Recycling failed photoelectrons for coherent photoemission from flat-band quantum materials

A photoemission experiment on a 2x1 surface reconstructed SrTiO₃¹ sample exhibited an anomalous peak in the low kinetic energy region of the spectrum, evading explanation ever since its observation. Incredibly, it was found that at low temperatures and with light of any frequency above a unique threshold, independent of the standard photoemission threshold, the peak became coherent beyond the experimental resolution and increasingly intense, eventually containing over 99% of all electrons photoemitted. Such an intense and coherent peak within the low kinetic energy region, usually characterized by a broad continuous signal, dwarfing both the work function cut-off and the primary photoemission spectrum, calls for an explanation beyond the conventional framework of secondary photoemission. We present a new model, hinging on the influence of flat bands on the steady state minority electron and hole distributions with Auger recombination between these populations, through which all the puzzling features of the experiment can be understood. Additionally, due to the unique threshold, our model predicts photoemission below the standard photoemission threshold under certain conditions. Finally, we develop a heuristic guide to predict if the anomalous behavior can manifest in other materials using first principles calculations which we use to identify a diverse set of materials.

1. Hong, C. et al. Anomalous intense coherent secondary photoemission from a perovskite oxide. Nature 617, 493-498 (2023).