Ground state and thermodynamic properties of the coupled double-Ising model: application to rare-earth tetraborides.

A combination of the standard Metropolis algorithm and the parallel tempering method is used to study ground-state and thermodynamic properties of the coupled double Ising (CDI) model on the Shastry-Sutherland lattice (SSL). This model is derived from the complex spin-electron model, in which electrons described by the Hubbard model and spins described by the Ising model interact via the anisotropic spin-dependent interaction of the Ising type, under the following three conditions: (a) the Hubbard interaction between electrons in the conduction band is sufficiently large; (b) the number of conduction electrons is equal to the number of the lattice points; (c) there is the easy axis spin anisotropy in the system. The model is solved numerically for the selected combinations of spins from the electron (s=1/2) and spin (S=1,3/2,2) Ising branch which represent very realistically the situation in TmB, ErBand HoBcompounds, where in addition all three conditions are simultaneously satisfied.