Tuning the superconducting dome in granular aluminum thin films

TL;DR

This study demonstrates that lowering substrate temperature during deposition enhances the critical temperature of granular aluminum films, revealing a tunable superconducting dome with potential for high kinetic inductance applications.

Contribution

It shows how substrate cooling increases Tc in granular aluminum, providing insights into tuning superconductivity in nanostructured films.

Findings

Maximum Tc reaches 3.27 K at cryogenic deposition temperatures

Superconducting transitions remain sharp at high resistivity levels

Thickness dependence persists even in films thicker than grains

Abstract

Granular aluminum, which consists of nanometer-sized aluminum grains separated by aluminum oxide, is a peculiar superconductor. Its phase diagram as function of normal-state resistivity features a superconducting dome with a maximum critical temperature Tc well above the Tc = 1.2 K of pure aluminum. Here we show how the maximum Tc of this superconducting dome grows if the substrate temperature during deposition is lowered from 300 K to cooling with liquid nitrogen (150 K and 100 K) and liquid helium (25 K). The highest Tc we observe is 3.27 K. These results highlight that granular aluminum is a model system for complex phase diagrams of superconductors and demonstrate its potential in the context of high kinetic inductance applications. This is augmented by our observation of comparably sharp superconducting transitions of high-resistivity samples grown at cryogenic temperatures and by a thickness dependence even for films substantially thicker than the grain size.