Theory for the negative longitudinal magnetoresistance in the quantum limit of Kramers Weyl semimetals

Negative magnetoresistance is rare in non-magnetic materials. Recently, negative magnetoresistance has been observed in the quantum limit of beta-Ag/Se, where only one band of Landau levels is occupied in a strong magnetic field parallel to the applied current. beta-Ag2Se is a material that hosts a Kramers Weyl cone with band degeneracy near the Fermi energy. Kramers Weyl cones exist at time-reversal invariant momenta in all synunorphic chiral crystals, and at a subset of these momenta, including the Gamma point, in non-symmorphic chiral crystals. Here, we present a theory for the negative magnetoresistance in the quantum limit of Kramers Weyl semimetals. We show that, although there is a band touching similar to those in Weyl semimetals, negative magnetoresistance can exist without a chiral anomaly. We find that it requires screened Coulomb scattering potentials between electrons and impurities, which is naturally the case in beta-Ag2Se.