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

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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.