Controlled Vaporization of the Superconducting Condensate in Cuprate Superconductors by Femtosecond Photoexcitation

We use ultrashort intense laser pulses to study superconducting state vaporization dynamics in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ ($x=0.1$ and 0.15) on the femtosecond time scale. We find that the energy density required to vaporize the superconducting state is $2.0\pm 0.8$ and $2.6\pm 1.0\mathrm{K}/\mathrm{Cu}$ for $x=0.1$ and 0.15, respectively. This is significantly greater than the condensation energy density, indicating that the quasiparticles share a large amount of energy with the boson glue bath on this time scale. Considering in detail both spin and lattice energy relaxation pathways which take place on the relevant time scale of $\sim {10}^{-12}\mathrm{s}$, the experiments appear to favor phonon-mediated pair-breaking mechanisms over spin-mediated pair breaking.

Figure 1

The photoinduced reflectivity $\Delta R/R$ in ${\mathrm{La}}_{1.9}{\mathrm{Sr}}_{0.1}{\mathrm{CuO}}_4$ ($T_c=30\mathrm{K}$) taken at various photoexcitation fluences (a) below and (b) above $T_c$. The data above $T_c$ are normalized with respect to $\mathcal{F}$ and fully overlap, showing that the response is linear. Below $T_c$ the two distinct relaxation components are marked as $A$ and $B$.

Figure 2

(a) The maximum amplitude $\Delta R_A/R$ at 4.5 K for $x=0.1$ (empty circles) and $x=0.15$ (full circles), and $\Delta R_B/R$ at 4.5 K (squares) and 32 K (triangles) for $x=0.1$ in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. The arrows mark the vaporization thresholds ${\mathcal{F}}_T=4.2\pm 1.7\mu \mathrm{J}/{\mathrm{cm}}^2$ and ${\mathcal{F}}_T=5.8\pm 2.3\mu \mathrm{J}/{\mathrm{cm}}^2$ for $x=0.1$ and $x=0.15$, respectively, obtained from the fit [23] (lines). (b) The $T$ dependence of $\Delta R_A/R$ for $x=0.1$. The dashed line is a fit to the data using Eq. (1).