CONICET | Buscador de Institutos y Recursos Humanos

We report on near normal infrared reflectivity spectra of ~550 nm thick transition metal and SiO2 cosputtered granular films having a wide range of metal fractions. Co0.85(SiO2)0.15 with conductivity well above the percolation threshold has a frequency and temperature behavior according to conducting metal oxides. The electron scattering rate displays an unique relaxation time characteristic of single type of carriers experiencing strong electron-phonon interactions. Using small polaron fits we individualize those phonons as glass vibrational modes. A film as Ni0.61(SiO2)0.39, with a metal fraction closer to the percolation threshold, undergoes a metal-non metal transition at ~77 K . As it is suggested by the scattering rate quadratic dependence we identify two carrier contributions associated to a Drude mode and a mid-infrared overdamped band. Disorder induced, the mid-infrared contribution drives the phase transition by thermal electron localization. Co0.51(SiO2)0.49 has the reflectivity of an insulator with a distinctive band at ~1450cm−1 originating in electron promotion, localization, and defect induced polaron formation. Angle dependent oblique reflectivity of globally insulating Co0.38(SiO2)0.62, Fe0.34(SiO2)0.66, and Ni0.28(SiO2)0.72, reveals a remarkable resonance at that band threshold due to the excitation by normal electric fields of electrons in metallic nanoparticles. At higher oblique angles, this localized plasma couples to SiO2 longitudinal optical Berreman phonons. Singular to a globally insulating phase, we believe that this Mie-like resonance might be a useful tool for tracking metal-insulator phase transitions in inhomogeneous materials.