Andreev reflection through a junction of a 3D topological insulator and an s-wave superconductor

Abstract

We examine the retro and specular Andreev reflections (ARs) through a junction consisting of a three-dimensional (3D) Bi2Se3 topological insulator (TI) and an s-wave superconductor (SC) using spatially-resolved nonequilibrium Green function formalism in Nambu space. A 3DTI Bogoliubov–de Gennes Hamiltonian is constructed. The retro (specular) AR is identified by the spatial distribution of charge (spin-y) current. When approaching the interface, unlike the specular AR, the retro AR exhibits a large amount of net charge flow near the top surface of the 3DTI. In the specular AR, a large parallel component of the spin-y flow is developed in the interface, which is not found in the retro AR. Diminished charge currents are found near the onset of the specular AR. The AR mechanism enhances the current-induced spin-y polarization flowing through the junction interface. In the TI phase, larger normal-transmission conductance in the normal-metal junction yields larger AR conductance in the corresponding SC junction. The Zeeman splitting perpendicular to the quintuple layers partially (completely) suppresses the in the finite (infinite thick) TI.