Adsorption Structures and Competition of Multiple Spin Exchange Interactions in Two-dimensional Quantum Solids.

在石墨表面吸附氦-3 （^3He）固体薄膜是二维自旋系统的理想材料。在^3He固体薄膜中，多自旋交换（MSE）相互作用的竞争主导了磁相互作用，使磁性变得复杂。近年来，波纹状吸附电位或吸附结构对MSE竞争的重要作用已得到认可。在这项研究中，我们首次测量了^3He固体薄膜在相应面密度下的热容，温度降至亚mk。我们观察到，在20多年的宽温度范围内，与其他面密度区一样，C∝T^α （α【近似等于】-1）的行为几乎成反比。这种行为是局域自旋系统在高温极限下的不寻常和反常的热容量，并归因于MSE相互作用的强竞争。α值被认为随着MSE的比较越小，越多的竞争越强。观测到的指数α绝对值在相称面密度区大于不相称面密度区，表明均方误差竞争非常强。这一观察结果与预测一致，即在相称结构中，吸附电位的波纹抑制了自旋交换，特别是低阶交换，并且MSE竞争应该比在不相称结构中更强。^3He薄膜通过衬底冷却的机理也不清楚。我们还用弛豫法测量了^3He薄膜与石墨衬底之间的热导率。测得的导热系数显示出与MSE相互作用强度相似的面密度变化。这一观察结果表明，测量的热导率是^3He薄膜本身的热导率，而不是界面的热导率，并且热量仅通过^3He薄膜和石墨衬底之间的局部点传递。提出了石墨基体中3^He自旋与磁性杂质之间的磁Kapitza机制。在磁场的初步测量中，观察到强烈的磁场依赖性，支持了这一假设。少

Helium-3 (^3He) solid films adsorbed on a graphite surface are considered as ideal materials of two-dimensional spin system. In the ^3He solid films, the competition of multiple spin exchange(MSE) interactions dominates the magnetic interactions and complicates the magnetism. Recently, the important roll of the corrugation of the adsorption potential, or the adsorption structures, on the MSE competition has been recognized.In this research we measured the heat capacity of the ^3He solid films in the commensurate areal density regime down to sub-mK temperature for the first time. We observed almost inversely proportional behavior, C ∝ T^α (α【approximately equal】 -1), in the wide temperature range more than two decades as in other areal density regimes. This behavior is unusual and anomalous as the heat capacity of localized spin systems in the high temperature limit, and is attributed to the strong competition of the MSE interactions. The α value is thought to be smaller as the MSE comp … More etition is stronger. The observed absolute value of the exponent α is larger in the commensurate areal density regime than in the incommensurate regime and shows the MSE competition is very strong. This observation agrees with the prediction that in the commensurate structure the corrugation of the adsorption potential suppresses spin exchanges, especially lower-order exchanges, and the MSE competition should be stronger than in incommensurate structures.The mechanisms of cooling ^3He films through substrates have not clearly understood either. We also measured the thermal conductivity between ^3He films and graphite substrates by the relaxation method. The measured thermal conductivity shows similar areal density variation with the strength of the MSE interaction. This observation indicates that the measured thermal conductivity is the thermal conductivity of the ^3He film itself, not of the interface, and the heat is transferred only by local spots between ^3He films and graphite substrates. It suggests the magnetic Kapitza mechanism between 3^He spins and magnetic impurities in graphite substrates. The preliminary measurements in magnetic fields, in which strong magnetic field dependence was observed, support this hypothesis. Less