Anomalous nuclear-spin heat capacities in submonolayer solid 3He adsorbed on graphite

Nuclear-spin heat capacities of submonolayer solid 3 He adsorbed on a graphite surface are measured down to 100 μK. a factor of 20 lower temperature than in previous works. This system is one of the most ideal two-dimensional quantum spin systems (S = ½). In a wide areal density region (6.1 nm - 2 ≤ρ ≤8.7 nm - 2 ), anomalous temperature dependencies of the heat capacity, CT α (-1.6≤α≤-0.7). are observed in a temperature range over two orders of magnitude (0.1 mK≤T≤20 mK) instead of the expected high-temperature behavior (α = -2) for localized spins. The a value shows a complicated density dependence which is accompanied by a density variation of a heat capacity isotherm at an extremely low temperature (= 0.2 mK). This anomaly is similar to the previously observed high-temperature behavior (α - 1) of the lowest density solid in the second layer. Although quantitative explanations are lacking for these anomalies, they are likely due to the high frustration caused by competing ferromagnetic and antiferromagnetic multiple-spin exchange interactions at least up to the six-spin exchange. We find that the excess heat-capacity (C e x ) due to the amorphous 3 He adsorbed on substrate heterogeneities is certainly not an origin of the anomalous behavior in C of the uniform submonolayer. Only at and below the density for the √3×√3 commensurate phase (ρ = 6.4 nm - 2 ) heat capacity bumps at around 20 mK are observed. We suggest the possibility of spin-polaron effects caused by delocalized vacancies to explain this anomaly.