Structure of the Interfacial Properties of Helium-3, Helium-4, and Helium-3/Helium-4 Mixtures: X-ray Scattering Studies

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9523281 Pershan This project employs x-ray scattering methods to measure the interfacial structures of 3He, 4He and 3He - 4He mixtures over a range of temperatures and composition, and under a variety of externally controlled geometries. The helium interfaces are interesting from a theoretical view because they represent the relatively rare realization of a well-defined, macroscopically oriented, inhomogeneous quantum fluid. The superfluid properties of bulk helium are fundamentally dependent on the liquid density. However, it is extremely difficult to vary the bulk density by significant fraction. In contrast, the density across the liquid/vapor interface varies from the uniform bulk value to essentially zero. The x-ray experiments are currently the only test of extant theories of the structural changes anticipated for mixture of the quantum fluids. In addition, it may be possible to measure the in-plane correlations for the two-dimensional electron fluid that can be stabilized on the surface of the helium. If successful, this would provide a tool to characterize the structure and positional correlations of the two-dimensional Wigner lattice formed by the layer of electrons trapped in the image potential at the helium surface. The experiments are of fundamental interest and will be of wider interest in the field of surface physics and fluid dynamics. %%% This project employs x-ray scattering methods to measure the interfacial structures of 3He, 4He and 3He - 4He fluid mixtures over a range of temperatures and composition, and under a variety of externally controlled geometries. These cryogenic fluids have many novel properties related to their "superfluidity". The arrangements of atoms forming the surface layers will be determined by x-ray methods using a beam of x-rays from a synchrotron light source. In another sub-project, it is possible to trap a film of e lectrons on the surface of the helium. The arrangement of electrons in this layer will also be sought. The experiments are of fundamental interest and will be of wider interest in the field of surface physics and fluid dynamics. ***