Ge as an ideal orbitronic platform: giant orbital Hall effect

Abstract: State-of-the-art developments in magnetic devices rely on manufacturing faster, more efficient memory elements. A significant development in this direction has been the discovery of orbital torques, which employ the orbital angular momentum of Bloch electrons to switch the magnetisation of an adjacent ferromagnet, and has motivated the search for \textit{orbitronic} materials displaying strong orbital dynamics exemplified, by the orbital Hall effect (OHE). In this work we propose Ge, as an optimal orbitronic platform. We demonstrate that holes in bulk Ge exhibit a giant OHE, exceeding that of the bulk states of topological insulators, and exceeding the spin-Hall effect by four orders of magnitude. The calculation is performed within the framework of the Luttinger model and the modern theory of orbital magnetisation, while incorporating recently-discovered quantum corrections to the OHE. Our study constitutes a fundamental milestone in applying the modern theory to a system with \textit{inversion symmetry}. Moreover, we argue that bulk Ge serves as an ideal testbed for the orbital torque resulting from a charge current, since the spin- and orbital-Edelstein effects in Ge are forbidden by symmetry. Our results provide a blueprint for producing strong orbital torques in magnetic devices with Ge, guiding future experimental work in this direction.