Abstract:
Recent femtosecond pump-probe experiments on Mott-Hubbard (MH) insulators reveal very fast charge recombination in the picosecond range. We present a theory of recombination of the excited bound holon-doublon pair - exciton in undoped cuprates, based on strong correlations and on the Hubbard and charge-transfer microscopic model which shows that such processes can be explained even quantitatively with the multi-magnon emission. We find that the recombination rate is exponentially dependent on the charge gap and on the exchange energy. On the other hand, the organic salts being a one-dimensional MH insulators require a substantial coupling to phonons and the multi-phonon mechanism for observed fast recombination. Related is the question of the theoretical approach and understanding of the transient response of charge carriers, as probed by absorption in pump-probe experiments. We develop a linear-response formalism for the time dependent optical conductivity within a strongly correlated system and consider as an example the relaxation of a single highly excited particle in the two-dimensional spin background.