Twofold symmetry of proximity-induced superconductivity in Bi2Te3/Bi2Sr2CaCu2O8+δ heterostructures revealed by scanning tunneling microscopy

We observe proximity-induced superconductivity in in situ prepared heterostructures constructed by topological insulator ${\mathrm{Bi}}₂{\mathrm{Te}}₃$ thin films and high-temperature cuprate superconductors ${\mathrm{Bi}}₂{\mathrm{Sr}}₂{\mathrm{CaCu}}₂{\mathrm{O}}_{8+\ensuremath{\delta}}$. The superconducting gap maximum is about 7.6 meV on the surface of ${\mathrm{Bi}}₂{\mathrm{Te}}₃$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the principal crystalline axes of ${\mathrm{Bi}}₂{\mathrm{Te}}₃$. This gap form is consistent with the ${\mathrm{\ensuremath{\Delta}}}_4y$ notation of the topological superconductivity proposed in such systems. Our results provide fruitful information on the possible topological superconductivity induced by the proximity effect in high-temperature superconducting cuprates.