Invited talk: Localized Nanoplasma Far Infrared Resonance in Transition Metal Granular Films

NESTOR E. MASSA

Kottayam, Kerala, India

Congreso; First International Conference on Nanostructred Materials and Nanocomposites (ICNM – 2009); 2009

comite organizador ad-hoc

We report on near normal and angle dependent specular infrared reflectivity spectra of ~550 nm thick Co and SiO2 cosputtered Cox(SiO2)1-x nanogranular films in the insulating regime. The reflectivity is characterized by well defined vibrational bands, an overdamped Drude contribution, due to carriers denoting the existence of conductivity critical paths not yet truncated, and a distinctive band at ~1450cm−1 originating in electron promotion, localization, and polaron formation. P-polarized angle dependent oblique reflectivity of globally insulating Co0.45(SiO2)0.55, reveals that in addition to those expected features there is a remarkable resonance at the threshold of the ~1450cm−1 band. Pointed out by Berreman,[1] the transverse magnetic (TM) P-polarized radiation has a wave vector with components parallel and perpendicular to the film surface allowing extra bands not seen in transverse electric S-polarization. In our case, the resonant band has a corresponding S-polarized sharp antiresonance, confirming its purely longitudinal character. That resonant peak has maximum intensity when the incident field only has a normal component. I.e., the radiation tangential component reaches the angle dependent null condition allowing for a collective electronic excitation, induced as a localized plasma, with origin in the metallic nanostructure. Being at or slighted lower energy of the edge denoting the electron promotion at higher energy levels by energetic enough photons, the origin of the electronic cloud is traced to carriers that are not able to overcome the metal-dielectric rough interface. We found that as the angle of incidence is increased the longitudinal highest frequency vibrational band merges with the sharp P-polarized resonance inducing broadening and softening of the net band profile. This is reminiscent to bulk oxides with  lattice modes undergoing the effects of strong electron-phonon interactions. Overall, it is important to emphasize that the three main features commented here for specific metal fractions, nominally, hopping conductivity-polaron formation-; mid gap band- shown as an overdamped oscillator, and the resonant condition are always present in our films. Its detection depends on the fractional amount of transitional metal nanoparticles, and thus, on the number of carriers. The evolution of transport properties relies on the behavior of insulating and metallic nanoaggregates as in highly correlated systems with coexistence of competing states in an inhomogeneous nanometric scale. The key for resonance unambiguous detection is the reduced number of free carriers making it almost singular to a globally insulating phase and thus a possible useful tool for tracking metal-insulator phase transitions in inhomogeneous materials.