Low Temperature Heat Capacities of Submonolayer Solid ³He

Abstract

Heat capacities (C) of ³ He submonolayer solids adsorbed on a graphite surface are measured down to 100 μK, a factor of twenty lower temperatures than previous work. At a real densities near the commensurate $\sqrt 3 \times \sqrt 3$ solid (6.4 nm ⁻² ), an anomalous temperature dependence, C ∝ 1/T, is observed in a wide temperature range over two orders of magnitude (0.1 ≤ T ≤ 20mK). Similar behavior was observed for the commensurate $\sqrt 7 \times \sqrt 7$ solid in the second-layer with the same density, suggesting a common microscopic mechanism. It is, however, a puzzle that the $\sqrt 3 \times \sqrt 3$ solid is ferromagnetic according to recent magnetization measurements by ISSP group, while the $\sqrt 7 \times \sqrt 7$ solid is known to be antiferromagnetic. Possible explanations for this conflict, such as vacancy effects or multiple-spin exchanges modulated by a substrate potential corrugation, are discussed.