Spin-Dependent Quasiparticle Reflection and Bound States at Interfaces with Itinerant Antiferromagnets

We find a novel channel of quasiparticle reflection from the simplest two-sublattice antiferromagnet (AF) on a bipartite lattice. Low-energy quasiparticles in a normal metal (N) experience spin-dependent retroreflection at AF/N interfaces. As a combined effect of antiferromagnetic and Andreev reflections, subgap Andreev states arise at an AF/superconductor (SC) interface. When the antiferromagnetic reflection dominates the specular one, Andreev bound states have almost zero energy on $\mathrm{A}\mathrm{F}/s$-wave superconductor ($s\mathrm{S}\mathrm{C}$) interfaces, whereas there are no low-energy subgap states on $\mathrm{A}\mathrm{F}/d$-wave superconductor ($d\mathrm{S}\mathrm{C}$) boundaries. For an $s\mathrm{S}\mathrm{C}/\mathrm{A}\mathrm{F}/s\mathrm{S}\mathrm{C}$ junction, the bound states are found to split, due to the finite width of the AF interlayer, and carry the supercurrent. The theory developed in the present Letter is based on a novel quasiclassical approach, which applies to interfaces involving itinerant antiferromagnets.