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

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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