Induced superconducting correlations in the quantum anomalous Hall insulator

Inducing Cooper pairing in a thin ferromagnetic topological insulator in the quantum anomalous Hall state (called quantum anomalous Hall insulator, QAHI) is a promising way to realize topological superconductivity with associated chiral Majorana edge states. However, finding evidence of superconducting proximity effect in a QAHI has proven to be a considerable challenge due to inherent experimental difficulties. Here we report the observation of crossed Andreev reflection (CAR) across a narrow superconducting Nb electrode contacting the chiral edge state of a QAHI, evinced by a negative nonlocal voltage measured downstream from the grounded Nb electrode. This is an unambiguous signature of induced superconducting pair correlation in the chiral edge state. Our theoretical analysis demonstrates that CAR processes of the chiral edge are not strongly dependent on the nature of the superconductivity that mediates them. Nevertheless, the characteristic length of the CAR process is found to be much longer than the superconducting correlation length in Nb, which suggests that the CAR is in fact mediated by superconductivity induced on the QAHI surface. The approach and results presented here provide a foundation for future studies of topological superconductivity and Majorana physics, as well as for the search for non-Abelian zero modes.