Polarons from first principles*

Polarons are among the most well-known quasiparticles in solid state physics, and are key to understanding fundamental concepts such as the electron mass enhancement in semiconductors and the formation of Cooper pairs in superconductors. Interest in polaron physics has been reignited by recent angle-resolved photoelectron spectroscopy studies, which revealed polaronic signatures in the band structures of several metal oxides and two-dimensional semiconductors. In this talk I will describe our recent work aimed at describing polarons and their spectroscopic signatures from first principles. In the first part of the talk I will outline a general many-body framework to compute and analyze polaron satellites in photoelectron spectra using the cumulant expansion approach [1,2]. I will discuss applications to titanium dioxide and europium oxide, and show that the calculations are able to reproduce very closely measured angle-resolved photoelectron spectra. In the second part of the talk I will address the question on how to compute the wavefunction of a polaron. I will describe a new approach to the polaron problem that overcomes some of the limitations of explicit supercell calculations [3]. This approach enables systematic calculations of wavefunctions and formation energies for both small and large polarons, and can be used to analyze the electron-phonon coupling mechanisms responsible for electron or hole self-trapping. I will illustrate these concepts using lithium fluoride and lithium oxide as examples, and I will discuss the connection with previous work on the polaron problem based on model Hamiltonians.
[1] C. Verdi, F. Caruso, F. Giustino, Nat. Commun. 8, 15769 (2017).
[2] J. M. Riley et al., Nat. Commun. 9, 2305 (2018).
[3] W. H. Sio, C. Verdi, S. Poncé, and F. Giustino, Phys. Rev. Lett. 122, 246403 (2019).