A new twist on polarons

Polarons emerge from the interaction between electrons and lattice vibrations in crystals, and play a prominent role in a variety of materials’ properties. For example, they underlie the optoelectronic properties of materials for photovoltaics and energy-efficient lighting, and their formation has been postulated as a precursor of unconventional forms of superconductivity. Recent experimental breakthroughs are giving unprecedented access to the microscopic nature of these quasiparticles, which motivates the development of a quantitative ab initio theory of polarons. After reviewing the state of the art in this area, I will focus on two recent developments.

In the first part of the talk, I will present first principles calculations of polarons in halide perovskites. I will show that these materials harbor a uniquely rich variety of polaronic species, including small polarons, large polarons, and twist-density waves. Surprisingly, some of these emergent quasiparticles support topologically nontrivial displacement fields, in analogy with Bloch points found in materials hosting magnetic and polar skyrmion lattices. These results showcase the potential of ab initio methods to discover new types of polaronic quasiparticles beyond existing simplified models.

In the second part of the talk, I will highlight the need for a unified theoretical framework to describe polarons across the coupling-strength scales. I will show that this can be achieved with a many-body formalism that includes both non-adiabatic electron-phonon interactions and static contributions from the polaronic displacements on an equal footing. After presenting proof-of-principle calculations on the polaronic contribution to the band gap renormalization in insulators, I will conclude by discussing new opportunities that such an ab initio many-body theory of polarons can bring.