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

1.核磁共振实验用连续波核磁共振方法研究了从第二层以下到-2.5层铁磁峰覆盖率以上的纯~3He薄膜。第一层磁化强度很好地符合居里定律，Weiss常数小于0.01mK。我们测量了铁磁第二层磁化强度随覆盖度的变化，温度远低于交换能J。初步分析表明，J随覆盖度变化不大，零温第二层磁化强度与第二层密度呈线性关系。2.热容量测量我们测量了在0.3-60mK温度范围内吸附在玻璃箔上的^3He薄膜的面密度，其中第二层固体^3He表现出反铁磁性。测量的热容似乎与格雷沃尔以前的测量结果并不定性地一致。我们的数据显示，在大约2mK和1/T^2的高温下，极大值是圆形的，而他在2.5mK和1/T的高温下观察到了相当尖锐的峰。尽管我们的数据与Elstner等人研究的三角形晶格上自旋为1/2的海森堡反铁磁体的理论计算结果是半定量一致的，但在任何固定温度下测量的热容都比理论热容小30%。由我们的数据计算出的核自旋熵从无限大温度到1mK减少了0.6Rln2，“缺失熵”似乎达到了0.4ln2。

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.