Ab-initio calculations for anharmonic polarons in hydrides

Hydrogen-rich materials or "hydrides" at high pressure reveal a host of interesting properties, among which record high critical temperatures for superconductivity. This recent discovery has put high-pressure hydrides in the spotlight. In this project, we focus on an aspect that makes these materials special: a very large phonon anharmonicity. Phonons are quantized lattice vibrations of the atoms in the crystal. When at atom is displaced out of its equilibrium position, it feels a restoring force that is usually approximated by a spring pulling it back to its lattice position. For hydrides, the force is no longer spring-like, but more complicated, and this is referred to as phonon anharmonicity. The electrons feel the lattice vibrations, and form an effective composite quasiparticle called a polaron, consisting of the electron taken together with the lattice deformation it induces. We combine the expertise of the Flemish partner, polaron physics, with that of the Austrian partner, first-principles calculation of phonons and electron-phonon interaction strength, to take into account phonon anharmonicity in the description of polarons in hydrides. This will lead to a better understanding of the normal state electronic and optical properties of the interesting class of materials that are the hydrides.

The FWF-funded research project "Ab-initio Calculations for Anharmonic Polarons in Hydrides" has led to groundbreaking advancements in the theoretical understanding of polarons-quasiparticles formed by the interaction of electrons with lattice vibrations in solids. This interdisciplinary project combined analytical models with advanced numerical simulations to study polarons in materials characterized by strong anharmonicity, such as quantum paraelectrics and halide perovskites, materials with significant implications for energy technologies including solar cells and solid-state cooling. Led by two core research groups based at the University of Vienna and the University of Antwerp, and supported by international collaborators, including experts from Japan, the project successfully developed novel computational tools, including state-of-the-art Quantum Monte Carlo algorithms and new analytical frameworks. These innovations pushed the frontiers of polaron research, enabling the accurate modeling of their behavior in materials containing light atoms and exhibiting nonlinear vibrational dynamics. The collaboration resulted in the publication of approximately 20 peer-reviewed articles in high-impact international journals, significantly enriching the scientific literature on quantum materials and condensed matter physics. In addition to its scientific output, the project had a strong community-building dimension, organizing several workshops and research meetings that fostered international exchange and collaboration. An important outcome of the project was the training and successful career advancement of early-stage researchers. Notably, PhD students Matthew Houtput and Thomas Hahn completed their doctoral work within the project framework and have continued to thrive in academia: Houtput secured an individual FWO fellowship, while Hahn joined the prestigious Flatiron Institute in New York as a postdoctoral researcher. From a fundamental standpoint, the project has shed light on the quantum dynamics of polarons under highly anharmonic conditions. Moreover, the development and public release of open-source software tools ensures that the broader scientific community can build on these findings, continuing to explore the complex and fascinating physics of polarons in functional quantum materials.