Excitons—quasiparticles formed by bound electron-hole pairs—are key to understanding a material’s optical and electronic properties. Researchers Siegfried Kaidisch, Amir Kleiner, Sivan Refaely-Abramson, and colleagues have developed exciton photoemission orbital tomography (exPOT), a computational framework that simulates and interprets experimental observations of excitons in complex materials. By bridging theoretical models with photoemission tomography, exPOT accurately captures exciton behavior, including electron-hole interactions. Applying the method to hexagonal boron nitride, the team demonstrates how exciton responses depend on light polarization and enables the study of excitons normally hidden from conventional techniques, providing new insights into these fundamental quasiparticles.

Figure 1. Exciton Photoemission Maps Electron-Hole Correlations in 2D Materials

First-Principles DFT and GW Analysis of 2D Materials

Researchers employ density functional theory (DFT) and many-body perturbation theory (GW) to probe the electronic structure, optical responses, and quasiparticle energies of two-dimensional (2D) materials such as graphene and transition metal dichalcogenides. Accurate modeling demands careful selection of basis sets and pseudopotentials to capture complex electron interactions. Studies examine the effects of stacking (heterostructures), defects, doping, and external stimuli like strain or electric fields on material properties. Investigated materials include graphene, MoS₂, WS₂, and hexagonal boron nitride. Methodological improvements focus on enhancing GW convergence, optimizing algorithms, and using real-space approaches for dielectric calculations. These efforts highlight that precise 2D material characterization requires moving beyond standard DFT to account for strong excitonic effects and challenges arising from reduced dimensionality. Figure 1 shows Exciton Photoemission Maps Electron-Hole Correlations in 2D Materials.

Exciton Photoemission Mapping Using Many-Body Perturbation Theory

Researchers have developed exciton photoemission orbital tomography (exPOT), a theoretical framework for analyzing how excitons—bound electron-hole pairs—emit photoelectrons in materials. The key innovation connects the Bethe-Salpeter Equation, which describes correlated electron-hole interactions, with photoemission tomography, enabling a detailed mapping of electronic structure. The study demonstrates that electron-hole correlations strongly influence exciton photoemission and that the resulting signals depend on the polarization of the pump pulse. Using hexagonal boron nitride as a model system, the framework also captures excitons with finite center-of-mass momentum, allowing the study of “momentum-dark” excitons.

By formulating the photoemission process within many-body perturbation theory, the approach provides an accurate description of electronic structure and photoelectron spectra. The method employs a Dyson orbital-like construction for Bloch functions, effectively reducing the many-electron problem to an effective one-electron picture. Measurements confirm that calculated photoelectron angular distributions reflect both intrinsic exciton properties and pump polarization, offering deeper insight into excitonic behavior and opening new avenues for studying complex materials.

Exciton Photoemission Orbital Mapping

Researchers have developed a computational framework, exciton photoemission orbital tomography (exPOT), to simulate and interpret exciton behavior in periodic systems. By linking the Bethe-Salpeter Equation with photoemission tomography, exPOT accurately models exciton photoemission while accounting for the effects of the probe pulse on matrix elements [1]. The framework reveals that electron-hole correlations strongly influence excitonic photoemission and introduce a dependence on the polarization of the pump pulse. Applied to hexagonal boron nitride, the method successfully captures excitons with finite center-of-mass momentum, including typically hard-to-observe momentum-dark excitons. The study also highlights how the exciton’s initial state affects the measured photoemission intensity, emphasizing the role of exciton superposition in interpreting experimental results.

References:

1. https://quantumzeitgeist.com/systems-exciton-photoemission-tomography-reveals-correlated-electron-hole-wave/

Cite this article:

Janani R (2025), Exciton Tomography Maps Electron-Hole Correlations in 2D Materials, AnaTechMaz, pp. 315