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

Abstract

Small polarons are excited by pulsed illumination (lambda =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 lambda =632.8 nm and at lambda =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 E-A=(0.58 /-0.02) eV and a frequency factor of Z=(5 /-3)x10(13) s(-1). Delayed double-pulse technique (lambda =532 nm pulse followed by a lambda =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 lambda =1064 nm with energy transfer via an unknown X center. Excitation and recombination show a single-exponential temporal evolution without any significant temperature dependence. (C) 2001 American Institute of Physics.