Ultrafast dynamics in the presence of antiferromagnetic correlations in electron doped cuprate La2 xCexCuO4±δ

We used femtosecond optical pump-probe spectroscopy to study the photoinduced change in reflectivity of thin films of the electron-doped cuprate La2-xCexCuO4 (LCCO) with dopings of x=0.08 (underdoped) and x=0.11 (optimally doped). Above Tc, we observe fluence-dependent relaxation rates that begin at a temperature similar to the one where transport measurements first show signatures of antiferromagnetic correlations. Upon suppressing superconductivity with a magnetic field, it is found that the fluence and temperature dependence of relaxation rates are consistent with bimolecular recombination of electrons and holes across a gap (2ΔAF) originating from antiferromagnetic correlations which comprise the pseudogap in electron-doped cuprates. This can be used to learn about coupling between electrons and high-energy (ω>2ΔAF) excitations in these compounds and set limits on the time scales on which antiferromagnetic correlations are static.