Abstract
We study the pump-probe response of three insulating cuprates and develop a model for its recombination kinetics. The dependence on time, fluence, and both pump and probe photon energies imply many-body recombination on femtosecond timescales, characterized by anomalously large trapping and Auger coefficients. The fluence dependence follows a universal form that includes a characteristic volume scale, which we associate with the holon-doublon excitation efficiency. This volume varies strongly with pump photon energy and peaks near twice the charge-transfer energy, suggesting that the variation is caused by carrier multiplication through impact ionization.
Highlights
Optical excitations and the processes that return them to equilibrium are well understood in conventional semiconductors, where the Coulomb interactions among carriers may be treated perturbatively [1,2]
We have shown that the insulating cuprates satisfy standard SRH recombination kinetics, but with much higher trapping and recombination rates than found in conventional semiconductors
Our results indicate that interactions influence multiple kinetic processes, and might provide an avenue for controlling nonequilibrium behavior in applications
Summary
Optical excitations and the processes that return them to equilibrium are well understood in conventional semiconductors, where the Coulomb interactions among carriers may be treated perturbatively [1,2]. Exceed those of conventional semiconductors by more than two orders of magnitude [2,19,20,21,22,23,24,25] Following Shockley, Read, and Hall (SRH) [26,27], we break the recombination process into two single-particle steps—carrier trapping, followed by recombination—but with much higher rates than found in conventional semiconductors By fitting this model to our measurements, we identify an additional many-body recombination channel that we associate with an Auger process, which we find to be anomalously fast. Experimentally it remains challenging to identify the physical origins of spectral and temporal changes in these systems, so the fluence dependence represents a potentially valuable new way to clarify these
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