Coulomb Blockade Effects in a Topological Insulator Grown on a High-Tc Cuprate Superconductor

TL;DR

This study investigates Coulomb blockade effects in Bi2Te3 topological insulator films grown on high-Tc cuprate superconductors, revealing that observed spectral gaps are due to Coulomb blockade rather than proximity-induced superconductivity.

Contribution

The paper demonstrates that the prominent spectral gap in Bi2Te3 on cuprates is caused by Coulomb blockade effects, challenging previous interpretations of proximity-induced superconductivity.

Findings

Spectral gap evolves into a Coulomb blockade gap in nanoscale islands.

Coulomb blockade explains the large gap features in tunneling spectra.

Additional interface barriers support Coulomb blockade interpretation.

Abstract

The evidence for proximity-induced superconductivity in heterostructures of topological insulators and high-Tc cuprates has been intensely debated. We use molecular beam epitaxy to grow thin films of topological insulator Bi2Te3 on a cuprate Bi2Sr2CaCu2O8+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 Bi2Sr2CaCu2O8+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.