CEQUINOR   05415
Unidad Ejecutora - UE
congresos y reuniones científicas
Invited Talk : Electron Dynamics in Transition Metal Granular Films
Erice, Sicilia, Italia
Congreso; 6th International Conference - Stripes08-bajo texto ver invitacion en PDF; 2008
Institución organizadora:
Organizing Committee-DIPARTIMENTO DI FISICA-Università di Roma “La Sapienza”
Electron Dynamics in Transition Metal Granular Films     Néstor E. Massa   Laboratorio Nacional de Investigación y  Servicios en Espectroscopía Optica- CEQUINOR, Universidad Nacional de La Plata, C.C. 962 , 1900 La Plata, Argentina.     Spatial heterogeneity is one the properties of highly correlated oxides in which the ions ensemble in aggregates with  dissimilar resistivities interact among them and by which the phase  mixture and the intrinsic disorder originate novel properties.  Here 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(SO2)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(SO2)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. Reminiscent of highly correlated systems, where, as currently reported in the literature, there is coexistence of competing states in an inhomogeneous metal-non metal nanometric scale, we believe that that Mie-like resonance might be a useful tool for tracking metal-insulator phase transitions in inhomogeneous materials.