Proximity-induced superconductivity in the topological surface state of thin Bi$_{2}$Se$_{3}$ films on Nb probed by ARPES

Topological superconductors (TSCs) are of great interest, since these systems could support novel electronic states such as Majorana fermions. A promising approach to realize TSCs is the preparation of artificial heterostructures involving a superconductor (SC) and a topological insulator (TI) in which the superconductivity is induced into the spin- and momentum-locked topological surface state by proximity coupling. By using angle-resolved photoemission spectroscopy, we have mapped the electronic band structure and determined the proximity-induced superconducting gaps for the simple case of prototypical TI Bi$_{2}$Se$_{3}$ on elemental s-wave SC Nb as functions of temperature and the thickness of single crystalline Bi$_{2}$Se$_{3}$ films. For both the bulk and the topological surface states coherence peaks and leading edge shifts of similar magnitude emerge at the Fermi level in the thickness rage of 4-10 QL. The study is an important step towards a comprehensive understanding of helical Cooper pairing in Dirac surface states and the optimization of artificial topological superconductors. Our smart, cleavage-based sample preparation technique is also applicable to other TI/SC heterostructures.