Time-resolved electron dynamics in antiferromagnetic CoO(001) thin films

Ultrathin antiferromagnetic CoO(001)-(1x1) films of 2 and 4 monolayers (ML) on Ag(001) are investigated by time- and angle-resolved two-photon photoelectron (2PPE) spectroscopy. Pump-probe spectra show unoccupied states between 3.6 and 4.0\,eV above the Fermi level ($E_{F}$), which are identified as image potential states with momentum-dependent lifetimes between 12 to 20\,fs. Electrons photoexcited across the band gap show lifetimes of 27\,fs at the bottom of the Co $3d$ $t_{2g}$-derived conduction band minimum. This lifetime is much shorter than in conventional semiconductors. Our observations point either to strong electron correlation effects as has been demonstrated for NiO or to an ultrafast relaxation pathway via metallic substrate states.