Odd-frequency triplet superconductivity at the helical edge of a topological insulator

Nonlocal pairing processes at the edge of a two-dimensional topological insulator in proximity to an s-wave superconductor are usually suppressed by helicity. However, the additional proximity of a ferromagnetic insulator can substantially influence the helical constraint and therefore open a new conduction channel by allowing for crossed Andreev reflection (CAR) processes. We show a one-to-one correspondence between CAR and the emergence of odd-frequency triplet superconductivity. Hence, nonlocal transport experiments that identify CAR in helical liquids yield smoking-gun evidence for unconventional superconductivity. Interestingly, we identify a setup---composed of a superconductor flanked by two ferromagnetic insulators---that allows us to favor CAR over electron cotunneling, which is known to be a difficult but essential task to be able to measure CAR.