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Description
Self-trapped polarons are quasiparticles that form in materials when a free electron or hole becomes localized within a self-induced lattice distortion. This autolocalization can modify the charge transport properties of a system, leading to a transition from band-like transport to thermally activated hopping.
Among first-principles approaches to polarons, the Variational Polaron Equations provide an efficient computational framework [1]. As implemented in the ABINIT software package, this approach explicitly treats the polaron-induced charge and structural forces, and enables sampling of the polaron energy landscape across distinct polaronic states. In this contribution, building on this framework, we demonstrate how the explicit treatment of polaron forces enables the variational optimization of minimum energy paths (MEPs) for polaron charge transfer using the string method [2]. Access to multiple polaronic states, together with the MEPs connecting them on the polaron energy landscape, provides a route to estimate hopping mobilities within the framework of transition state theory [3].
These capabilities are demonstrated for rutile TiO₂, a prototypical semiconductor known to exhibit electron polaron self-trapping, whose conductivity measurements display clear signatures of polaronic hopping transport [4].
References
[1] V. Vasilchenko, M. Giantomassi, S. Poncé, X. Gonze, Phys. Rev. B, 112, 014314 (2025)
[2] W. E, W. Ren, E. Vanden-Eijnden, The Journal of Chemical Physics, 126, 164103 (2007)
[3] N. Deskins, M. Dupuis, Phys. Rev. B, 75, 195212 (2007)
[4] S. X. Zhang et al., J. Appl. Phys. 102, 013701 (2007)