Superconducting gap in ${\mathrm{BaFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ from temperature-dependent transient optical reflectivity

Temperature and fluence dependence of the 1.55-eV optical transient reflectivity in ${\mathrm{BaFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ was measured and analyzed in the low and high excitation density limit. The effective magnitude of the superconducting gap of $\sim 5$ meV obtained from the low-fluence-data bottleneck model fit is consistent with the angle-resolved photoemission spectroscopy results for the $\gamma$- and $\beta -\mathrm{hole}$ Fermi surfaces. The superconducting state nonthermal optical destruction energy was determined from the fluence dependent data. The planar optical destruction energy density scales well with $T_{\mathrm{c}}^2$ and is found to be similar in a number of different layered superconductors.

Figure 1

Photoinduced reflectivity transients $\mathrm{\Delta }R/R$ in ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}$)${}_2$ measured in the superconducting state (a) and normal state (b) as a function of the pump fluence.

Figure 2

Temperature dependence of the transient reflectivity $\mathrm{\Delta }R/R$ at low (a) and high (b) fluence. Note that the data above the critical temperature collapse on a single curve in a wide temperature range at both fluencies. The superconducting state response is obtained by subtracting the average of the transients from the 34–50-K interval at low (c) and high (d) fluence.

Figure 3

(a) The fluence dependence of the transient amplitude at $T=7$ K. The red line is the saturation model [9] fit discussed in text. (b) The dominant relaxation time in the superconducting state. The data from Co doped [8] Ba-122 are shown for comparison. The continuous red line is the Rothwarf-Taylor fit [6] while the dashed lines indicate the high-$\mathcal{F}$ trend. (c) The 7-K transients with the 32-K normal-state response subtracted. (d) Similar data as in (c) in Co doped Ba-122 from [8] shown for comparison.

Figure 4

The planar [18] SC state optical destruction energy density as a function of $T_{\mathrm{c}}^2$ for some iron based pnictides compared to NbN [20], ${\mathrm{MgB}}_2$ [21], and (La,${\mathrm{Sr})}_2{\mathrm{CuO}}_4$ [9]. The inset shows $U_{\mathrm{d}}$ in pnictides normalized to the Fe content.

Figure 5

The temperature dependence of the SC response amplitude for low (a) and high (b) excitation density. The lines are fits discussed in text, where the shaded region in (a) was excluded from the bottleneck model fits. (c) The low excitation SC response relaxation time as a function of temperature. (d) Temperature dependence of the normal-state transient reflectivity amplitude. The dashed line is a bottleneck fit with $T$-independent gap. Transients at a few characteristic temperatures are shown in the inset.