Distinct Pseudogap and Quasiparticle Relaxation Dynamics in the Superconducting State of Nearly Optimally Doped ${\mathrm{SmFeAsO}}_{0.8}{\mathbf{F}}_{0.2}$ Single Crystals

We use femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in a nearly optimally doped pnictide superconductor with $T_c=49.5\mathrm{K}$. Multiple relaxation processes are evident, with distinct superconducting state quasiparticle recombination dynamics exhibiting a $T$-dependent superconducting gap, and a clear “pseudogaplike” feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude ${\Delta }_{\mathrm{PG}}=61\pm 9\mathrm{meV}$ above $T_c$. Both the superconducting and pseudogap components show saturation as a function of fluence with distinct saturation fluences 4 and $40\mu \mathrm{J}/{\mathrm{cm}}^2$, respectively.

Figure 1

Temperature dependence of the photoinduced reflectivity $\Delta R/R$ at the pump fluence $3\mu \mathrm{J}/{\mathrm{cm}}^2$. Below $T_c$, the response of the superconducting state is clearly seen. In the inset, the temperature dependence of the magnetization in the superconducting state indicates high quality of the single crystals.

Figure 2

(a) The low-fluence photoinduced reflectivity at different $T$ in the temperature region where superconductivity appears. The thin lines represent scans above $T_c$, while the thick dotted trace shows the high-fluence trace at 4.2 K linearly scaled to the fluence of the other traces. (b) The photoinduced reflectivity response above $T_c$. The thick dotted trace shows the high-fluence response at 200 K linearly scaled to the fluence of the other traces. The inset in (b) shows the dependence of the $T$-independent component $\Delta R_A/R$ as a function of $\mathcal{F}$ at 270 K.

Figure 3

The normalized photoinduced reflectivity (a) at $T=4.2\mathrm{K}$ (the legend is in the same order as traces at 1-ps delay) and (b) at $T=75\mathrm{K}$ for different laser fluences $\mathcal{F}$. Component $A$ was subtracted in both plots. The thin lines are exponential fits. The insets show the amplitude of the SC and PG components as a function of pump laser fluence $\mathcal{F}$.

Figure 4

(a) The photoinduced reflectivity due to the superconducting state response at different $T$ below $T_c$. To obtain the traces, the averaged response above $T_c$ [thin traces in Fig. 2a] was subtracted and the result was smoothed to reduce noise. The thin lines represent a two-exponential fit. (b) The photoinduced reflectivity response above $T_c$ showing the PG response. The trace at 4.2 K shows the raw data with only component $A$ subtracted showing unambiguously the simultaneous presence of the SC and the PG components below $T_c$.

Figure 5

(a) The superconducting state relaxation time ${\tau }_{\mathrm{SC}}$ and amplitude of the photoinduced reflectivity $\Delta R_{\mathrm{SC}}/R_{\mathrm{SC}}$ in the superconducting state. $\Delta R_{\mathrm{SC}}/R_{\mathrm{SC}}$ is shown as a function of $T$ for two laser fluences: 3 (circles) and $15\mu \mathrm{J}/{\mathrm{cm}}^2$ (diamonds). The line is a fit using the expression [Eq. (1)] for the photoinduced reflectivity in the strong excitation limit. (b) The amplitude and relaxation time ${\tau }_{\mathrm{PG}}$ of the “pseudogap” component above $T_c$ for $\mathcal{F}=3\mu \mathrm{J}/{\mathrm{cm}}^2$. The fit to the data uses the model [6] with a $T$-independent gap. The inset shows the possible recombination pathways across ${\Delta }_{\mathrm{SC}}$ and ${\Delta }_{\mathrm{PG}}$. The red arrows indicate the initial very fast relaxation. The PG relaxation may proceed via 2 and then 1 or directly to the ground state 3.