Magnetic field tuning of the valley population in the Weyl phase of Nd₂Ir₂O₇

Materials hosting Dirac valleys serve as promising candidates for novel electronic devices. Valley polarization, the population control of distinguishable valleys, is yet the most formidable challenge in this area. The frustrated magnet Nd₂Ir₂O₇, where strong correlations together with spin-orbit coupling play a crucial role, is predicted to be a Weyl semimetal and to host topological pairs of bulk Dirac-like valleys. Here we use an external magnetic field to manipulate the localized rare earth 4f moments coupled to the 5d electronic bands. Low energy optical spectroscopy reveals that a field of only a few teslas suffices to create charge compensating pockets of holes and electrons in different regions of momentum space, thus introducing a valley population shift that can be tuned with the field. This demonstrates that rare earth elements can act as boosters of an external magnetic field, making these materials good candidates for ``valleytronics" devices.