We have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor ${\mathrm{PuCoGa}}_5$. The deduced electron-phonon coupling constant is incompatible with the measured superconducting transition temperature $T_c=18.5\mathrm{K}$, which speaks against phonon-mediated superconductivity. Upon lowering the temperature, we observe an order-of-magnitude increase of the quasiparticle relaxation time in agreement with the phonon bottleneck scenario—evidence for a hybridization gap in the electronic density of states. The modification of photoinduced reflectance in the superconducting state is consistent with the heavy character of the quasiparticles that participate in Cooper pairing.

• Received 26 February 2010

DOI:https://doi.org/10.1103/PhysRevLett.104.227002