Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of superconducting iron pnictide Ca(Fe_0.944Co_0.056)_2As_2

Abstract

We report temperature evolution of coherently excited acoustic and optical phonon dynamics in superconducting iron pnictide single crystal Ca(Fe_0.944Co_0.056)_2As_2 across the spin density wave transition at T_SDW ~ 85 K and superconducting transition at T_SC ~20 K. Strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range of 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T_SC indicating a coupling of these low energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T ~ 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T_SDW.

6 pages, 4 figures (revised manuscript)

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• Signatures of superconducting and pseudogap phases in ultrafast transient reflectivity of Ca(Fe0.927Co0.073)2As2
• Anomalous temperature dependence of optical and acoustic phonons in Bi${}_2$Se${}_3$ arising from stacking faults
• Nonequilibrium carrier and phonon dynamics in the ferrimagnetic semiconductor Mn${}_3$Si${}_2$Te${}_6$
• Probing superconductivity and pairing symmetry by coherent phonons in multiorbital superconductors
• Detecting isotropic density and nematic fluctuations using ultrafast coherent phonon spectroscopy