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

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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