Theoretical Understanding of Photon Spectroscopies in Correlated Materials In and Out of Equilibrium

TL;DR

This paper reviews the theoretical frameworks for photon spectroscopies in correlated materials, covering equilibrium and out-of-equilibrium techniques, emphasizing recent developments and future challenges in the field.

Contribution

It provides a comprehensive overview of theoretical methods for photon spectroscopies in quantum materials, including equilibrium and ultrafast out-of-equilibrium approaches.

Findings

Advances in numerical methods have improved understanding of spectroscopic signals.

Theoretical models now better describe ultrafast and transient phenomena.

Future challenges include modeling complex out-of-equilibrium states.

Abstract

Photon-based spectroscopies have had a significant impact on both fundamental science and applications by providing an efficient approach to investigate the microscopic physics of materials. Together with the development of synchrotron X-ray techniques, theoretical understanding of the spectroscopies themselves and the underlying physics that they reveal has progressed through advances in numerical methods and scientific computing. In this review, we provide an overview of theories for angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering applied to quantum materials. First, we discuss methods for studying equilibrium spectroscopies, including first-principles approaches, numerical many-body methods and a few analytical advances. Second, we assess the recent development of ultrafast techniques for out-of-equilibrium spectroscopies, from characterizing equilibrium properties to generating transient or metastable states, mainly from a theoretical point of view. Finally, we identify the main challenges and provide an outlook for the future direction of the field.