Figure 1

Fast, intensity-dependent relaxation of the normalized reflectivity transients $(\Delta R/R{)}_{\mathrm{norm}}$ in the superconducting state. The decay rate of $\mathbf{(}\Delta R(t)/R{\mathbf{)}}_{\mathrm{norm}}$ systematically decreases with decreasing intensity at 7 K (a) [from bottom to top: $\Phi =0.7\mu \mathrm{J}/{\mathrm{cm}}^2$ (light green), 2.2, 4.4, 7.0, 8.9, 12.8 to $16.1\mu \mathrm{J}/{\mathrm{cm}}^2$ (black)] but not at 60 K (b).Reuse & Permissions

Figure 2

Slow relaxation of $(\Delta R/R{)}_{\mathrm{norm}}$ in the superconducting state. (a) $(\Delta R/R{)}_{\mathrm{norm}}$ at 7 K measured at three different fluences show no intensity dependence in the slow dynamics despite their marked intensity dependence at short times. (b) Temperature dependence of $(\Delta R/R{)}_{\mathrm{norm}}$ near $T_c$ obtained at $\Phi =4.4\mathrm{J}/{\mathrm{cm}}^2$. We note a sharp decrease in the offset across the transition (i.e., 37 to 42 K) due to switching off of the long-time component with loss of superconductivity. Above $T_c$ there is an upturn of the signal evident at short times due to stimulated Brillouin scattering [27].Reuse & Permissions

Figure 3

Decay-rate analysis of data with a view towards gap symmetry. (a) Illustration of the three rates in the Rothwarf-Taylor model: Bare QP recombination rate ($Bn$), pair creation rate (${\gamma }_{\mathrm{pc}}$), and escape rate (${\gamma }_{\mathrm{esc}}$). ($n$: QP, $N$: Bosons) (b) The initial decay rate (${\gamma }_{r0}$) as a function of pump fluence at various temperatures. The linear dependence of the recombination rate on initial population is a hallmark of bimolecular recombination. Dashed lines show linear fits to data. The thermal decay rate at each temperature was obtained by extrapolating the fits to zero fluence. (c) The initial relaxation rate ${\gamma }_{r0}$ plotted as a function of temperature for five representative pump fluences along with the thermal rate ${\gamma }_{\mathrm{th}}$ (orange diamonds). Below $T_c$, ${\gamma }_{r0}$ strongly depends on the excitation density. There is a sharp transition at 37 K, although a discernible intensity dependence persists above $T_c$. The dashed line shows $\sim T^2$ dependence as a guide. (d) $|(\Delta R/R{)}_{\max }|$ plotted as a function of $\Phi$ for four temperatures, each offset for clarity. Dashed lines show a slope of 1.Reuse & Permissions

Figure 4

(a) Schematic representation of the electron and hole bands crossing the Fermi level in the superconducting state. (b) Interband recombination of QPs in the $\alpha$ bands can occur with their counterparts in the electron bands. As a result, a Cooper pair, an optical phonon and an antiferromagnetic spin fluctuation are produced.Reuse & Permissions