Fractional AC Josephson effect in a topological insulator proximitized by a self-formed superconductor

Josephson junctions with topological weak links should support Majorana bound states, yet are challenging to fabricate without damaging the topological material. Here, the authors evaporate a noble metal onto a (BiSb) ₂Te₃ topological insulator that chemically reacts, creating a self-formed superconductor while minimizing damage. The interface between the superconductor and untouched (BiSb) ₂Te₃ has adequate interface transparency, and the two materials have similar work functions. Under microwave irradiation, they observe suppressed first and third Shapiro steps, which supports the presence of Majorana bound states.