Odd-Frequency Triplet Superconductivity at the Helical Edge of a Toplogical Insulator

Non-local pairing processes at the edge of a two-dimensional topological insulator in proximity to an $s$-wave superconductor are usually suppressed by helicity. However, 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, non-local 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.