Anisotropic angle-dependent Andreev reflection at the ferromagnet/superconductor junction on the surface of topological insulators

Abstract

We theoretically demonstrate that a ferromagnetic/superconductor junction on the surface of three-dimensional topological insulators (3D TIs) has an anisotropic angle-dependent Andreev reflection when the in-plane magnetization has a component perpendicular to the junction. In the presence of in-plane magnetization, the Dirac cone's location adjusts in the $k$-space, whereas its out-of-plane component induces a gap. This movement leads to the anisotropic angle-dependent Andreev reflection and creates transverse conductance flows parallel to the interface. Also, an indirect gap induces in the junction, which removes the transport signatures of Majorana bound states. Because of the full spin-momentum locking of Dirac fermions on the surface of 3DTIs, a torque that called \textit{Andreev Transfer Torque} (ATT) imposes on the junction. Moreover, we propose a setup to detect them experimentally.