Poor-man's Majorana edge mode enabled by specular Andreev reflection

It is known that the surface of a three-dimensional topological insulator (3D TI) supports a chiral Majorana edge mode at the interface between a superconductor and a magnetic insulator. The complexity of the materials combination is such that this state has not yet been observed. Here we show that a helical Majorana edge mode appears even in the absence of the magnetic insulator, if the Fermi level of the massless surface electrons is at the Dirac point. Specular Andreev reflection of Dirac fermions is at the origin of the effect. The simplified geometry may favor experimental observation of the helical Majorana mode, although it lacks the topological protection of its chiral counterpart.