Frustrated square-lattice magnetism from indirect exchange through a 3D superconductor

We study the magnetic ordering of a square lattice of magnetic adatoms deposited on the surface of a 3D superconductor without spin-orbit coupling. The adatoms experience an effective long-ranged indirect exchange interaction that can be frustrated upon varying the lattice spacing. The frustration allows the adatom system to break the lattice rotation symmetry at low temperatures through an Ising phase transition, while maintaining the spin-rotation symmetry. Near special values of the lattice spacing the susceptibility is peaked along a continuous manifold in momentum space, making the system an attractive candidate for the study of spin liquid physics and 2D topological superconductivity.