The Metal-Insulator Transition in Metal Transition Granular Films

We study with infrared reflectivity the concentration and temperature dependence of the regime change from metallic to insulating in granular films made of transition metals embedded in SO$_{2}$. The TM$_{x}$(SO$_{2}$)$_{1-x}$ (TM=Fe, Ni, Co), (0.25$ \leq $x$ \leq $0.85) systems yield spectra typical of conducting oxides where hopping carriers undergo electron-phonon interactions with localization enhanced by nanoparticles and substrate roughness. The distinct Drude component, extending beyond 1.3 eV in the metallic state, undergoes a dramatic change in intensity due to the progressive reduction of carriers critical paths as the transition temperature is reached in the glassy matrix. At the intermediate conducting state for x$\sim$ 0.55, about the percolation threshold, a well defined reflectivity edge and band, considered fingerprint for small polarons, emerges in addition to the vibrational bands. A very good agreement is found between the measured optical conductivity and current small polaron models. This, in addition to underlying the importance of polarization effects, provides grounds toward a quantitative microscopic description of transport properties. It also adds toward an understanding of a non-magnetic factor in the magnetoresistance and extraordinary Hall coefficient enhancements.