Andreev reflection without Fermi surface alignment in high- T _c van der Waals heterostructures

We address the controversy over the proximity effect between topological materials and high-T c superconductors. Junctions are produced between Bi₂Sr₂CaCu₂O_8+δ and materials with different Fermi surfaces (Bi₂Te₃ and graphite). Both cases reveal tunneling spectra that are consistent with Andreev reflection. This is confirmed by a magnetic field that shifts features via the Doppler effect. This is modeled with a single parameter that accounts for tunneling into a screening supercurrent. Thus the tunneling involves Cooper pairs crossing the heterostructure, showing that the Fermi surface mismatch does not hinder the ability to form transparent interfaces, which is accounted for by the extended Brillouin zone and different lattice symmetries.