Carrier relaxation dynamics in intra-gap states: the case of the superconductor Y Ba_2 Cu_3O_7-delta and the charge-density-wave semiconductor K_0.3MoO_3

Abstract

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 intra-gap states. The data on YBa_2Cu_3O_7-delta near optimum doping and K_0.3$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 intra-gap 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 intra-gap states, while the similarity of the response in the two materials appear to exclude spin and vortex excitations.

arXiv comment3 postscript figures are included, will be published in Phys. Rev. B

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