1208.3742 (Sunil Kumar et al.)

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

Sunil Kumar, L. Harnagea, S. Wurmehl, B. Buchner, A. K. Sood

Using femtosecond time-resolved differential reflectivity measurements we investigate temperature and fluence-dependence of quasiparticle kinetics in iron pnictide CaFe2As2 single crystals exhibiting a spin density wave (SDW) and concurrent structural phase transition at T_SDW ~ 165 K. The fluence-dependence clearly shows that saturation effects dominate the behavior of the transient reflectivity signal at pump-fluences higher than ~110 \mu J/cm2. Using pump-fluence well in the linear regime, we have analyzed the temperature-dependence of three-component carrier relaxation dynamics where the amplitudes and time-constants of the individual relaxation components show significant changes in the vicinity of T_SDW. In the SDW phase, using phonon-bottleneck model, temperature-dependence of the fastest electronic relaxation process yields the zero-temperature charge gap to be 2\Delta(0)/k_BT_SDW ~ 1.6\pm0.2 whereas, in the normal state, an electron-phonon coupling constant of ~0.2 has been estimated from the linear temperature-dependence of the time-constant. Very low-frequency coherent oscillations with frequency in the gigahertz range are also observed which are attributed to coherent excitation of an acoustic phonon mode in the crystal. These measurements of acoustic phonons give the temperature-dependence of elastic modulus on either side of the phase transition temperature T_SDW.

View original: http://arxiv.org/abs/1208.3742