Observation of a coherent optical phonon in the iron pnictide superconductor $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ ($x=0.06$ and 0.08)

We report the observation of a coherent lattice oscillation in a pnictide superconductor. A coherent fully symmetric optical phonon was detected in $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ ($x=0.06$ and $x=0.08$) using time-resolved pump-probe reflectivity with 40 fs time resolution. The analysis of the phonon parameters for various excitation fluences reveals no evident difference below and above the critical temperature, suggesting that the $A_{1g}$ mode is not involved in the superconducting phase transition.

Figure 1

Time-resolved reflectivity curve on $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, $x=0.08$ at 10 K.

Figure 2

(Color online) Coherent optical lattice oscillation on $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, $x=0.08$ in the superconducting and metallic phases at 10 and 25 K, respectively. The curve at 10 K is vertically offset for clarity. The solid line is the oscillatory component of the fitting function.

Figure 3

(Color online) Phonon (a) damping time and (b) amplitude as a function of the pump fluence in $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ $x=0.08$ for the superconducting and the metallic phases. The fitting line going through zero is shown on the amplitude graph. (c) Phonon amplitude versus temperature in $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, $x=0.06$, for a pump fluence of $4.5\text{mJ}/{\text{cm}}^2$.