Electron retroreflection and spin beam splitting in a twisted graphene bilayer

We theoretically investigate the various reflection processes in a twisted graphene bilayer-based normal conductor/superconductor junction. It is found that the special spinor wave functions in strongly doped superconductor region lead the Andreev reflection to be suppressed completely. For the 100% electron → electron reflection, except the traditional specular reflection, electrons can undergo retroreflection, which is sensitive to the band contour and can be used to confirm the van Hove singularities in the twisted graphene bilayer. By depositing a ferromagnetic insulator on the N region, we further find that electrons can be spatially separated with spin-down electrons specular reflected and spin-up electrons retroreflected, or vice versa. It provides a new mechanism to realize a spin beam splitter.