To study how Andreev reflection (AR) occurs between a superconductor and a three-dimensional topological insulator (TI), we use superconducting Nb tips to perform point-contact AR spectroscopy at 4.2 K on as-grown single crystals of Bi2Se3. Scanning tunneling spectroscopy and scanning tunneling microscopy are also used to characterize the superconducting tip and both the doping level and surface condition of the TI sample. The point-contact measurements show clear spectral signatures of AR, as well as a depression of zero-bias conductance with decreasing junction impedance. The latter observation can be attributed to interfacial Rashba spin-orbit coupling, and the presence of bulk bands at the Fermi level in our samples suggests that bulk states of Bi2Se3 are involved in the observed AR.

To study how Andreev reflection (AR) is affected by itinerant antiferromagnetism, we perform dwave AR spectroscopy with superconducting YBa2Cu3O 7−δ on TiAu and on variously-oxidized Nb (NbOx) samples. X-ray photoelectron spectroscopy is also used on the latter to measure their surface oxide composition. Below the Néel temperatures (TN ) of both TiAu and NbOx, the conductance spectra show a dip-like structure instead of a zero-bias peak within the superconducting energy gap; for NbOx, higher-oxidized samples show a stronger spectral dip at zero bias. These observations indicate that itinerant antiferromagnetic order suppresses the AR process. Interestingly, the spectral dip persists above TN for both TiAu and NbOx, implying that spin fluctuations can also suppress AR. Our results suggest that d-wave AR spectroscopy may be used to probe the degree of spin ordering in itinerant antiferromagnets.