Anatomy of Spin and Current Generation from Magnetization Gradients in Topological Insulators and Rashba Metals

Abstract

We explore the spin density and charge currents arising on the surface of a topological insulator and in a 2D Rashba metal due to magnetization gradients. For topological insulators a single interconversion coefficient controls the generation of both quantities. This coefficient is quantized to a value proportional to the vorticity of the Dirac point which constitutes a hallmark of parity anomaly at finite density. As such, it also unveils a robust route to disentangle and detect the protected states of a topological insulator on a given surface. In stark contrast, Rashba metals do not exhibit such anomalies since they contain an even number of helical branches. Nonetheless, also these are governed by quantized responses which, however, are not protected against weak disorder. Furthermore, we find that for Rashba metals the interconversion coefficients demonstrate discontinuities and a nontrivial interplay upon varying the chemical potential, the strength of the spin-orbit coupling, and a pairing gap. Our results have implications for the binding between magnetic skyrmions and superconducting vortices, the emergence of Majorana zero modes, and pave the way for superconducting diode effects mediated by out-of-plane magnetization gradients.