Antiparticle of exciton in semimetals

An emergent quantized field enriches quantum many-body systems. We propose an antiparticle analog of the exciton in semimetals as an emergent collective mode in interacting electron systems. We show that interband excitations in semimetals are generally comprised of both excitons and antiparticles of excitons. These two stand for two distinct interband collective modes in semimetals, having different energies and opposite conserved charges. The conserved charge here is a quantity conjugate to a joint U(1) symmetry of two electrons' bands associated with the interband excitations. The opposite charges foster fertile scattering processes among the interband collective modes. In spin-polarized systems, they also suggest possible experimental detections of the antiparticles. We clarify that the effective theory of the interband excitations is given by a generalized Klein-Gordon theory. Our theory provides a comprehensive understanding of excitonic spectra in generic semimetals, bringing broader insight into electronic collective phenomena in solids.