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We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phononsin undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-densitywave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165K using femtosecond time-resolved pump-probe spectroscopy. The contributions intransient differential reflectivity arising from exponentially decaying QPrelaxation and oscillatory coherent acoustic phonon mode show large variationsin the vicinity of T_SDW. From the temperature-dependence of the QPrecombination dynamics in the SDW phase, we evaluate a BCS-like temperaturedependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW,whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 hasbeen estimated from the linear temperature-dependence of the QP relaxationtime. The long-wavelength coherent acoustic phonons with typical time-period of~100 ps have been analyzed in the light of propagating strain pulse modelproviding important results for the optical constants, sounds velocity and theelastic modulus of the crystal in the whole temperature range of 3 K to 300 K.

Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition