Ultrafast transient response and electron-phonon coupling in the iron-pnictide superconductor $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$

The transient response of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, $x=0.08$ was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the presence of two distinct decay channels: a faster one, of less than 1 ps in the considered pump fluence range and a slower one, corresponding to lattice thermalization and lasting $\cong 6\text{ps}$. Our analysis indicates that the fast relaxation should be attributed to preferential scattering of the electrons with only a subset of the lattice-vibration modes with a second moment of the Eliashberg function $\lambda ⟨{\omega }^2⟩\cong 64{\text{meV}}^2$. The simultaneous excitation of a strong fully symmetric $A_{1g}$ optical phonon corroborates this conclusion and makes it possible to deduce the value of $\lambda \cong 0.12$. This small value for the electron-phonon coupling confirms that a phonon-mediated process cannot be the only mechanism leading to the formation of superconducting pairs in this family of pnictides.

Figure 1

(Color online) Time-resolved reflectivity curves on $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$: (a) at $T_i=10\text{K}$ (SC phase) for different pump fluences (an offset has been applied for clarity); (b) for $F=1.3\text{mJ}/{\text{cm}}^2$ as a function of $T_i$, across $T_c$. All the curves have been normalized to show the same peak value after the excitation pulse.

Figure 2

(Color online) (a) Fitting by a biexponential decay function and two components of the fit, for $T_i=10\text{K}$ and $F=1.3\text{mJ}/{\text{cm}}^2$; (b) Relaxation times ${\tau }_1$ and ${\tau }_2$ as a function of the maximum electronic temperatures.

Figure 3

(Color online) Three temperatures model simulations of the transient reflectivity curves. (a) $T_i=25\text{K}$, $F=1.3\text{mJ}/{\text{cm}}^2$; (b) $T_i=10\text{K}$, $F=1.3\text{mJ}/{\text{cm}}^2$; (c) $T_i=10\text{K}$, $F=2\text{mJ}/{\text{cm}}^2$; and (d) $T_i=10\text{K}$, $F=3.2\text{mJ}/{\text{cm}}^2$. The difference between experimental curve and simulation are shown in black.