Charge Recombination in Undoped Cuprates

TL;DR

This paper provides a theoretical analysis of charge recombination in undoped cuprates, explaining the rapid picosecond carrier lifetime through a mechanism involving Mott excitons and multimagnon emission.

Contribution

It introduces a minimal microscopic model for charge recombination in cuprates, combining the Hubbard model with a decay mechanism via multimagnon emission, supported by numerical calculations.

Findings

Recombination involves metastable Mott excitons decaying via multimagnon emission.

Decay rate depends exponentially on the ratio of the Mott-Hubbard gap to exchange coupling.

Theoretical results align with experimental picosecond carrier lifetimes.

Abstract

We theoretically analyse the process of charge recombination in the planar Mott-Hubbard insulators with the aim to explain short picosecond-range lifetime of photoexcited carriers, experimentally studied via pump-probe experiments on the undoped cuprates. The recombination mechanism consists of two essential ingredients: the formation of a metastable s-type bound holon-doublon pair, i.e. the Mott exciton, and the decay of such an excitonic state via the multimagnon emission. In spite of the large gap that requires many bosons to be emitted, latter process is fast due to large exchange scale and strong charge-spin coupling in planar systems. As the starting microscopic model we consider the single-band Hubbard model, and then more realistic three-band model for cuprates, both leading to the same minimal one. The decay rate of the exciton is evaluated numerically via the Fermi golden rule, having consistency also with the direct time-evolution calculation. The decay rate reveals exponential dependence on the ratio of the Mott-Hubbard gap and the exchange coupling - the result qualitatively reproduced also within a toy exciton-boson model.