Electronic structure of the heavy-fermion superconductor PuCoGa$_{5}$ probed via the dynamics of photoinduced quasiparticles.

We studied the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa$_{5}$ using pump-probe spectroscopy. The relaxation time of photoinduced reflectance in the normal state allows an estimate of the electron-phonon coupling constant $\lambda $ =0.20-0.26, which is incompatible with the measured T$_{c}$ and speaks against phonon-mediated superconducting pairing. Upon lowering the temperature in the normal state (T$>$T$_{c})$, we find an order-of-magnitude increase of the relaxation time -- evidence for the presence of a hybridization gap in the electronic density of states. The relaxation slows down due to a coupling of quasiparticle population to a slowly decaying non-equilibrium high-frequency phonon population. A kinetic model that describes the coupled quasiparticle and phonon populations allows the estimate of the hybridization gap magnitude to be $\sim $90 K. Our measurement provides the first evidence for the presence of a hybridization gap in PuCoGa$_{5}$. The observed dynamics in the superconducting state confirms that the heavy quasiparticles detected in the normal state also participate in the superconducting pairing.