Gap-Dependent Quasiparticle Dynamics and Coherent Acoustic Phonons in CaFe 2 As 2 across Spin Density Wave Phase Transition Sunil KUMAR 1 , Luminita HARNAGEA 2 , Sabine WURMEHL 2 , Bernd BUCHNER 2 , and A. K. SOOD 1  1 Department of Physics, Indian Institute of Science, Bangalore 560012, India 2 Leibniz-Institute for Solid State and Materials Research, D-01171 Dresden, Germany (Received January 23, 2013; accepted February 21, 2013; published online March 25, 2013) We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe 2 As 2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature T SDW  165 K using femtosecond time-resolved pump–probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T SDW . From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ð1:6  0:2Þk B T SDW , whereas, much above T SDW , an electron–phonon coupling constant of 0:13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of 100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 to 300 K. KEYWORDS: ultrafast spectroscopy, iron pnictide, spin density wave, quasiparticle dynamics, acoustic phonons 1. Introduction Since the recent discovery of high temperature super- conductivity in iron pnictides 1) a lot of interest has been generated, both theoretically and experimentally, to under- stand the magnitude, nature and symmetry of energy gaps in the spin-density wave (SDW) and superconducting (SC) states of these compounds. 2–6) Parent compounds of the iron pnictide family show a spin-ordered phase below the SDW phase transition temperature T SDW and superconductivity evolves either by electron or hole-doping at a lower temperature. Therefore, the parent compounds are being studied using various experimental methods to understand the role of spin fluctuations, spin–phonon and electron– phonon interactions. The picture on the nature and presence or absence of charge gaps opening at or below the magneto- structural transition in the parent compound so far remains inconclusive. On one side, the temperature-dependent resistivity indicates metallic behavior in the SDW phase. 7,8) Similarly, strong orbital-dependent reconstruction of the Fermi surface across the magneto-structural transition and metallic SDW state 9) or absence of charge gap 10,11) was derived from angle resolved photoemission spectroscopy (ARPES). On the other hand, in many infrared absorption 3,4) and ARPES 6) studies two charge gaps with 2 0 =k B T SDW ranging between 3.5 to 11 have been reported. In the last couple of years, a few ultrafast time-resolved spectroscopic studies have been reported on the nature of charge gaps in the parent SDW iron pnictides: BaFe 2 As 2 , 12) SmFeAsO 13) and SrFe 2 As 2 . 14) The transient reflectivity measurements have shown a bi-exponential dynamics where a fast component could be described by quasiparticle recombination dynamics across a charge gap within the phonon bottleneck model giving band gap value of 2 0 =k B T SDW  5 and 7.2. 13,14) Our present study addresses the temperature and fluence- dependent QP dynamics and coherent acoustic phonons in parent CaFe 2 As 2 single crystals investigated by measuring transient differential reflectivity using femtosecond pump– probe spectroscopy which has not been carried out hitherto. The QP dynamics involves three distinct exponentially decaying relaxation channels with decay times varying from sub-picosecond to hundreds of picoseconds where drastic changes occur in the amplitudes and corresponding decay-times at T SDW . Such a behavior is clearly an indi- cation of charge gap opening in the spin density wave phase. By analyzing the temperature-dependence of the fast relaxation component using phonon bottleneck model 15,16) we derive the zero-temperature gap value of 2 0 ð1:6  0:2Þk B T SDW . In the high temperature normal metallic phase of the sample, linear temperature-dependence of the QP relaxation time has been used to estimate the coupling strength of electrons with the optical phonons to be 0:13. Further, our experimental results clearly show detection of very low-frequency (GHz) coherent oscillations superimposed on slowly exponentially decaying back- ground. These are attributed to coherent excitation of an acoustic phonon mode launched by laser induced electronic and/or thermal stress at the sample surface. Analysis of the observed mode using strain pulse propagation model in the ultrafast ultrasonics 17) yields temperature dependence of the optical constants, sound velocity and hence the elastic modulus across the SDW phase transition. Moreover, from the phonon-amplitude peaking at a temperature higher than T SDW , we infer strong magneto-elastic coupling between spin-fluctuations in the normal paramagnetic phase and the lattice, a behavior similar to that observed in multiferroic manganites. 18,19) 2. Experimental Details Single crystals of CaFe 2 As 2 were grown by high temperature solution growth technique using Sn flux and characterized as reported earliar. 8) Spin density wave phase transition along with a concurrent structural (S) transition from high symmetry tetragonal phase to orthorhombic symmetry at low temperatures were established to occur at T SDW ðT S Þ 165 K. The crystals with c-axis perpendicular Journal of the Physical Society of Japan 82 (2013) 044715 044715-1 FULL PAPERS #2013 The Physical Society of Japan http://dx.doi.org/10.7566/JPSJ.82.044715