Enhanced Cooper pairing versus suppressed phase coherence shaping the superconducting dome in coupled aluminum nanograins Article Swipe
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· 2016
· Open Access
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· DOI: https://doi.org/10.1103/physrevb.93.100503
· OA: W2138035602
Deterministic enhancement of the superconducting (SC) critical temperature\n$T_c$ is a long-standing goal in material science. One strategy is engineering\na material at the nanometer scale such that quantum confinement strengthens the\nelectron pairing, thus increasing the superconducting energy gap $\\Delta$, as\nwas observed for individual nanoparticles. A true phase-coherent SC condensate,\nhowever, can exist only on larger scales and requires a finite phase stiffness\n$J$. In the case of coupled aluminium (Al) nanograins, $T_c$ can exceed that of\nbulk Al by a factor of three, but despite several proposals the relevant\nmechanism at play is not yet understood. Here we use optical spectroscopy on\ngranular Al to disentangle the evolution of the fundamental SC energy scales,\n$\\Delta$ and $J$, as a function of grain coupling. Starting from well-coupled\narrays, $\\Delta$ grows with progressive grain decoupling, causing the\nincreasing of $T_c$. As the grain-coupling is further suppressed, $\\Delta$\nsaturates while $T_c$ decreases, concomitantly with a sharp decline of $J$.\nThis crossover to a phase-driven SC transition is accompanied by an optical gap\npersisting above $T_c$. These findings identify granular Al as an ideal\nplayground to test the basic mechanisms that enhance superconductivity by\nnano-inhomogeneity.\n
