Heat capacity of kagome 3He monolayer film on graphite

Abstract

The heat capacities of second layer 3He-4He mixture solid films are measured. The total areal densities are adjusted to be the 4/7 phase commensurate to the underlying 4He first layer, and the fractions of 3He in the second layer are adjusted to be almost three-fourths of the amount of second-layer helium atoms. According to the recent theoretical prediction of the adsorption structure of the 4/7 phase by Takagi, 3He atoms in such a 3He-4He mixture film are expected to be a nuclear spin system of the pure kagome lattice. The heat capacity measurements of such 3He-4He films between 0.1 and 80 mK reveal double peak structures. The higher-temperature peaks appear at around 30 mK; these are thought to arise from the replacement of 3He and 4He atoms. The magnitude of the higher-temperature peak strongly supports the adsorption structure proposed by Takagi, and not the one previously proposed by Elser, and gives strong supporting evidence of the realization of a pure kagome 3He lattice. The lower-temperature peaks must arise from the short-range spin ordering of 3He nuclear spins on the kagome lattice. As compared to the heat capacity of the pure 3He in the 4/7 phase, i.e., in the triangular phase, slower short-range ordering is observed in the kagome lattice, which agrees with the theoretical predictions.