Point contact Andreev reflection on topological insulator-superconductor SmB₆/YB₆ heterostructures

Topological insulator-superconductor (TI-SC) heterostructures have attracted great interest for exploring topological superconductivity which has been predicted to creation of Majorana Fermions as well as exotic superconductivity. We have previously demonstrated the superconducting proximity effect in in-situ deposited SC-TI Nb/SmB₆ heterostructures as a result of Nb superconductivity induced in the surface state of SmB₆. Here, to probe the proximity-induced superconductivity in the surface state of SmB₆, we fabricated inverted TI-SC SmB₆/YB₆ heterostructures in-situ and investigated the superconductivity with point contact spectroscopy measurements. In the limit of SmB₆ layer thickness going to zero, the YB₆ thin film exhibits tunneling-like spectra with a finite Z (barrier parameter), whereas a thin SmB₆/YB₆ bilayer exhibits clear doubling of the conductance within the gap of YB₆, indicating occurrence of near perfect Andreev reflection with Z ~ 0. We will discuss the results within the context of the Fermi velocity mismatch, helical Cooper pairing, and the topological proximity effect which can influence the observed transition from the tunneling-like regime to the Andreev reflection regime.