Degeneracy of electronic states and the "anomalous" kinetics of a nonlinear response of high-temperature superconductors in pump–probe spectroscopy

It is shown that all the spectral–temporal and temperature anomalies of a nonlinear response of high-temperature superconductors (HTSCs) observed in the pump–probe spectroscopy at high excitation levels can be interpreted taking into account contributions of interband electronic transitions to the dielectric constant of an excited HTSC film with a "frozen" energy gap in the electronic spectrum. A model is constructed which explains all these "anomalies" by a drastic decrease in the rates of three-particle nonradiative recombination of excess carriers under strong degeneracy conditions. It is found that this situation takes place due to the specific distribution of the density of electronic states in a HTSC almost immediately after the "opening" of the energy gap in the electronic spectrum.