Coulomb gap induced by electronic correlation and enlarged superconducting gap in laterally confined Pb islands grown on SrTiO3

TL;DR

This study reveals how quantum size effects and electronic correlations influence the superconducting and Coulomb gaps in nano-sized Pb islands on SrTiO3, highlighting the interplay between size, correlation, and superconductivity.

Contribution

It demonstrates the emergence of an enlarged superconducting gap without increased transition temperature and the size-dependent Coulomb gap in nano-sized Pb islands, linking quantum size effects to electronic correlations.

Findings

Enlarged superconducting gap with a high BCS ratio (~6.22) observed.

Coulomb gap size depends on the lateral size of Pb islands.

Quantum size effects influence electronic correlations in nano-superconductors.

Abstract

We report high-resolution scanning tunneling microscopy (STM) study of nano-sized Pb islands grown on SrTiO3, where three distinct types of gaps with different energy scales are revealed. At low temperature, an enlarged superconducting gap ({\Delta}s) emerges while there is no enhancement in superconducting transition temperature (Tc), giving rise to a larger BCS ratio 2{\Delta}s/kBTc ~ 6.22. The strong coupling here may originate from the electron-phonon coupling on the metal-oxide interface. As the superconducting gap is suppressed under applied magnetic field or at elevated temperature, Coulomb gap and pseudogap appear, respectively. The Coulomb gap is sensitive to the lateral size of Pb islands, indicating that quantum size effect is able to influence electronic correlation, which is usually ignored in low-dimensional superconductivity. Our experimental results shall shed important light on the interplay between quantum size effect and correlations in nano-sized superconductors.