Quantum-size effect induced Andreev bound states in ultrathin metallic islands proximitized by a superconductor

TL;DR

This paper reports the experimental observation of Andreev bound states in ultrathin metallic islands on a superconductor, revealing quantum size effects through spatial oscillations and energy symmetry, advancing understanding of superconductor-normal metal heterostructures.

Contribution

It provides the first direct detection of ABSs in ultrathin metallic islands with quantum confinement effects, supported by a theoretical model explaining the coupling influence on ABS energies.

Findings

Observation of in-gap ABSs with symmetric energies about the Fermi level.

Spatial oscillations of ABS wave functions demonstrate quantum size effects.

Coupling strength critically influences ABS energy levels.

Abstract

While Andreev bound states (ABSs) have been realized in engineered superconducting junctions, their direct observation in normal metal/superconductor heterostructures-enabled by quantum confinement-remains experimentally elusive. Here, we report the detection of ABSs in ultrathin metallic islands (Bi, Ag, and SnTe) grown on the s-wave superconductor NbN. Using high-resolution scanning tunneling microscopy and spectroscopy, we clearly reveal in-gap ABSs with energies symmetric about the Fermi level. While the energies of these states show no position dependence, their wave functions exhibit spatial oscillations, demonstrating a quantum size effect. Both the energy levels and spatial distribution of the ABSs can be reproduced by our effective model in which a metallic island is coupled to the superconducting substrate via the proximity effect. We demonstrate that the coupling strength plays a critical role in determining the ABS energies. Our work introduces a novel physical platform for implementing ABSs, which hold promise for significant device applications.