Spin-spin Relaxation Time Measurements of 2D $^3$He on Graphite

TL;DR

This study investigates the spin-spin relaxation time ($T_2$) and magnetic susceptibility ($$) of second-layer $^3$He on graphite, revealing Fermi fluid behavior, a maximum $T_2$ at intermediate density, and frequency-dependent relaxation influenced by substrate heterogeneity.

Contribution

It provides detailed measurements of $T_2$ and $$ across densities and frequencies, highlighting the effects of substrate heterogeneity and distinguishing between fluid and solid $^3$He behaviors.

Findings

$(T)$ shows Fermi fluid behavior with no self-condensation.

$T_2$ exhibits a maximum around $ ho=3$ nm$^{-2}$, influenced by substrate heterogeneity.

$1/T_2$ depends linearly on frequency, likely due to magnetic field inhomogeneity.

Abstract

Spin-spin relaxation time ($T_2$) and magnetic susceptibility ($\chi$) of the second layer $^3$He adsorbed on Grafoil, exfoliated graphite, preplated with a monolayer $^4$He are studied by pulsed-NMR in a density range of $0.68 \leq \rho \leq 5.28$ nm$^{-2}$. The temperature dependence of $\chi(T)$ and $\chi(T = 0)$ show Fermi fluid behaviour and no evidence of self-condensation are found even at the lowest density $\rho = 0.68$ nm$^{-2}$. Density dependence of $T_2$ at $f = 5.5$ MHz shows a broad maximum of 5.7 ms around $\rho = 3$ nm$^{-2}$. Since the decrease of $T_2$ in dilute side can not be expected in the ideal 2D fluid, it can be understood as the relaxation caused by a small amount of solid $^3$He at heterogeneity of the substrate. We also measured the Larmor frequency dependence of $T_2$ at $\rho = 5.28$ nm$^{-2}$. $1/T_2$ has a $f$-linear dependence similarly to the earlier study on a first layer solid $^3$He. From a comparison between our result and the earlier one, this linearity is almost independent of the particle motion. Now, it could be caused by a microscopic magnetic field inhomogeneity arisen from the mosaic angle spread and diamagnetism of the graphite substrate.