Amplitude Fluctuations Driven by the Density of Electron Pairs within Nanosize Granular Structuters inside Strongly Disordered Superconductors: Evidence for a Shell-Like Effect

Motivated by the recent observation of the shell effect in a nanoscale pure superconductor by Bose {\em et al} [Nat. Mat. {\bf 9}, 550 (2010)], we explore the possible shell-like effect in a strongly disordered superconductor as it is known to produce nanosize superconducting puddles (SPs). We find a remarkable change in the texture of the pairing amplitudes that is responsible for forming the SP, upon monotonic tuning of the average electron density, $\langle n \rangle$, and keeping the disorder landscape unaltered. Both the spatially averaged pairing amplitude and the quasiparticle excitation gap oscillate with $\langle n \rangle$. This oscillation is due to a rapid change in the low-lying quasiparticle energy spectra and thereby a change in the shapes and positions of the SPs. We establish a correlation between the formation of SPs and the shell-like effect. The experimental consequences of our theory are also discussed.

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