Mott transition in granular aluminum

TL;DR

This paper demonstrates the presence of free spins in granular aluminum films and observes a Mott transition characterized by increased electron effective mass and spin-flip scattering as grains become decoupled, indicating a metal-insulator transition.

Contribution

It provides direct experimental evidence of free spins and characterizes the Mott transition in granular aluminum through multiple measurement techniques.

Findings

Presence of free spins in granular Al films confirmed by μSR.

Observation of a Mott transition with increased electron effective mass.

Magneto-resistance shows Heavy-Fermion like behavior near transition.

Abstract

The presence of free spins in granular Al films is directly demonstrated by $\mu$SR measurements. A Mott transition is observed by probing the increase of the spin-flip scattering rate of conduction electrons as the nano-size metallic grains are being progressively decoupled. Analysis of the magneto-resistance 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 $\rho \approx 50,000~\mu\Omega~cm$, at which a metal to insulator transition is known to exist. As this transition is approached the magneto-resistance exhibits a Heavy-Fermion like behavior, consistent with an increased electron effective mass.