Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of the superconducting iron pnictide Ca(Fe0.944Co0.056)2As2
1 Department of Physics and Center for Ultrafast Laser Applications, Indian Institute of Science Bangalore 560012, India
2 Leibniz-Institute for Solid State and materials Research - Dresden, D-01171 Dresden, Germany, EU
Received: 19 September 2012
Accepted: 20 November 2012
We report the temperature evolution of coherently excited acoustic and optical phonon dynamics in the superconducting iron pnictide single crystal Ca(Fe0.944Co0.056)2As2 across the spin density wave transition at TSDW ∼ 85 K and the superconducting transition at TSC ∼20 K. The 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 from 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below TSC 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 TSDW.
PACS: 78.47.J- – Ultrafast spectroscopy (<1 psec) / 74.70.Xa – Pnictides and chalcogenides / 74.25.Kc – Phonons
© EPLA, 2012