Coulomb blockade effects in tunneling measurements of a topological insulator grown on a high-T_c cuprate superconductor

We use molecular-beam epitaxy to grow thin films of topological insulator Bi₂Te₃ on a high-T_c superconductor 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. 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 the Bi₂Te₃ films 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.