STUDIES ON STATIC AND DYNAMIC STRUCTURE OF EXPANDING FLUID ALKALI METALS BY USING SYNCHROTRON RADIATION

自从Wigner的开创性工作以来，低密度电子气的反常行为得到了广泛的研究。现代多电子理论预言，相互作用的电子气具有负的压缩性，当电子密度足够低时，静态介电函数(DF)变为负值。然而，负df的低密度电子气是否真的存在仍然是一个悬而未决的问题。我们在Spring-8用同步辐射将电子密度降低到理论预测的负值范围，测量了膨胀流体Rb的X射线衍射(XD)、小角X射线散射(SAXS)和非弹性X射线散射(IXS)。XD测量清楚地观察到，尽管平均原子间距离随着膨胀而增加，但原子间距离却减小，这表明在这种情况下，类似电荷之间的吸引相互作用得到了增强。SAXS测量还发现，密度起伏开始出现在df变为负值的密度处。从IXS测量中，我们得到了动态结构因子和色散关系。结果表明，在动量传递较小的区域，由色散关系估算的声速比密度为负值的绝热声速大。这些发现表明，观测到的结构特征是三维电子气压缩不稳定性和负df存在的证据。负df特别有趣，因为电子相互吸引，电子对可以在具有负df的电子气介质中产生，这使得没有声子辅助的新型超导体成为可能。

Since Wigner's pioneering work, the anomalous behaviors of low-density electron gas has been extensively investigated A modern many-electron theory has predicted that interacting electron gas suffers a negative compressibility and the static dielectric-function (DF) becomes negative when electron density is sufficiently reduced. However, a problem has remained unsolved as to whether the low-density electron gas with negative DF really exists or not. We have measured x-ray diffraction (XD), small-angle x-ray scattering (SAXS) and inelastic x-ray scattering (IXS) of expanding fluid rubidium using synchrotron radiation at SPring-8 by reducing electron density down to the range where the DF has been theoretically predicted to become negative. It was clearly observed by the XD measurement that the interatomic distance decreases in spite of the fact that mean interatomic distance increases with expansion, suggesting that an attractive interaction among like charges, ions in this case, is enhanced. It was also found by the SAXS measurement that density fluctuations start to appear at the density where the DF becomes negative. From the IXS measurement we have obtained the dynamic structure-factor and the dispersion relation as a function of density. It was revealed that the sound velocity estimated from the dispersion relation at the small momentum-transfer region becomes large compared with the adiabatic sound velocity at the density where the DF becomes negative. These findings indicate that the observed structural features are evidence of the compressional instability and the existence of negative DF of three dimensional electron gas. The negative DF is especially interesting because electrons attract each other and electron-pairs can be generated in a medium of electron gas with negative DF, which gives a possibility of a new type of superconductors without assistance of phonon.