Feng Rao,Distinguished Professorat the School of Materials, Shenzhen University, published a research paper in Science on 14 June, 2019. (“Catching structural transitions in liquids”.Rao et al., Science 364, 1032–1033, 2019.Link to the paper:https://science.sciencemag.org/content/364/6445/1032)
Phase-change random-access memory is a key ingredient for nonvolatile memory and neuro-inspired computing devices.It exploits the ability of chalcogenide phase-change materials (PCMs) to rapidly switch between logical states “0” (glassy) and “1” (crystalline). The “0” state is reached through fast quenching of the melted PCM. In this process, the diffusivity and viscosity change by ~17 orders of magnitude, most of which happens over a narrow temperature range. What is happening inside the cooling liquid to make this possible has so far remained elusive, because the process, which requires measurement times in nanoseconds, is intercepted by crystallization. Zalden et al. overcame this challenge by using femtosecond pulse x-ray diffraction and captured a liquid-liquid transition (LLT) in PCM liquids during superfast cooling.
Femtosecond x-ray diffraction and absorption spectroscopies can be used to probe into liquid states in the already commercialized Ge2 Sb2 Te5 and the recently designed Sc0.2Sb2 Te3 PCMs, in which nucleation, rather than growth (as in AIST and Ge15Sb85), plays a dominant role for nanoseconds operation. In general, LLTs appear to occur often, in systems from water to PCMs to metallic glass-forming liquids. However, LLTs are difficult to confirm experimentally—and have also defied a systematic understanding as to which material systems exhibit them, and when, and why. In all cases, LLTs should feature a clear structural indicator. The approach of Zalden et al. opens a new avenue for investigating the complex and swift structural changes in highly dynamic liquids. This should ultimately aid in optimizing the performance of PCM-based devices.
This workwas supported by theNational Natural Science Foundation of China, the Major Provincial Basic Research Program of Guangdong,and the Science and Technology Foundation of Shenzhen.Professor Feng Rao is the first author andcorrespondingauthor of thispaper. ProfessorWei ZhangfromXi’an Jiaotong Universityand ProfessorEvan MafromJohns Hopkins Universityare the co-correspondingauthors.
Professor Feng Rao has previously published a paper on high-performance cache type ultra-fast (sub-nanosecond) phase change memory materialScSbTe in Science (“Reducing the stochasticity of crystal nucleation to enable sub-nanosecond memory writing”.Rao et al., Science 358, 1423–1427, 2017). The ultrafast (sub-nanosecond) crystallization and non-volatile properties of ScSbTe materials also benefit from potential LLPT-induced kinetics (fragile-to-strong crossover).