Ultrafast electron dynamics upon above band-gap excitation in epitaxial LaFeO$_3$(001) thin films

TL;DR

This study investigates ultrafast electron dynamics in epitaxial LaFeO₃(001) thin films using advanced spectroscopic techniques, revealing multiple unoccupied states and biexponential decay pathways following above band-gap excitation.

Contribution

It provides a detailed surface-science analysis of LaFeO₃ thin films, mapping electronic states and uncovering complex electron decay mechanisms not previously characterized.

Findings

Identification of three unoccupied electronic states associated with Fe and La orbitals.

Observation of a biexponential decay in conduction band electrons with time constants of 39 fs and 1100 fs.

Demonstration of two independent electron decay pathways in LaFeO₃ thin films.

Abstract

Strong electron correlations in perovskite oxides give rise to rich and often unexpected electronic phenomena. In this study, we present a comprehensive surface-science investigation of epitaxial thin films of the charge-transfer insulator LaFeO$_3$(001). The characterization includes low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and photoemission spectroscopy. We map both the occupied and unoccupied electronic states using two-photon photoemission (2PPE) spectroscopy. Furthermore, we probe electron dynamics through an ultraviolet-ultraviolet (UV-UV) pump-probe experiment, exciting electrons from hybridized O~$2p$/Fe $3d$ states to Fe minority-spin states above the band gap. Our results reveal three distinct unoccupied states, which we assign to Fe $t_{2g\downarrow}$, Fe $e_{g\downarrow}$, and La $5d$ orbitals. Notably, the conduction band minimum exhibits a biexponential decay with time constants of 39\,fs and 1100\,fs, suggesting the presence of two independent decay pathways.