Spin-excitons in high magnetic fields

Spin-Excitons are sharp branches of dispersive triplet excitations found with energies within the gap of paramagnetic Kondo Insulators, which exist for a restricted range of wave-vectors around Q ̲ a = ( π , π , π ) . The application of a high applied magnetic field is expected to drive the non-magnetic Kondo Insulating State to become unstable. The instability occurs through different routes, which depend on the strength of the quasi-particle interactions. One route towards instability, which occurs for strong quasi-particle interactions, is by a transition to a field-induced antiferromagnetic state. The second mode of instability, which occurs for weak interactions, is simply due to the magnetic field closing the hybridization gap. In this route, the non-magnetic Kondo insulator becomes unstable to a spin-polarized metallic state. Here we examine the effects of an applied magnetic field on spin-excitons. In the presence of a weak magnetic field, the excitations split into three branches, where the splittings of the dispersion relation are proportional to the applied field. The three branches exist for different ranges of Q ̲ and have different intensities. For strong interactions, the lowest-energy branch, which has the weakest intensity and exists for the smallest Q ̲ range, completely softens at the transition to the antiferromagnetic phase. This lower branch of excitations is found to disappear at the boundary separating the two modes of instability, leaving the upper two branches intact.