Collective Mode Dynamics in Superfluid 3He

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w:\awards\awards96\num.doc 9701710 Lee This experimental project is related to the internal degrees of freedom of Cooper pairs in superfluid 3He, which are largely resonsible for the fascinating behavior of this liquid including the occurrence of collective modes. The collective modes observed in ultrasound experiments are sometimes known as Cooper Pair Vibrations. The experiments in this project mainly deal with the non-linear excitations of two well characterized collective modes of 3He B, namely the real squashing mode and the squashing mode. The studies will take advantage of the fact that the line widths of these modes become very narrow at low temperatures where the quasi-particle scattering rates are small. The planned investigations will emphasize saturation studies and a search for phonon echoes, in analogy to pulsed nuclear magnetic resonance effects. Pulse time of flight studies with large excitation pulses will be employed. In addition, a search for soliton behavior and in particular self induced transparency will be undertaken using similar techniques. Because of the large coupling between the squashing mode and ultrasound, it is expected that it will be more advantageous to study the squashing collective mode. In addition, an experimental approach to find magnetic resonance signals corresponding to transition between Zeeman sublevels of both the squashing and real squashing collective modes in magnetic fields. %%%% This experimental project is devoted to the superfluid behavior present in the liquid formed by cooling helium atoms of isotope mass three, at low temperatures. Liquid helium when cooled below 4.2 Kelvin degrees (about -270 Centigrade) becomes a superfluid, which flows with no viscosity, permitting persistent flow patterns such as rotating inside a container. The superfluid formed by the isotope o f helium of mass 3 is even more interesting in its properties, which are the subject of this experimental project. In liquid helium three the superfluid is associated with "pairing" of the helium atoms, and the pairs of helium atoms have a characteristic rotational motion about each other, in contrast to pairing of electrons in conventional superconductors in which the pairs do not have "orbital angular momentum". The consequences of this unconventional pairing in the helium three superfluid include collective modes, known as squashing and real squashing modes, which are the subject of this research, which will be carried out in part using nuclear magnetic resonance methods. Liquid helium three, one of the simplest liquids known, is a testing ground for developing a thorough understanding of the properties of condensed matter. Even with its simplicity it offers complex behavior which may, if correctly understood, provide insight into the behavior of very different materials, including high temperature superconductors, in which unconventional pairing may also occur in the superfluid (superconducting) phase. This experimental work is carried out in part by graduate and postdoctoral students who receive an excellent training in physics and in other areas also, in the pursuit of the cryogenic measurements using nuclear magnetic resonance and other sophisticated methods. This is an excellent preparation for a career in industrial research, government laboratories or in academia. ****