Electron dynamics in films made of transition metal nanograins embedded in SiO2: Infrared reflectivity and nanoplasma infrared resonance

Abstract

We report on near normal infrared reflectivity spectra of ~550 nm thick films made of cosputtered transition metal nanograins and SiO 2 in a wide range of metal fractions. Co 0.85 (SiO 2 ) 0.15 ,with conductivity well above the percolation threshold has a frequency and temperature behavior according to what it is find in 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. Ni 0.61 (SiO 2 ) 0.39 , with a metal fraction closer to the percolation threshold, undergoes a metal-non metal transition at ~77 K.Here, as it is suggested by the scattering rate nearly quadratic dependence, we identify two relaxation times (two carrier contributions) associated to a Drude mode and a mid-infrared overdamped band, respectively. Disorder induced, the mid-infrared contribution drives the phase transition by thermal electron localization. Co 0.51 (SiO 2 ) 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 Co 0.38 (SiO 2 ) 0.62 , Fe 0.34 (SiO 2 ) 0.66, and Ni 0.28 (SiO 2 ) 0.72 , reveals a remarkable resonance at that band threshold. We understand this as due to the excitation by normal to the film electric fields of defect localized electrons in the metallic nanoparticles. At higher oblique angles, this localized nanoplasma couples to SiO 2 longitudinal optical Berreman phonons resulting a band peak softening reminiscent to the phonon behavior undergoing strong electron-phonon interactions. Singular to a globally insulating phase, we believe that this resonance might be a useful tool for tracking metal-insulator phase transitions in inhomogeneous materials.

High temperature far-infrared dynamics of orthorhombic NdMnO3: emissivity and reflectivity

“…(pseudocubic Pmcm Z=1) [12] Overall, we find that our 1-emissivity spectra show remarkable similarities with the known behavior bulk disordered conducting oxides undergoing a metal to insulating phase transition (Fig. 7 in [21]). It suggests that a quantitative analysis of the high temperature mid-infrared real part of the optical conductivity, eq.…”

“…Overall, we find that our 1-emissivity spectra show remarkable similarities with the known behavior of bulk disordered conducting oxides undergoing a metal to insulating phase transition (figure 7 in [21]). This suggests that a quantitative analysis of the high temperature mid-infrared real part of the optical conductivity, equation (7), within a small polaron context, may help to identify phonon groups involved in main carrier-phonon interactions.…”

h−ErMnO3 absorbance, reflectivity, and emissivity in the terahertz to mid-infrared from 2 to 1700 K: Carrier screening, Fröhlich resonance, small polarons, and bipolarons

“…81 To reproduce the full mid-infrared spectral region, we then allowed the possibility of more than one vibrational contribution using an empirical approach shown successful when applied to other distorted perovskites and glassy systems. 82,83,84 This results in the convoluted addition of bell shaped uncorrelated individual contributions in the region from 1000 cm -1 to 9000 cm -1 , each calculated at a phonon frequency j associated to the strength j at a temperature T.…”

“…26 A bipolaron is made of two small polarons in association as two carrier self-trapped localized bound pair, in which the common potential exceeds their Coulomb repulsion. 84 In this regime the confinement energy of small bipolarons is double relative to that in small polarons because the presence of the second electron, i.e., the depth of the well that self-traps both carriers is twice as deep. Their effective mass is expected to be also much larger than the hundred-fold increments of the electron mass expected for small polarons.…”

Spectral density analysis of the optical properties of Ni-Al 2 O 3 nano-composite films

“…The influence of tunnelling charge transfer phenomena on infrared and/or optical absorption has to our knowledge, not been studied. Recently it has been suggested that the infrared properties of nanocomposites should be described using concepts used for transition metal oxides 43 , implying that a purely electrostatic theory may be insufficient. It is conceivable that such processes could "steal" resonance strength from the SDF, thus leading to a lower computed than experimental volume fraction of a nanocomposite.…”