arXiv:1001.1047v2 [cond-mat.supr-con] 8 Jan 2010 Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO$_{1}$$_{−}$ x F x single crystals T. Mertelj $^{1}$, P. Kusar$^{1}$, V.V. Kabanov$^{1}$, L. Stojchevska$^{1}$, N.D. Zhigadlo$^{2}$, S. Katrych $^{2}$, Z. Bukowski$^{2}$, J. Karpinski$^{2}$, S. Weyeneth3 and D. Mihailovic 1 $^{1}$Complex Matter Dept., Jozef Stefan Institute, Jamova 39, Ljubljana, SI-1000, Ljubljana, Slovenia $^{2}$Laboratory for Solid State Physics, ETH Zürich, 8093 Zürich, Switzerland and $^{3}$Physik-Institut der Universität Zürich, 8057 Zürich, Switzerland (Dated: November 20, 2018) We investigate the quasiparticle relaxation and low-energy electronic structure in undoped Sm- FeAsO and near-optimally doped SmFeAsO 0 . 8 F 0 . 2 single crystals - exhibiting spin-density wave (SDW) ordering and superconductivity respectively - using pump-probe femtosecond spectroscopy. In the undoped single crystals a single relaxation process is observed, showing a remarkable critical slowing down of the QP relaxation dynamics at the SDW transition temperature T SDW ≃ 125 K. In the superconducting (SC) crystals multiple relaxation processes are present, with distinct SC state quasiparticle recombination dynamics exhibiting a BCS-like T -dependent superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K indicating the existence of a pseudogap of magnitude 2Δ$_{PG}$ ≃ 120 meV above T c . From the pump-photon energy dependence we conclude that the SC state and PG relaxation channels are independent, implying the presence of two sepa- rate electronic subsystems. We discuss the data in terms of spatial inhomogeneity and multi-band scenarios, finding that the latter is more consistent with the present data. tronic structure, investigating possible multi-component response as a sign of phase separation and to obtain de- tailed information about the quasiparticle (QP) dynam- ics in the normal, SDW and superconducting states. The discovery of high-temperature superconductivity in iron-based pnictides (IP) 1–3 has attracted a great deal of attention recently. Contrary to the cuprate supercon- ductors, where a single band with a high degree of corre- lations is believed to be sufficient starting point for the description of the electronic properties, there is a clear theoretical 4 and experimental 5 evidence that in IP sev- eral bands cross the Fermi energy ( ϵ$_{F}$ ). $^{58}$The implica- tions of the presence of several bands at ϵ$_{F}$ in IP are still under intense investigation. In the undoped state two SDW gaps were detected by optical spectroscopy 6 in 122 compounds (AFe$_{2}$As$_{2}$ A=Ba,Sr) presumably originating from different bands crossing ϵ$_{F}$ . In LaFeAsO (La-1111) a similar optical conductivity suppression was observed $^{7}$, but no analysis in terms of SDW gaps was performed. In the superconducting state multiple superconducting gaps were detected 8–11 corresponding to different bands crossing ϵ$_{F}$ . In addition to superconducting gaps also the presence of a pseudogap was reported by NMR 12,13 and point-contact Andreev spectroscopy 14 in La-1111. Time resolved spectroscopy has been very instrumen- tal in elucidating the nature of the electronic excitations in superconductors, particularly cuprates, by virtue of the fact that different components in the low-energy ex- citation spectrum could be distinguished by their differ- ent lifetimes $^{15–29}$. Moreover, the relaxation kinetics can give us valuable information on the electronic density of states 16 and electron-phonon coupling $^{30}$. Extensive and systematic experiments on cuprates have also given infor- mation on the behavior of the pseudogap for charge exci- tations, complementing the information obtained on spin excitations from NMR and other spectroscopies $^{17,19,25}$. In this work we present a time-resolved femtosec- ond spectroscopy study of undoped and near-optimally doped SmFeAsO$_{1}$ − x F x single crystals with x = 0 and x ≃ 0 . 2 with the aim of elucidating the low energy elec- In Fig. 1 we plot temperature dependence of Δ R/R transients in undoped SmFeAsO. The only discernible difference of the response at different pump-photon ener- gies is the presence of a coherent phonon oscillation with the frequency 5.1 THz (170 cm − $^{1}$) at 295K, at ℏ ω$_{P}$ = 1 . 55 The crystals were flux grown at high pressure at ETH in Zurich 32 and were approximately ∼ 120 x ∼ 80 µ m in size. For optical measurements the crystals were glued on a sapphire window mounted in an optical liquid-He flow cryostat. Optical experiments were performed using the stan- dard pump-probe technique, with 50 fs optical pulses from a 250-kHz Ti:Al$_{2}$O$_{3}$ regenerative amplifier seeded with an Ti:Al$_{2}$O$_{3}$ oscillator. We used the pump pho- tons with either doubled ( ℏ ω$_{P}$ = 3 . 1 eV) or fundamen- tal ( ℏ ω$_{P}$ = 1 . 55 eV) photon energy and the probe pho- tons with 1.55 eV photon energy. The pump and probe polarizations were perpendicular to each other and ori- ented with respect to the the crystals to obtain the max- imum amplitude of the response at low temperatures. The pump and probe beam diameters were determined by measuring the transmittance of calibrated pinholes mounted at the sample place $^{31}$. I. EXPERIMENTAL A. Undoped, spin-density-wave ordered SmFeAsO