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

TL;DR

This study uses time-resolved photoelectron spectroscopy to explore ultrafast electron dynamics in ultrathin antiferromagnetic CoO(001) films, revealing short-lived unoccupied states and rapid electron relaxation processes.

Contribution

It provides the first detailed measurement of electron lifetimes and relaxation pathways in CoO thin films using time-resolved spectroscopy.

Findings

Unoccupied image potential states with 12-20 fs lifetimes

Electron lifetimes in conduction band are around 27 fs

Evidence suggests strong correlation effects or ultrafast relaxation pathways

Abstract

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.