Modern ab-initio approaches to the quantum many-body problem open unprecedented possibilities to compute, analyze, and predict materials properties of complex systems ranging from bulk semiconductors over correlated systems to nanomaterials. Whereas density-functional theory with state-of-the-art functionals allows for efficient calculations of larger systems, first-principles many-body perturbation theory provides a deeper physical understanding of many-body effects, such as electronic quasiparticle excitations, dynamical screening, or excitons. The increasing complexity of materials systems for technological applications, for instance in optoelectronics or photovoltaics, demands for continuous development of novel efficient and accurate theoretical methods along with improved computational schemes to predict electronic and optical properties.
In this presentation, I will focus on two examples: First, it will be demonstrated how ab-initio many-body calculations and state-of-the-art synchrotron experiments can go hand in hand to unravel the electronic screening mechanisms in strongly correlated CuO. The dynamical screening of the electron-electron interaction is a key quantity in many-body perturbation theory. We elucidate the crucial role played by d-d excitations in renormalizing the band gap and dissect the contributions of different excitations to the electronic self-energy which is illuminating concerning both the general theory and this prototypical material.
Second, it will be shown how theoretical modeling of material properties can drive technological innovation. Silicon is known as a notoriously bad light emitter, due to its indirect band gap. Making silicon a direct-gap efficient ligth-emitter would enable the integration of microelectronics and optoelectronics, which is expected to revolutio-nize various fields of technology, such as communication, sensing, and imaging. We predict hexagonal silicon-germanium alloys to be direct-gap materials with a dipole-active absorption edge and optical oscillator strengths comparable to III-V semiconductors. The band gap, and therefore the emission frequency, can be tuned with alloy composition. Our findings are supported by recent data from our experimental collaborators, which are promising towards the development of silicon-based nanolasers.