content_image/380344.jpg content_image/380334.jpg
of the Mattis-Bardeen formula, 47 

where ℏ ω is the probe-photon energy and Δ ( T ) the su- perconducting gap. By using the BCS-gap temperature dependence with 2Δ$_{0}$ / k$_{B}$T$_{c}$ = 3 . 5 we obtain an excellent fit to the observed temperature dependence. Unfortu- nately the shape of the temperature dependence (5) is a very weak function of 2Δ$_{0}$ / k$_{B}$T$_{c}$ and one can not reliably distinguish the contributions from different gaps 48 and reliably determine 2Δ$_{0}$ / k$_{B}$T$_{c}$ . 

<img src='content_image/380344.jpg'>

## D. Normal state response in SmFeAsO 0 . 8 F 0 . 2 

The temperature dependence of the component-B am- plitude is consistent with a bottleneck due to the relax- ation over a T -independent gap 16 (see Fig. 9) with a magnitude 2Δ$_{PG}$ = 120 meV as noted previously. 41 The sub-ps part of component A on the other hand is tem- perature independent suggesting finite density of states at ϵ$_{F}$ . This is consistent with heat capacity measure- ments in polycrystalline SmFeAsO$_{1}$ − x F $_{x}$45,46 where a fi- nite Sommerfeld constant in the superconducting state suggests a finite density of states at ϵ$_{F}$ . Due to very sim- ilar pump-photon energy dispersion of components A and B we believe that they originate from the same electronic states which have a soft-gapped density of states at ϵ$_{F}$ . Saturation of component B amplitude with increasing F indicates that the pseudogap can be destroyed so it is not a simple band-structure effect. 

The pump-photon energy dispersion similar to that of components A and B is not observed for component C. Electronic states involved in the relaxation related to components A and B must therefore be different than for component C. This confirms that the relaxation below T$_{c}$ does not proceed via a cascade but rather through dis- tinct parallel channels as suggested previously. 41 These 

<img src='content_image/380334.jpg'>

channels correspond to two distinct electronic subsystems which are weakly coupled on the sub-ps timescale. One exhibits the superconducting gap(s) and the other a pseu- dogap. 

A possible origin of the distinct electronic subsystems could be a chemical phase separation of the doped F. However, the superconducting transition is rather narrow (see Fig. 4) so the presence of weakly-superconducting fluorine-poor regions in which SDW is suppressed giv- ing rise to additional pseudo-gapped electronic subsystem is also unlikely. More importantly undoped SmFeAsO shows virtually no pump-photon energy dispersion which the superconducting sample does, so a simple chemical phase separation to doped and undoped regions is very unlikely despite similarity (see Fig. 6) between compo- nent A and the undoped SmFeAsO room-temperature transients. Moreover, there is no divergent signal at 125 K (or anywhere near that temperature) which can be attributed to the presence of the undoped phase in the superconducting sample. We therefore believe that both electronic subsystems are intrinsic to the SC material 

Apart from the chemical phase separation an intrin- sic electronic phase separation akin to that proposed for the cuprates 49 could be origin of the distinct electronic subsystems. The existence of intrinsic electronic phase separation has been reported in 122 systems, 50,51 how- ever at present the issue is still rather controversial. 

In the case of the spatially homogeneous electronic state different electronic subsystems would correspond to different bands crossing ϵ$_{F}$ . This would imply that the inter-band scattering between parts of the Fermi surface corresponding to different electronic subsystems is negli- gible on a timescale of a few hundred fs, since excitation photon at 1.5 eV only weakly excites components A and B. Further, since both components exist in the supercon- ducting state even at low F the part of the Fermi surface corresponding to components A and B has to remain un- gapped or pseudo-gaped in the superconducting state. 

Another possibility for a weakly coupled electronic sub- system are Sm crystal-field levels. The energy of the levels in SmFeAsO$_{1}$ − x F x is in the range of 20-60 meV as determined indirectly from heat capacity fits. 52,53 In- volvement of the crystal-field levels could explain the strong ℏ ω$_{P}$ -dependence of components A and B and de- coupling from the other low lying electronic states. How- ever, the observation of a pseudogap by NMR in La- 1111 12,13 (which has no crystal field level structure at low energy), and the weak ℏ ω$_{P}$ -dependence in undoped SmFeAsO point against such a scenario. 

Finally, let us briefly compare our results in Sm-1111 to femtosecond spectroscopy in (Ba,K)-122. 54,55 There is a marked difference in the magnitude of the relaxation time in the superconducting state, which increases with decreasing temperature beyond 60 ps in optimally doped (Ba,K)-122 54 and remains T -independent in Sm-1111 at ∼ 5 ps. Similarly, the excitation fluence dependence of the relaxation time, which is absent in Sm-1111 is pro- nounced in (Ba,K)-122. 55 This suggests different relax- 

