Ground states of an Ising model on an extended Shastry-Sutherland lattice and the 1/2-magnetization plateau in some rare-earth-metal tetraborides

A complete solution of the ground-state problem for an Ising model on the Shastry-Sutherland lattice with an additional interaction along the diagonals of "empty" squares in an applied magnetic field is presented. A rigorous proof is given that this interaction gives rise to a plateau at one-half of the saturation magnetization. Such a fractional plateau has been observed in some rare-earth-metal tetraborides, in particular in strong Ising magnets ErB4 (where it is the only one) and TmB4 (where it is the broadest one), but its origin has remained unclear. Our study sheds new light on the solution of this problem.