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

Bryan Rachmilowitz,Shang Gao,Ilija Zeljkovic,Genda Gu,Zheng Ren,He Zhao,Christian Flindt,Konrad H Thomas,Ruidan Zhong,Hong Li,John Schneeloch,John Marangola

doi:10.1038/s41535-020-00274-6

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.

Highlights

When a superconductor (SC) is interfaced with a normal, nonsuperconducting material, Cooper pairs are able to tunnel across the interface, and the normal material can become superconducting via the superconducting proximity effect (SPE)
As the proximity-induced superconducting gap at the exposed bare surface of the normal material is expected to decrease with its thickness[22], we explore a range of Bi2Te3 thicknesses, from ~10 quintuple layers (QLs) down to a partial coverage of a single QL film
Our measurements resolve the outstanding controversy between angle-resolved photoemission spectroscopy (ARPES) and STM measurements of molecular-beam epitaxy (MBE)-grown heterostructures of topological materials and cuprates, by shedding light on an overlooked aspect of the underlying physics in these systems rooted in the Coulomb blockade (CB) effects

Summary

Introduction

When a superconductor (SC) is interfaced with a normal, nonsuperconducting material, Cooper pairs are able to tunnel across the interface, and the normal material can become superconducting via the superconducting proximity effect (SPE). Interest in the SPE has been brought to the forefront by the discovery of 2D materials predicted to harbor exotic electronic phenomena if interfaced with superconductors, such as Majorana modes in topological superconductors[1] and superluminescence in proximitized p-n junctions[2]. While the majority of SCs used have led to well-established platforms for proximity studies, the experiments using cuprates, which exhibit significantly larger superconducting gap (ΔSC) and Tc18, have given unexpectedly conflicting results[12,13,14,15,16,17]. On the other hand, tunneling measurements have observed a gap in dI/dV spectra in both Bi17 and Bi2Te316 grown on Bi-2212, interpreted to arise due to proximity-induced superconductivity in the topological material

Methods

Results

Conclusion