Finite-temperature properties of the easy-axis Heisenberg model on frustrated lattices

Motivated by recent experiments on a compound displaying Ising-type short-range correlations on the triangular lattice, we study the anisotropic easy-axis spin-21 Heisenberg model on the triangular and kagome lattices by performing numerical calculations of finite-temperature properties, in particular of static spin structure factor and of thermodynamic quantities, on systems with up to 36 sites. On the triangular lattice, the low-temperature spin structure factor exhibits long-range spin correlations in the whole range of anisotropies, whereas thermodynamic quantities reveal a crossover upon increasing the anisotropy, most pronounced in the vanishing generalized Wilson ratio in the easy-axis regime. In contrast, on the kagome lattice, the spin structure factor is short range, and thermodynamic quantities evolve steadily between the easy-axis and the isotropic cases, consistent with the interpretation in terms of a spin liquid.