Bonding, Fluctuations and Dynamics in Supercritical Metallic Fluids Using Synchrotron Radiation

原子是如何凝聚成金属和半导体的？为了理解固态物理中的这个重要问题，我们在SPring-8上使用同步辐射研究了膨胀流体金属和半导体的静态和动态结构。目前的结果总结如下。(1)液体和膨胀流体硒（Se）采用高分辨率非弹性x射线散射技术，我们发现聚合物分子构成的液体硒的动态结构因子有特殊的缩小。膨胀流体Se在高温高压下发生非金属-金属（NM-M）转变。我们观察到过渡区小角散射强度明显增加。同时，集体模的激发能的弥散速度是绝热声速的两倍。这些结果提示在纳米-纳米跃迁过程中会产生特征波动。(2)液态As2Se3中、短程结构在高温高压下发生NM-M转变。我们观察到小角散射强度的增加，但其分布与膨胀流体Se不同。同时，在NM-M跃迁过程中，局部结构的密度依赖性未见异常。(3)流体硫(S)的x射线衍射测量非金属液体S膨胀为气相，没有发生NM-M转变。我们发现，在高温高压下，第一配位壳层在超临界区保持了良好的分离。此外，我们还观察到S二聚体分子可能聚集形成Si簇的密度区域。为了深入了解纳米-微米跃迁过程中的特征波动，我们计划研究纳米-微米跃迁过程中动态声速相对于绝热声速大幅加速的机理。

How are atoms condensed into metals and semiconductors? To understand this important question in solid state physics, we have studied static and dynamic structures for expanded fluid metals and semiconductors using synchrotron radiation at SPring-8. The present results are summarized as below.(1) Liquid and expanded fluid selenium (Se)Using high-resolution inelastic X-ray scattering technique, we found a peculiar narrowing of dynamic structure factor in liquid Se constituted of polymeric molecules. Expanded fluid Se undergoes the nonmetal-metal (NM-M) transition at high temperature and high pressure. We observed a clear increase of small angle scattering intensity in the transition region. At the same time excitation energy of the collective mode disperses twice as fast as the adiabatic sound velocity. These results hint that characteristic fluctuations are generated in the NM-M transition.(2) Short and medium range structures in liquid As_2Se_3Liquid As2Se3 undergoes the NM-M transition at high temperature and high pressure. We observed the increase of small angle scattering intensity but the profile was different from that in expanded fluid Se. Meanwhile no anomaly in the density dependence of the local structure was observed at the NM-M transition.(3) X-ray diffraction measurements of fluid sulfur (S)Nonmetallic liquid S is expanded into the vapor phase without the NM-M transition. We found that the well-separated first coordination shell is preserved in the supercritical region at high temperature and high pressure. Furthermore we observed the density region in which S dimeric molecules may be aggregate to form Si clusters.We plan to investigate the mechanism of the large speed-up of the dynamic sound velocity compared to the adiabatic one observed in the NM-M transition in order to understand deeply the characteristic fluctuations in the NM-M transition.