Proximity-induced odd-frequency superconductivity in a topological insulator

At an interface between a topological insulator (TI) and a conventional superconductor (SC),
superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular,
the induced superconductivity by an s-wave SC in a TI can develop an order parameter with a p-wave
component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi2Se3 proximity coupled to Nb. From depthresolved magnetic field measurements below the superconducting transition temperature of Nb, we observe
a local enhancement of the magnetic field in Bi2Se3 that exceeds the externally applied field, thus
supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency
superconducting state. Our experimental results are complemented by theoretical calculations supporting
the appearance of such a component at the interface which extends into the TI. This state is topologically
distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our
knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We
thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting
states.