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.