Ultrafast carrier relaxation through Auger recombination in the topological insulator Bi1.5Sb0.5Te1.7Se1.3

Ultrafast carrier dynamics have great significance for our understanding of the transport properties of the surface state in topological insulator (TI) materials. We report midinfrared pump-probe measurements on the intrinsic TI material Bi1.5Sb0.5Te1.7Se1.3 and show that the change in photoinduced reflectivity can be decomposed into a fast negative part and a slow positive part. Calculations of the dielectric function made at various carrier temperatures and densities reveal that the fast negative component corresponds to the disappearance of the phase-space filling effect due to hot carriers around the probe energy and the decay component corresponds to the recombination of carriers near the band edge. The ratio of the fast negative component to the slow positive component is larger in the excitations conducted at the higher carrier densities, which suggests that the carrier temperature increases through Auger recombination. A qualitative analysis using rate equations reinforces this assumption, so we conclude that Auger recombination is the main cause of the population relaxation at carrier densities higher than 1018 cm(-3) and that we determined the Auger coefficient for Bi1.5Sb0.5Te1.7Se1.3 as C = 0.4 x 10(-26) cm(6)/s.