Angle resolved photoemission spectroscopy study of excitons in quantum materials

Abstract

Excitons, which are bound states consisting of electron-hole pairs, have garnered significant attention in condensed matter physics research over the past decade. Due to the key characteristics of charge neutrality, mobility, and photoelectric conversion, excitons have diverse applications in technologies, such as transistors, photodetectors, light-emitting diodes, and solar cells. The optical method has been commonly used to study excitons in semiconductors, but it falls short in providing momentum information. Consequently, angle-resolved photoemission spectroscopy (ARPES) has gained attention in recent years for its ability to offer direct insights into momentum space. This article reviews the recent progress of ARPES in the study of excitons in quantum materials, focusing on both stimulated and spontaneous excitons. For stimulated excitons in semiconductors, we review research involving two-photon time-resolved ARPES techniques, which detect exciton bound states directly within the semiconductor band gap, as well as single-photon ARPES studies that excite stimulated excitons manifested as side energy bands between conduction bands and valence bands. Regarding spontaneous excitons, we review ARPES studies that have been crucial in revealing the physical properties of excitons condensation and band folding effects. Finally, we offer a perspective on future research directions in this field.