Regensburg 2007 – scientific programme

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DF: Fachverband Dielektrische Festkörper

DF 9: Electric, Electromechanical and Optical Properties I

DF 9.3: Talk

Thursday, March 29, 2007, 11:00–11:20, H48

Polaronic charge transport in reduced LiNbO₃ investigated by means of time-resolved ESA spectroscopy — •Christoph Merschjann, Bettina Schoke, Daniela Conradi, Mirco Imlau, and Manfred Wöhlecke — Department of Physics, University of Osnabrück, D-49069 Osnabrück

Charge-transport processes are investigated in differently reduced LiNbO₃ single crystals by means of time-resolved excited-state-absorption (ESA) spectroscopy. The detected transient light-induced absorption (α_li) upon intense pulsed laser illumination (λ = 532 nm, τ = 8 ns) originates from four distinct types of metastable small polarons. From the temporal and spectral characteristics of α_li one is able to distinguish between bound Nb_Li⁴⁺ and free Nb_Nb⁴⁺ polarons, Nb_Li⁴⁺:Nb_Nb⁴⁺ bipolarons, and bound O⁻ hole polarons. Among other results we find competing photochromic effects in the blue spectral range in reduced LiNbO₃. Furthermore, non-exponential relaxations of α_li are observed, whose time constants strongly depend on the state of reduction and purity of the samples. The excitation and recombination phenomena of photo-induced small polarons can be described by a charge transport model, based on a random-walk of excited charge carriers. The presented experimental method is an ideal complement to EPR and electrical measurements, yielding time-resolved spectra at room temperature.

Supported by the Deutsche Forschungsgemeinschaft (projects IM 37/2-2 and TFB 13-04).