Thermodynamic evidence for two-dimensional He-3 tunnelling excitations

摘要

The heat capacity of the second layer solid of He-4 adsorbed on graphite has been measured after doping with He-3 atoms, in the temperature range 2-80 mK. In a series of experiments the He-3 coverage was held constant at 1.0 and 0.7 nm(-2), respectively, while the He-4 coverage was progressively increased. Heat capacity isotherms show a distinct maximum over the range of coverages for which the second layer commensurate solid is expected to be stable. In this region the heat capacity exhibits a broad maximum near 50 mK, at a value close to k(B) per He-3 atom. At higher temperatures the heat capacity decreases slowly, while at lower temperatures it tends to zero faster than linearly. Our interpretation is that the excitations are tunnelling He-3 quasiparticles in the host 2D crystalline He-4 matrix. In this case the bandwidth and effective mass are determined by the sum of all possible cyclic permutations causing a He-3 atom to hop from a particular site. The low temperature behaviour indicates a breakdown of the Fermi liquid description arising either from interactions between the excitations or localisation. (C) 2003 Elsevier Science B,V. All rights reserved. [References: 4]