We used femtosecond optical pump-probe spectroscopy to study the photoinduced change in reflectivity of thin films of the electron-doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ (LCCO) with dopings of $x=0.08$ (underdoped) and $x=0.11$ (optimally doped). Above $T_c$, 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 ($2{\mathrm{\Delta }}_{\mathrm{AF}}$) 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 ($\omega >2{\mathrm{\Delta }}_{\mathrm{AF}}$) excitations in these compounds and set limits on the time scales on which antiferromagnetic correlations are static.

• Received 17 August 2016
• Revised 9 January 2017

DOI:https://doi.org/10.1103/PhysRevB.95.115125