This thesis reports the results of transverse pulsed nuclear magnetic resonance (NMR) experiments performed on superfluid ³He in a new class of highly uniform silica aerogel materials. In these structurally uniform aerogels the experimentally determined superfluid transitions can be exceedingly narrow, < 0.1% of the superfluid transition temperature, T ca, thus opening up the possibility for more detailed investigations into the role of disorder on the unconventional superfluid phases of ³He. Of particular interest is the effect of anisotropic disorder on the relative phase stability of the various p-wave states of ³He and on the orientation of the superfluid order parameter. Both isotropic and anisotropic aerogels were grown at Northwestern and methods for engineering and characterizing their microstructure are described.
From measurements of the nuclear magnetic susceptibility and NMR frequency shifts of ³He in an isotropic aerogel the two primary superfluid states are definitively identified as impure versions of the isotropic and axial p-wave states. The theoretically predicted destruction of long-range orbital order, known as the Larkin-Imry Ma effect, which is expected to lead to a suppression of the frequency shift in the impure axial state is not observed.
We have also studied superfluid ³He in a uniformly anisotropic aerogel, where the anisotropy was engineered via growth induced "stretching". In contrast to the isotropic aerogel, the stretched aerogel stabilizes anisotropic superfluid states which otherwise would not exist. Measurements of the nuclear magnetic susceptibility show that the superfluid phases in the stretched aerogel are equal-spin pairing (ESP) states. Near Tca, the superfluid phase is identified as the axial p-wave state from the tip angle dependence of the NMR frequency shift. Furthermore, these frequency shift data are consistent with the direction of the orbital part of axial state order parameter, [special characters omitted], being perpendicular to both the external magnetic field and the stretching axis. At sufficiently low temperature there is a transition to a different ESP state that is marked by a reduced frequency shift and broadened NMR spectrum. From our analysis of the NMR line shape, we conclude that this superfluid phase is consistent with a disordered axial state composed of domains of [special characters omitted].
|Advisor:||Halperin, William P.|
|Commitee:||Koch, Jens, Sauls, James A.|
|Department:||Physics and Astronomy|
|School Location:||United States -- Illinois|
|Source:||DAI-B 73/07(E), Dissertation Abstracts International|
|Subjects:||Low Temperature Physics, Condensed matter physics|
|Keywords:||Aerogels, Helium-3, Quenched disorder, Superfluidity|
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