Experimental Studies at Ultralow Temperatures

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This work focuses on two problems in the properties of low temperature quantum fluids and solids: the two dimensional ground state of thin films of He-3, and the superfluid state of highly disordered He-3 contained in aerogel. These problems are quite different, but share both the techniques of ultralow temperature refrigeration and the use of SQUIDs for wide band NMR. They also deal directly with thenature of phase transitions in these systems. In the case of He-3 films, the examination of the nature of ordering by two-dimensional He-3 in very low fields will demand an improved substrate, and directlyaddress the controversial issue of the existence of a finite temperature phase transition at zero field. Using SQUID NMR techniques allows the study of these films over a wide range of fields including the zero field limit.The second effort will constrain the identification of the superfluid order parameter in the presence of impurities by the use of longitudinal resonance, a probe which was essential in the originalidentification of the bulk superfluid phases. In this case, SQUID NMR techniques will be used to look for the predicted longitudinal NMR resonance. This experiment will test the standard picture where the "dirty" superfluids are seen as merely suppressed versions of their "clean" counterparts. Next-generation instrumentation of high performance SQUID systems surpassing commercial state of the art and techniques for SQUID NMR and the millidegree low temperature techniques produce an excellent vehicle for training graduate students at the Ph.D. and Master's level, as well as undergraduate students.%%%The project centers on interesting fundamental physics that occurs at temperatures close to absolute zero. He-3 systems provide excellent models for a wide range of physical behavior that has analogsat more conventional temperatures. The advantage of the He-3 systems is their relative simplicity allowing them to be understood theoretically to a much greater degree, and thus provide the connection between theory and experiment that is needed to advance the understanding of condensed matter systems. The first experiment involves the study of He-3 films that behave as though they are two-dimensional. There are well known theories (within a set of models where interactions are isotropic) that say these films cannot order magnetically above absolute zero because of their two-dimensionality. On the other hand, very small anisotropic distortions may have a highly disproportionate effect resulting in magnetic phase transitions. An earlier version of this study showed that this may be the case, however there remain many questions thatmust be answered before this system can be fully understood. The second project deals with the nature of the superfluid in liquid He-3. This liquid is intrinsically the purest liquid known, but placing it inside aerogel has the effect of introducing disorder (dirt) that modifies the ordered state. The nature of this ordered statewithin the disordered medium will be studied using SQUID NMR. This NMR technique has many advantages for these experiments. It also has potential applications as wide ranging as non-destructive testing andmedical imaging and diagnostics. These experimental projects provide an excellent way to train graduate and undergraduate students in a wide variety of experimental and analytical techniques.***