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An NMR study of the statics and dynamics of thin helium films
Abstract
The results of nuclear magnetic resonance (NMR) on thin $\sp3\rm{He}$-$\sp4$He mixture films at temperatures 24mK $\leq T \leq$ 650mK which are adsorbed to Nucleopore are reported. The nuclear magnetic susceptibility, the relaxation times T$\sb1$ and T$\sb2$, and the spin diffusion coefficient, D, were measured used pulsed NMR techniques in a 2 Tesla field. The $\sp4$He coverages investigated ranged from 0.137 $\leq n\sb4\leq$ 0.534 atoms A$\sp2$ with a fixed submonolayer $\sp3$He coverage of 0.00746 $\leq n\sb3 \leq$ 0.00749 atoms A$\sp2.$ At $n\sb4$ = 0.391 atoms/A$\sp2$ measurements were taken with the $\sp3$He coverage ranging 0.00749 $\leq n\sb3 \leq$ 0.0179 atoms/A$\sp2$. We present the $\sp3$He magnetization as a function of the $\sp4$He coverage. The magnetization is degenerate for temperatures below the Fermi temperature, T$\sb{F}$, and from the degenerate magnetization the hydrodynamic mass over a range of $\sp4$He coverages is obtained. Variational and density functional descriptions of the film are considered. The diffusion data are seen to rise rapidly, from 10$\sp{-8}$ to 10$\sp{-3}\ \rm{cm}\sp2$/sec, as the $\sp4$He coverage is increased from 0.19 to 0.39 atoms/A$\sp2$, a range of just 2.5 layers. For $T < T\sb{F}$ the temperature dependence of a degenerate Fermi gas is not seen; $D \not= T\sp{-2}$. For all coverages T$\sb1$ is two orders of magnitude larger than T$\sb2$. Two regimes are seen. For coverages $n\sb4 <$ 0.23 the temperature dependence of T$\sb1$ and T$\sb2$ are consistent with $\omega\tau\sb{c} \gg$ 1. A signature of the completion of the second layer of the $\sp4$He is seen in T$\sb{1}$. For coverages $n\sb4 >$ 0.23, $\omega\tau\sb{c} \ll$ 1 and the temperature dependence correlates with the superfluid areal density. Activated behavior is seen which probes higher bound states of the $\sp3$He in the $\sp4$He film.
Subject Area
Condensed matter physics|Plasma physics
Recommended Citation
Sprague, Donald T, "An NMR study of the statics and dynamics of thin helium films" (1993). Doctoral Dissertations Available from Proquest. AAI9316717.
https://scholarworks.umass.edu/dissertations/AAI9316717