The Be K-edge in beryllium oxide and chalcogenides: soft x-ray absorption spectra from first-principles theory and experiment

Ab Initio Quantum-Mechanical Predictions of Semiconducting Photocathode Materials

Ab initio Quantum-Mechanical methods are well-established tools for material characterization and discovery in many technological areas. Recently, state-of-the-art approaches based on density-functional theory and many-body perturbation theory were successfully applied to semiconducting alkali antimonides and tellurides, which are currently employed as photocathodes in particle accelerator facilities. The results of these studies have unveiled the potential of ab initio methods to complement experimental and technical efforts for the development of new, more efficient materials for vacuum electron sources. Concomitantly, these findings have revealed the need for theory to go beyond the status quo in order to face the challenges of modeling such complex systems and their properties in operando conditions. In this review, we summarize recent progress in the application of ab initio many-body methods to investigate photocathode materials, analyzing the merits and the limitations of the standard approaches with respect to the confronted scientific questions. In particular, we emphasize the necessary trade-off between computational accuracy and feasibility that is intrinsic to these studies, and propose possible routes to optimize it. We finally discuss novel schemes for computationally-aided material discovery that are suitable for the development of ultra-bright electron sources toward the incoming era of artificial intelligence.

Ultrafast X-Ray Absorption Spectroscopy of Strongly Correlated Systems: Core Hole Effect

In recent years, ultrafast pump-probe spectroscopy has provided insightful information about the nonequilibrium dynamics of excitations in materials. In a typical experiment of time-resolved x-ray absorption spectroscopy, the systems are excited by a femtosecond laser pulse (pump pulse) followed by an x-ray probe pulse after a time delay to measure the absorption spectra of the photoexcited systems. We present a theory for nonequilibrium x-ray absorption spectroscopy in one-dimensional strongly correlated systems. The core hole created by the x ray is modeled as an additional effective potential of the core hole site, which changes the spectrum qualitatively. In equilibrium, the spectrum reveals the charge gap at half-filling and the metal-insulator transition in the presence of the core hole effect. Furthermore, a pump-probe scheme is introduced to drive the system out of equilibrium before the x-ray probe. The effects of the pump pulse with varying frequencies, shapes, and fluences are discussed for the dynamics of strongly correlated systems in and out of resonance. The spectrum indicates that the driven insulating state has a metallic droplet around the core hole. The rich structures of the nonequilibrium x-ray absorption spectrum give more insight into the dynamics of electronic structures.