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

The interplay of superconductivity and topological surface states which are protected by time-reversal symmetry provides a platform for exploring new quantum phenomena, such as Majorana zero modes that may find application in fault-tolerant quantum computation. Here, by growing high-quality topological insulator Bi$_2$Se$_3$ films on a $\textit{d}$-wave superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi$_2$Se$_3$ films with a large pairing gap up to 15 meV. Interestingly, distinct from the $\textit{d}$-wave pairing of Bi2212, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant $\textit{s}$-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes.