Topological band structure of excitons in octochlore KTb$_{3}$F$_{10}$: an analog of the Luttinger Semimetal

RESUMO

Understanding the symmetry and topology of band structures in crystalline solids has been among the defining themes of a generation of condensed matter physicists, with implications across many different platforms including electronic bands, photonic crystals, phonons and magnetic excitations. In the latter systems, there is usually magnetic long-range order and time reversal symmetry breaking. Here we further enrich the possible band structures available to magnetic systems by examining dispersive crystal field excitations above a ground state that is a simple product of singlets. In these systems, there is an interplay of magnetic interactions and band topology with both crystal and time reversal symmetries. Using high-resolution neutron spectroscopy we reveal the dispersion relations of bosonic excitons in KTb$_{3}$F$_{10}$, an experimental realization of the octochlore lattice. We show that there are pinch point features in the scattering intensity. The dominant coupling in this system is the long-range magnetostatic dipolar interaction, whose singularity at the zone center is passed onto the scattering intensity. However, the pinch points survive when the range of the dipoles is truncated to finite shell number in the form of a three-band quadratic band touching protected by cubic symmetry. This model, which is related to the Luttinger semi-metal, provides a general long wavelength model for the emergence of pinch point scattering intensity encompassing also the true long-range dipolar model.