Enhanced Cooper pairing versus suppressed phase coherence shaping the superconducting dome in coupled aluminum nanograins

Abstract

The development of the fundamental superconducting (SC) energy scales---the SC energy gap $\mathrm{\ensuremath{\Delta}}$ and the superfluid stiffness $J$---of granular aluminum, i.e., thin films composed of coupled nanograins, is studied by means of optical THz spectroscopy. Starting from well-coupled grains, $\mathrm{\ensuremath{\Delta}}$ grows as the grains are progressively decoupled, causing the unconventional increase of ${T}_{c}$ with sample resistivity. When the grain coupling is suppressed further, $\mathrm{\ensuremath{\Delta}}$ saturates while the critical temperature ${T}_{c}$ decreases, concomitantly with a sharp decline of $J$, delimiting a SC dome in the phase diagram. This crossover to a phase-driven SC transition is accompanied by an optical gap surviving into the normal state above ${T}_{c}$. We demonstrate that granular aluminum is an ideal testbed to understand the interplay between quantum confinement and global SC phase coherence due to nanoinhomogeneity.