The effects of disorder on superflow in the quantum solid helium-4

The existence of a superfluid-like phase in solid helium was predicted in the late 1960's. The first convincing experimental evidence of such a phase was the non-classical rotational inertia (NCRI) measurements published in 2004 by Kim and Chan. Uncovering the exact microscopic mechanism giving rise to this effect is the subject of this dissertation. The majority of the work concentrates on exploring the effect of various types of disorder on NCRI and performing some of the first simultaneous measurements of crystal quality and NCRI.;We have measured the effect of 3He impurities in solid 4He crystals, from isotopically pure 4He (<1ppb 3He) up to 30 ppm. We find that the onset temperature and the broadness of the transition increase continuously with the 3He concentration. We have also studied ultra-high purity 4He crystals down to temperatures of ∼1 mK finding no additional features in the period or dissipation.;The sample quality dependence of NCRI was measured by growing samples with various methods known to produce crystals of high or low quality. The addition of disorder is observed to increase the magnitude of NCRI and the onset temperature. Higher quality crystals were found to be more reproducible from sample to sample. In some crystals, annealing was found to reduce the NCRI and dissipation. However, there was always appears to be some non-zero minimum NCRI.;The first combined sample characterization and torsional oscillator measurements were made for solid helium confined within aerogel. Torsional oscillator measurements were made at Penn State and a complimentary x-ray scattering experiment was performed at the APS synchrotron facility. Although the samples were highly disordered (with an average grain size of 1000 A) the NCRI signals were comparable with those from high quality crystals, the data were also highly reproducible, as with the high quality samples.;A set of complimentary torsional oscillator and heat capacity measurements with helium confined within thin slab geometries were undertaken to test a confinement effect reported by Rittner and Reppy. Rather than a confinement effect, we find that the data are more consistent with a quenching effect (a dependence on how rapidly the samples are grown).;Day and Beamish reported an increase in the shear modulus of solid helium with a dependence on temperature, frequency, etc. which match the torsional oscillator experiments. In collaboration with Syshchenko and Beamish, we have studied the shear modulus and NCRI for both 3He and 4He crystals. The 4He solid exhibits an anomaly in the NCRI and shear modulus. The solid 3He only shows a shear modulus increase for the hcp phase and no NCRI effect for either the bcc or hcp crystal phases.;We have made the first simultaneous NCRI and optical measurements. These measurements enable us to determine crystal quality, orientation, etc. The experimental details will be presented along with some preliminary data because the experiments are currently in progress.