Speaker
Description
I will discuss the interplay between electron-phonon coupling and electron-electron correlations in multi-orbital Hubbard models. I will show that in a two-orbital Hubbard model coupled with a Jahn-Teller phonon mode, polaronic effects and Mott physics can either compete or coexist according to the ratio between electron-phonon coupling and Hund's exchange [1]. Indeed this follows from the fact that the Jahn-Teller phonon can coexist with the effect of the Hubbard U, while it competes with the Hund's coupling, as discussed some time ago in theories for alkali-doped fullerides [2]. For a half-filled system, this interplay leads to two spectacularly different Mott insulators, a standard high-spin Mott insulator with frozen phonons which is stable when the Hund's coupling prevails, and a low-spin insulator favored by phonons, where the characteristic features of Mott insulators and bipolarons coexist. The two phases are separated by a sharp boundary along which an intermediate solution emerges as a kind of compromise between the two insulators [1]. I will then discuss the stability of this picture with doping, the role of the phonon frequency and the relevance of these results for some materials. I will finally touch on the interplay between electron-phonon coupling and exciton condensation.
This presentation features results obtained in collaboration with A. Scazzola, A. Amaricci, S. Giuli and G. Mazza.
[1] A. Scazzola, A. Amaricci and M. Capone, Phys. Rev. B 107, 085131 (2023)
[2] Y. Nomura, S. Sakai, M. Capone and R. Arita, Science Advances 1, e1500568 (2015)
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