Shaping a superconducting dome: Enhanced Cooper-pairing versus suppressed phase coherence in coupled aluminum nanograins

TL;DR

This study investigates how nanostructuring aluminum into coupled nanograins affects superconductivity, revealing a complex interplay between pairing strength and phase coherence that influences the critical temperature.

Contribution

It provides new insights into the mechanisms of enhanced superconductivity in granular aluminum by disentangling the roles of pairing energy and phase stiffness through optical spectroscopy.

Findings

Superconducting gap $$ increases with grain decoupling.

Critical temperature $T_c$ peaks at intermediate coupling.

Optical gap persists above $T_c$, indicating phase fluctuations.

Abstract

Deterministic enhancement of the superconducting (SC) critical temperature $T_c$ is a long-standing goal in material science. One strategy is engineering a material at the nanometer scale such that quantum confinement strengthens the electron pairing, thus increasing the superconducting energy gap $\Delta$, as was observed for individual nanoparticles. A true phase-coherent SC condensate, however, can exist only on larger scales and requires a finite phase stiffness $J$. In the case of coupled aluminium (Al) nanograins, $T_c$ can exceed that of bulk Al by a factor of three, but despite several proposals the relevant mechanism at play is not yet understood. Here we use optical spectroscopy on granular Al to disentangle the evolution of the fundamental SC energy scales, $\Delta$ and $J$, as a function of grain coupling. Starting from well-coupled arrays, $\Delta$ grows with progressive grain decoupling, causing the increasing of $T_c$. As the grain-coupling is further suppressed, $\Delta$ saturates while $T_c$ decreases, concomitantly with a sharp decline of $J$. This crossover to a phase-driven SC transition is accompanied by an optical gap persisting above $T_c$. These findings identify granular Al as an ideal playground to test the basic mechanisms that enhance superconductivity by nano-inhomogeneity.