Doping dependence of femtosecond quasiparticle relaxation dynamics in Ba(Fe,Co)2As2single crystals: Evidence for normal-state nematic fluctuations

Abstract

We systematically investigate the photoexcited (PE) quasiparticle (QP) relaxation and low-energy electronic structure in electron doped Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ single crystals as a function of Co doping, $0\ensuremath{\le}x\ensuremath{\le}0.11$. The evolution of the photoinduced reflectivity transients with $x$ proceeds with no abrupt changes. In the orthorhombic spin-density-wave (SDW) state, a bottleneck associated with a partial charge-gap opening is detected, similar to previous results in different SDW iron pnictides. The relative charge gap magnitude $2\ensuremath{\Delta}(0)/{k}_{\mathrm{B}}{T}_{\mathrm{s}}$ decreases with increasing $x$. In the superconducting (SC) state, an additional relaxational component appears due to a partial (or complete) destruction of the SC state proceeding on a sub-0.5-picosecond timescale. From the SC component saturation behavior the optical SC-state destruction energy, ${U}_{\mathrm{p}}/{k}_{\mathrm{B}}=0.3$ K/Fe, is determined near the optimal doping. The subsequent relatively slow recovery of the SC state indicates clean SC gaps. The $T$ dependence of the transient reflectivity amplitude in the normal state is consistent with the presence of a pseudogap in the QP density of states. The polarization anisotropy of the transients suggests that the pseudogap-like behavior might be associated with a broken fourfold rotational symmetry resulting from nematic electronic fluctuations persisting up to $T\ensuremath{\simeq}200$ K at any $x$. The second moment of the Eliashberg function, obtained from the relaxation rate in the metallic state at higher temperatures, indicates a moderate electron phonon coupling, $\ensuremath{\lambda}\ensuremath{\lesssim}0.3$, that decreases with increasing doping.