Chiral Majorana fermion modes on the surface of superconducting topological insulators

The surface of superconducting topological insulators (STIs) has been recognized as an effective p+/- ip superconductivity platform for realizing elusive Majorana fermions. Chiral Majorana modes (CMMs), which are different from Majorana bound states localized at point defects, possess many exotic properties. Here we predict that CMMs can be achieved in experiments by depositing a ferromagnetic insulator overlayer on top of the STI surface. We simulate this heterostructure by employing a realistic tight-binding model and show that the CMM appears on the edge of the ferromagnetic islands and can be directly probed by STM only after the superconducting gap is inverted by the exchange coupling between the ferromagnet and the STI. In addition, multiple CMMs can be generated by tuning the chemical potential of the topological insulator. These results can be applied to both proximity-effect-induced superconductivity in topological insulators and intrinsic STI compounds such as PbTaSe₂, BiPd and their chemical analogues, providing a route to engineering CMMs in those materials.