Asymmetric Fraunhofer Spectra in a Bi₂Se₃ Josephson Junction

Josephson junctions with topological insulators as their weak link (S–TI–S junctions) are predicted to host Majorana fermions. But the details of S–TI–S current-phase relations and their interplay with magnetic fields are not completely understood. We fabricate a Bi₂Se₃ junction with NbTi leads and measure Fraunhofer patterns of the junction with applied in-plane fields. We observe Faunhofer patterns with aperiodic node spacings. These asymmetries appear even at zero parallel field and for temperatures up to 1 K. The anomalous features are compared to asymmetric Fraunhofer patterns expected for finite Cooper pair momentum shifts as well as geometric effects, withthe conclusion that geometric effect can dominate. These results are important for differentiating geometrical phase shifts from those caused by Cooper pair momentum shifting, Majorana mode signatures, or other unconventional superconducting behavior.