Competing effects of surface phonon softening and quantum size effects on the superconducting properties of nanostructured Pb

TL;DR

This study investigates how surface phonon softening and quantum size effects influence the superconducting transition temperature and critical field in nanostructured lead, revealing a near-constant TC despite size reduction.

Contribution

It demonstrates that phonon softening enhances electron-phonon coupling, compensating quantum size effects and maintaining TC in nanostructured Pb.

Findings

TC remains nearly constant from 65 to 7nm particle size

Upper critical field HC2 is significantly enhanced in small particles

Electron-phonon coupling increases as particle size decreases below 22nm

Abstract

The superconducting transition temperature (TC) in nanostructured Pb remains nearly constant as the particle size is reduced from 65 to 7nm, below which size the superconductivity is lost rather abruptly. In contrast, there is a large enhancement in the upper critical field (HC2) in the same size regime. We explore the origin of the unusual robustness of the TC over such a large particle size range in nanostructured Pb, by measuring the temperature dependence of the superconducting energy gap in planar tunnel junctions of Al/Al2O3/nano-Pb. We show that below 22nm, the electron phonon coupling strength increases monotonically with decreasing particle size, and almost exactly compensates for the quantum size effect, which is expected to suppress TC.