The ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ and GeTe compounds are of interest for applications in phase change memories. In the reset process of the memory the crystal is rapidly brought above the melting temperature $T_m$ by Joule heating and then the liquid phase rapidly cools down leading to the formation of the amorphous phase. Since the liquid above $T_m$ is metallic and the amorphous phase is semiconducting a semiconductor-to-metal transition occurs in the supercooled liquid. Based on density functional simulations, we estimated the metal-semiconductor transition temperature $T_{\text{M-SC}}$ by monitoring the opening of a band gap in the supercooled liquid phase. Due to previous evidence on the importance of the van der Waals (vdW) interaction in describing the liquid phase of these materials, we used both the revised Vydrov-van Voorhis functional which includes vdW nonlocal interactions and the Perdew-Burke-Ernzerhof functional without vdW corrections. The estimated $T_{\text{M-SC}}$ is about 100–150 K higher with the former than with the latter framework for both compounds. By including vdW interactions the estimated $T_{\text{M-SC}}$ is closer to $T_m$ than to the glass transition temperature for both systems. The analysis of the structural properties as a function of temperature suggests a correlation between the metal-semiconductor transition and a Peierls distortion. However, the data support more a continuous structural transformation than the presence of a first order liquid-liquid phase change associated the metal-semiconductor transition.

中文翻译：

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Ge2Sb2Te5和GeTe化合物过冷液相中的金属-半导体过渡

这 ${\mathrm{通用电器}}_{\mathrm{2个}}{锑}_{\mathrm{2个}}{特}_5$GeTe化合物和GeTe化合物对相变存储器的应用很感兴趣。在存储器的复位过程中，晶体迅速升高到熔化温度以上${Ť}_{米}$通过焦耳加热，然后液相迅速冷却，导致形成非晶相。由于以上液体${Ť}_{米}$金属是金属，而非晶相则是半导体，在过冷的液体中会发生半导体到金属的转变。基于密度泛函模拟，我们估算了金属-半导体的转变温度${Ť}_{\text{M-SC}}$通过监测过冷液相带隙的打开。由于以前有证据表明范德华（vdW）相互作用在描述这些材料的液相中的重要性，因此我们同时使用了修正的Vydrov-van Voorhis功能（包括vdW非局部相互作用）和没有vdW的Perdew-Burke-Ernzerhof功能更正。估计${Ť}_{\text{M-SC}}$对于这两种化合物，前者的骨架比后者的骨架高约100–150K。通过包括vdW交互，估计${Ť}_{\text{M-SC}}$ 更接近 ${Ť}_{米}$而不是两个系统的玻璃化转变温度。根据温度对结构特性的分析表明，金属-半导体跃迁与Peierls畸变之间存在相关性。但是，与金属-半导体跃迁相关的一阶液相-液相变化的存在相比，数据更能支持连续的结构转变。