Gapful electrons in a vortex core in granular superconductors
We calculate the quasiparticle density of states (DoS) inside the vortex core in a granular superconductor, generalizing the classical solution applicable for dirty superconductors. A discrete version of the Usadel equation for a vortex is derived and solved numerically for a broad range of parameters. Electron DoS is found to be gapful when the vortex size $ξ$ becomes comparable to the distance between neighboring grains $l$. Minigap magnitude $E_g$ grows from zero at $ξ\approx 1.4 l$ to third of superconducting gap $Δ_0 $ at $ξ\approx 0.5 l$. The absence of low-energy excitations is the main ingredient needed to understand strong suppression of microwave dissipation recently observed in a mixed state of granular Al.
cond-mat.supr-con↗