Fully gapped topological surface states in Bi$_2$Se$_3$ films induced by a $d$-wave high-temperature superconductor*

Topological insulators are a new class of materials which are insulating in bulk but exhibit robust conducting surface states protected by time-reversal symmetry. The coupling between such symmetry-protected surface states and symmetry-broken states (for example, superconductivity) may lead to novel quantum phenomena, such as Majorana zero modes which are crucial for fault-tolerated quantum computation. Using molecular beam epitaxy, we have successfully grown high quality topological insulator Bi$_2$Se$_3$ films on high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. In this talk, I will present our recent work on superconducting proximity effect in Bi$_2$Se$_3$ films induced by high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. Using angle-resolved photoemission spectroscopy, we observe a proximity-induced gap up to 15 meV on the topological surface states of Bi$_2$Se$_3$ [1]. Bi$_2$Se$_3$/Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ heterostructure not only provides new opportunities for investigating the intriguing coupling between a topological insulator thin film and a d-wave superconductor, but also may be a new system for realizing Majorana zero modes. \\[4pt] [1] Eryin Wang et.al, {\it Nature Physics.}{\bf 9}, 621 (2013).