Coulomb blockade effects in a topological insulator grown on a high-T _c cuprate superconductor

The evidence for proximity-induced superconductivity in heterostructures of topological insulators and high-T_c cuprates has been intensely debated. We use molecular-beam epitaxy to grow thin films of topological insulator Bi₂Te₃ on a cuprate Bi₂Sr₂CaCu₂O_8+x, and study the surface of Bi₂Te₃ using low-temperature scanning tunneling microscopy and spectroscopy. In few unit-cell thick Bi₂Te₃ films, we find a V-shaped gap-like feature at the Fermi energy in dI/dV spectra. By reducing the coverage of Bi₂Te₃ films to create nanoscale islands, we discover that this spectral feature dramatically evolves into a much larger hard gap, which can be understood as a Coulomb blockade gap. This conclusion is supported by the evolution of dI/dV spectra with the lateral size of Bi₂Te₃ islands, as well as by topographic measurements that show an additional barrier separating Bi₂Te₃ and Bi₂Sr₂CaCu₂O_8+x. We conclude that the prominent gap-like feature in dI/dV spectra in Bi₂Te₃ films is not a proximity-induced superconducting gap. Instead, it can be explained by Coulomb blockade effects, which take into account additional resistive and capacitive coupling at the interface. Our experiments provide a fresh insight into the tunneling measurements of complex heterostructures with buried interfaces.