Carrier-relaxation dynamics in intragap states: The case of the superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and the charge-density-wave semiconductor ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$

The unusual slow carrier relaxation dynamics—observed in femtosecond pump-probe experiments on high-temperature superconductors and recently also in a charge-density-wave system—is analyzed in terms of a model for relaxation of carriers in intragap states. The data on ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ near optimum doping and ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$ are found to be described very well with the model using a BCS-like gap which closes at $T_c.$ From the analysis of the data we conclude that a significant intragap density of localized states exists in these materials, which can be clearly distinguished from quasiparticle states by the time-resolved optical experiments because of the different time and temperature dependences of the photoinduced transmission or reflection. Localized charges are suggested to be the most likely origin of the intragap states, while the similarity of the response in the two materials appear to exclude spin and vortex excitations.