Crossover from Dispersive to Diffusive Energy Transport
Excitation energy transport in disordered solids is one of the relaxation phenomena studied intensively during the past ten years. Especially transport in glasses has attracted considerable interest1, but also some porous systems2, polymers3 and disordered crystals4 have been investigated. Disordered crystals offer the possibility to model complex relaxation behavior in a distinct and well-defined manner. For this reason we have chosen a chemically mixed crystal (CMC) of p-dichloro-(DCB) in p-dibromobenzene (DBB) as a representative example of a substitutionally disordered crystal. In recent publications we have shown that triplet excitation energy transport exhibits dispersive character5 as a function of doping concentration. Instead of using isotopically mixed crystals6 CMC show due to the formation of induced energy funnels7, 8 a richer energy landscape which gives rise to a broad waiting time distribution for jumps of excitation energy described by random walk processes9. Phosphorescence measurements indicated10 that at temperatures above about 6 K transport becomes strongly temperature dependent. This gave rise to a comparison of transport in disordered crystals with relaxation processes in spin glasses11. Within this concept energy transport exhibits as a function of temperature a kind of glass transition which is manifested in a cross-over from ergodic to non-ergodic behavior11. Above the glass transition, the phosphorescence decay of the excitation energy can be described by a stretched exponential Kohlrausch-Williams-Watts (KWW) behavior. In this communication we like to report on the temperature dependence of the parameters β and τ, which enter the KWW description via the phosphorescence intensity IPhos, ~exp[-(t/τ)β].
KeywordsRandom Walk Spin Glasses11 Energy Transport Energy Separation Phosphorescence Intensity
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