Lifetime of small polarons in strontium-barium-niobate single crystals doped with cerium or chromium

Abstract

Small polarons are excited by pulsed illumination (λ=532 nm) of strontium–barium–niobate single crystals doped with 0.66 mol % cerium or with 0.025 mol % chromium. The dark decay of the polarons is observed by monitoring the light-induced absorption at λ=632.8 nm and at λ=785 nm. The relaxation fulfills a stretched-exponential behavior over at least five decades. Using the Arrhenius law the temperature dependence yields the activation energy EA=(0.58±0.02) eV and a frequency factor of Z=(5±3)×1013 s−1. Delayed double-pulse technique (λ=532 nm pulse followed by a λ=1064 nm pulse) reveals a distance dependent recombination rate of the small polarons, i.e., the lifetime of a created polaron is a function of the distance to the next available deep electron trap. In SBN:Cr the R branch can be excited by pulses of λ=1064 nm with energy transfer via an unknown X center. Excitation and recombination show a single-exponential temporal evolution without any significant temperature dependence.