Mott transition in granular aluminum

Abstract

A Mott transition in granular Al films is observed by probing the increase of the spin-flip scattering rate of conduction electrons as the nanosize metallic grains are being progressively decoupled. The presence of free spins in granular Al films is directly demonstrated by $\ensuremath{\mu}\mathrm{SR}$ measurements. Analysis of the magnetoresistance in terms of an effective Fermi energy shows that it becomes of the order of the grains electrostatic charging energy at a room temperature resistivity ${\ensuremath{\rho}}_{300\phantom{\rule{4pt}{0ex}}\mathrm{K}}\ensuremath{\approx}50000\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\ensuremath{\Omega}\phantom{\rule{4pt}{0ex}}\mathrm{cm}$, at which a metal to insulator transition is known to exist. As this transition is approached the magnetoresistance exhibits a heavy-fermion-like behavior, consistent with an increased electron effective mass.