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

The development of the fundamental superconducting (SC) energy scales—the SC energy gap Δ 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, Δ grows as the grains are progressively decoupled, causing the unconventional increase of T_c with sample resistivity. When the grain coupling is suppressed further, Δ 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.