情况。我们将澄清与LLT（或LLCP）相关的SER击穿是否与在接近Tg的较低温度下发生的SER击穿具有相同的起源。调查将扩展到过冷液体制度低于Tm掩盖快速结晶。由于过冷液体的弛豫动力学与结晶动力学密切相关，因此将使用时间分辨泵浦和探测激光反射率技术来确定那些组合物的晶体生长速度，其中在QENS实验中将观察到SER击穿。低于Tm的LLT的表现（例如扩散率和粘度异常）应反映在生长速度的温度依赖性中，并通过拟议的POT实验进行检测。结果可以帮助澄清LLT（和脆弱的强过渡）在非金属系统中的存在，允许识别LLT在结晶动力学中的签名，并提供深入了解SER故障的机制。

The central objective of the proposed research is to gain insight into the supercooled and equilibrium liquid-state behaviors of the metalloid glassforming systems, and to understand the origin of the breakdown of the Stokes-Einstein relation (SER) above the melting temperature Tm. Furthermore, we will explore the possible liquid-liquid transitions (LLTs) in these alloys and reach a better understanding of the relationship between the liquid-state behaviors (likely including LLTs), crystallization kinetics and glass transitions.We propose to use quasi-elastic neutron scattering (QENS) experiments for determining the temperature dependence of mean self-diffusion coefficients and the α-relaxation times in the liquid metalloid glassforming systems. The systems include those technologically important phase-change materials (PCMs) such as GeTe, Ge2Sb2Te5, Ge15Sb85, Sb2Te3, and AIST, as well as the non-PCMs metalloid systems such as GeSe, As2Te3 and As2Se3. By measuring the diffusivities and α-relaxation times, the validity of the SER can be studied experimentally near and above the Tm. The results will allow us to determine the circumstances of validity of the SER in PCMs and non-PCMs and gain insight into the liquid dynamics on different microscopic length-scales. In the cases of the SER breakdowns, the parameters for a modified ‘fractional’ SER model will be determined for describing experimental data. An anomalous breakdown of the SER near Tm would provide supporting evidence for the hypothesis of the existence of a LLT below Tm hidden by crystallization in PCMs. The latter may play an important role in the ultrafast phase switching for non-volatile memory devices applications.Furthermore, by investigating those systems which have both the SER breakdown and a LLT above Tm, we aim to establish a quantitative relation between the SER breakdowns and the LLTs. This helps understand the cases where the LLT is below Tm hidden by crystallization. We will clarify whether the SER breakdown associated with a LLT (or LLCP) has the same origin as the ones that happen at lower temperature close Tg. The investigation will be extended towards the undercooled liquid regime below Tm obscured by fast crystallization. Since the relaxation dynamics of the undercooled liquid is closely related to crystallization kinetics, the crystal growth velocity of those compositions, where a SER breakdown would be observed in the QENS experiments, will be determined using the time-resolved pump-and-probe laser reflectivity technique. The manifestation of a LLT below Tm (e.g. diffusivity and viscosity anomalies) should be reflected in the temperature dependence of growth velocity and detected by the proposed POT experiments. The results can help clarify the existence of LLTs (and fragile-strong transitions) in metalloid systems, allowing for identifying a signature of LLTs in crystallization kinetics and providing insight into the mechanism of the SER breakdowns.