Doping dependence of femtosecond quasi-particle relaxation dynamics in Ba(Fe,Co)_2As_2 single crystals: possible evidence for normal state nematic fluctuations

TL;DR

This study explores how doping affects ultrafast quasiparticle relaxation in Ba(Fe,Co)2As2, revealing evidence of nematic fluctuations in the normal state and detailing the evolution of electronic gaps and electron-phonon interactions.

Contribution

It provides new insights into doping-dependent quasiparticle dynamics and nematic fluctuations in iron-based superconductors using femtosecond spectroscopy.

Findings

Charge gap decreases with doping.

Nematic fluctuations persist up to 200 K.

Electron-phonon coupling is moderate and decreases with doping.

Abstract

We systematically investigate the photoexcited (PE) quasi-particle (QP) relaxation and low-energy electronic structure in electron doped Ba(Fe_{1-x}Co_{x})_{2}As_{2} single crystals as a function of Co doping, 0<= x <=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 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_p/k_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 point symmetry resulting from nematic electronic fluctuations persisting up to T~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, lambda <~0.3, that decreases with increasing doping.