Michael Helle with Arnold Dahm

Anisotropic Diffusion of Isotopic Impurities in HCP Helium

4 He crystallizes at low temperature and pressure into an hcp lattice. Vacancies in the 4 He lattice propagate along the basal and transverse planes of the crystal as waves in straight lines. 3 He impurities can diffuse in the crystal by tunneling into these delocalized vacancies. It is the spins of a dilute concentration of 3 He that is used to measure the diffusion of vacancies. Prior attempts to measure the diffusion coefficients and their temperature dependence have yielded results that vary significantly. NMR techniques will be used to measure these diffusion coefficients. This will be accomplished by placing the crystal in a gradient coil and saturating a given slice of spins with a large rf pulse. This will create a one to one ratio between spins up and spins down making the NMR signal vanish. As 3 He atoms diffuse into/out of the slice the signal will recover, allowing the measurement of the diffusion coefficient in a direction normal to the field gradient. The diffusion rate will be measured along three orthogonal field gradients to determine whether 3 He diffuses anisotropically in an attempt to explain the discrepancy between previous results.