Odd-frequency superconductivity at the helical edge

Björn Trauzettel, Wuerzburg University

We prove the existence of odd-frequency pairing at the edge of a quantum spin Hall system in proximity to an s-wave superconductor. If the s-wave pairing (in one region of the system) is accompanied by a proximity-induced ferromagnetic exchange interaction (in another region of the system) then a peculiar equal-spin triplet pairing sets in. This pairing can be nicely understood on the basis of unconventional bound states - some of which with Majorana character. In fact, the odd-frequency pairing and the Majorana bound states come hand in hand. We further predict a unique connection between the odd-frequency pairing and finite crossed Andreev reflections at the helical edge. This correspondance enables us to unambiguously detect odd-frequency superconductivity in topological materials.