Zero Point Vacanty and Nuclear Magnetism of Adsorbed Two Dimensional Quantum Solid

吸附在石墨表面的亚单层^3He固体薄膜是最理想的二维量子自旋体系之一。该系统的磁性被理解为由多个自旋交换相互作用的竞争所控制。然而，竞争的细节尚不清楚。考虑到吸附电位的波纹对竞争有重要影响，采用路径积分蒙特卡罗模拟计算吸附电位，确定了结构相图。从√<3>×√<3>相开始，随着密度的增加，依次出现条纹畴壁结构、蜂窝畴壁结构、蜂窝笼结构和不相称结构。这些转变是二级结构相变。所得的结构相图可以定性地解释许多实验观察结果。特别是随着密度的增加，AFM - FM的突然变化是最令人困惑的，这可以解释为由于结构相从相称结构向不相称结构（C-IC转变）转变而引起的竞争的突然变化。另一方面，我们已经测量了这些系统的热容低至100 μK，并观察到在宽密度区和超过两个数量级的宽温度范围内，这些系统的热容几乎与温度成反比。在任何局域自旋系统中，高温极限下的热容必须与温度的平方成反比。观测到的指数与预期正常值的差值-2被认为是一个挫折参数，它反映了多自旋交换相互作用的竞争强度。热容指数随密度的变化表现出复杂的行为。这种行为也可以用我们提出的相图来解释。在低于√<3>×√<3>相的密度范围内，竞争变得更弱，并且在相同密度范围内，流体没有热容贡献，只观察到突起，这些事实有力地支持了零点空缺的存在，但没有观察到直接证据。少

The submonolayer ^3He solid film adsorbed on a graphite surface is one of the most ideal two-dimensional quantum spin system. The magnetism of this system is understood to be governed by the competition of the multiple spin exchange interactions. However, the detail of the competition had not been understood. We have considered that the corrugation of the adsorption potential plays an important role on the competition, and determined the structural phase diagram based on the calculation of the adsorption potential by the path integral Monte Carlo simulations. With increasing density from the √<3>×√<3> phase, striped domain wall structure, honeycomb domain wall structure, honeycomb cage structure and incommensurate structure appear successively. These transitions are the second order structural phase transitions. The obtained structural phase diagram can explain many experimental observations qualitatively. Especially, the sudden AFM - FM change with increasing density, which was the bi … More g puzzle, can be explained as the sudden change of competition due to the structural phase transition from the commensurate structure to the incommensurate one (C-IC transtion).On the other hand, we have measured the heat capacities of these systems down to 100 μK, and observed anomalous temperature dependence, which are almost inversely proportional to the temperature, in a wide density regime and in a wide temperature range more than two orders of magnitude. In any localized spin system, the heat capacity at the high temperature limit must be inversely proportional to the square of temperature. The difference of the observed exponent from the expected normal value, -2, are thought to be a frustration parameter, which show the strength of the competiton of the multiple spin exchange interactions. The observed heat capacity exponent shows complicated behavior with the density. This behavior can also be explained by our proposed phase diagram. The facts that at the lower density range than the √<3>×√<3> phase the competition become weaker and that the no heat capacity contribution from the fluid but the bumps are observed at same density range, strongly support the existence of the zero point vacancy, for which no direct evidence had been observed. Less