Anomalous nuclear-spin heat capacities in submonolayer solid (formula presented) adsorbed on graphite

Abstract

Nuclear-spin heat capacities of submonolayer solid (formula presented) a factor of 20 lower temperature than in previous works. This system is one of the most ideal two-dimensional quantum spin systems (formula presented) In a wide areal density region (formula presented) are observed in a temperature range over two orders of magnitude (formula presented) instead of the expected high-temperature behavior (formula presented) for localized spins. The (formula presented) value shows a complicated density dependence which is accompanied by a density variation of a heat capacity isotherm at an extremely low temperature (formula presented) This anomaly is similar to the previously observed high-temperature behavior (formula presented) 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 (formula presented) due to the amorphous (formula presented) 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 (formula presented) commensurate phase (formula presented) 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. © 2002 The American Physical Society.