Superconductivity of lanthanum revisited: enhanced critical temperature in the clean limit

TL;DR

This study investigates how the superconducting energy gap and critical temperature of lanthanum thin films depend on thickness and purity, revealing enhanced superconductivity in ultra-clean, bulk-like samples.

Contribution

It demonstrates that high-purity lanthanum films exhibit a 40% higher critical temperature than previously reported, emphasizing the importance of sample purity.

Findings

Critical temperature decreases linearly with inverse film thickness.

Bulk-like lanthanum films show 40% higher $T_c$ than literature values.

Surface contamination affects surface state and superconducting properties.

Abstract

The thickness dependence of the superconducting energy gap $\Delta_{\rm{La}}$ of double hexagonally close packed (dhcp) lanthanum islands grown on W(110) is studied by scanning tunneling spectroscopy, from the bulk to the thin film limit. Superconductivity is suppressed by the boundary conditions for the superconducting wavefunction at the surface and W/La interface, leading to a linear decrease of the critical temperature $T_c$ as a function of the inverse film thickness. For thick, bulk-like films, $\Delta_{\rm{La}}$ and $T_c$ are 40% larger as compared to literature values of dhcp La measured by other techniques. This finding is reconciled by examining the effects of surface contamination as probed by modifications of the surface state, suggesting that the large $T_c$ originates in the superior purity of the samples investigated here.