Public thesis defense Vasilii Vasilchencko - IMCN
sst |
Formation and transport of polarons from a variational ab initio approach
Friday, September 4th, 2026 – 4pm – Auditorium SUD03 – Place Croix du SUD – 1348, Louvain-la-Neuve
Polarons are quasiparticles formed when charge carriers interact strongly with atomic vibrations in a material. This interaction can alter a carrier’s energy and effective mass or even trap it within a local distortion of the crystal lattice. Charge transport may then occur via thermally activated “hops” between localized states rather than via conventional band-like motion. Understanding these effects is important for explaining the electronic properties of many materials.
In this thesis, we develop a first-principles computational framework for studying the formation and motion of self-trapped polarons. The method is based on a variational expression for the polaron binding energy that avoids artificial self-interaction of the charge. We introduce an efficient optimization algorithm and validate it using both a general theoretical model and realistic calculations of polar materials.
Our approach can identify several stable polaron configurations within a single system. It reveals complex polaron energy landscapes with multiple minima, representing distinct arrangements of polaron charge and lattice distortion.
We also extend the framework to determine the most favourable paths and transition states between these configurations. This enables us to describe thermally activated polaron hopping and estimate charge-transfer rates. Applications to lithium fluoride (LiF) and rutile titanium dioxide (TiO₂) demonstrate that the method provides a reliable description of polaron dynamics. For TiO₂, the calculated transport properties agree well with experimental measurements. Overall, this work provides a robust foundation for predicting polaron formation and transport in materials from first principles.
Jury members
Prof. Xavier Gonze (UCLouvain), Supervisor
Prof. Arnaud Delcorte (UCLouvain), Chairperson
Prof. Samuel Poncé (UCLouvain), Secretary
Prof. Benoît Hackens (UCLouvain)
Prof. Matthieu Verstraete (University of Utrecht, The Netherlands)
Prof. Cesare Franchini (University of Vienna, Austria)