Time-Resolved Study of Pseudogap and Superconducting Quasiparticle Dynamics in Ca_0.82La_0.18Fe_{1 – x}Ni_xAs₂

We use femtosecond time-resolved optical reflectivity to study the photoexcited quasiparticle (QP) dynamics in the iron-based 112 type superconducting (SC) samples Ca _0.82 La _0.18 Fe _{1 – x} Ni_xAs ₂, with x = 0 and 0.024. In the parent sample, a fast and a slow relaxation emerge at temperatures below the magnetic-structure (MS) transition T _ms ≈ 50 K and the SC transition T _c ≈ 33 K, respectively. The latter obviously corresponds to an SC QP dynamics, which is further confirmed in the x = 0.024 sample with T _c ≈ 25 K. The former suggests that a partial of photoexcited QP relaxation through a pesudogap (PG) channel, which is absent in the doped x = 0.024 sample without the MS transition. (paper)