Abstract:
Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states in which electrons have their spin locked at a right angle to their momentum under the protection of time reversal symmetry. Topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking such as superconductivity can lead to new quantum phenomena and devices. However, the existence of the superconducting states (Cooper pairs) in TI's surface has not been obtained to date experimentally. Here, we report for the first time the realization of a superconducting TI/Superconductor (SC) heterostructure characterized with Cooper pairs tunneling into TI through superconducting proximity effect, by successful growing Bi2Se3 thin films on superconductor NbSe2 substrate with atomically sharp and electronic transparent interface. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we unambiguously observed the Cooper pairs at Bi2Se3 surface (Bi2Se3/NbSe2 interface) where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana Fermions in condensed matter physics.