2-Dimensional Nuclear Magnetism of Adsorbed ^3He Thin Films

点击翻译按钮获取中文摘要

1.NMR ExperimentsContinuous wave NMR was used to study pure ^3He films at coverage from below 2nd layr completion to above the ferromagnetic peak coverage at -2.5 layrs. The films were adsorbed on Grafoil and were cooled to temperatures as low as 0.075mK.The first layr magnetization followed Curie law very well, with a Weiss constant of less than 0.01mK.We measured the ferromagnetic 2nd layr magnetization to temperatures far below the exchange energy (J) as a function of coverage. Preliminary analysis indicated that J did not vary strongly with coverage, and that the zero temperature 2nd layr magnetization was linear in 2nd layr density. Both of these properties are consistent with there being two phases in coexistence., only one of which is ferromagnetic.2.Hear Capacity MeasurementsWe have measured heat capacities of ^3He thin films adsorbed on Grafoil at temperatures between 0.3 and 60mK for areal densities where the 2nd layr solid ^3He shows antiferromagnetism.. The measured heat capacities do not seem to agree qualitatively with previous measurements by Graywall. Our data show rounded maxima at about 2mK and 1/T^2 behavior at high temperatures, while he observed rather sharp peaks at 2.5mK and 1/T behavior at high temperatures. Although our data agree semi-quantitatively with theoretical calculations for the spin-1/2 Heisenberg antiferromagnet on the triangular lattice studied by Elstner et al., the magnitude of measured heat capacities is smaller than the theoretical one by 30% at any fixed temperature. Nuclear spin entropy calculated from our data decreases by 0.6Rln2 from the infinite temperature to 1mK.The "missing entropy" seems to reach about 0.4ln2.