Speaker
Description
A widespread belief treats electron–phonon and electron–electron (Hubbard) interactions as mutually exclusive, assuming a direct competition in which one interaction dominates while the other becomes irrelevant.
In this talk, I will discuss how this scenario—although justified in some materials—is challenged in many modern quantum systems, ranging from alkali-metal-doped fullerides [1] to magic-angle twisted bilayer graphene [2].
To shed light on this interplay, I will start from simplified models, comparing and contrasting the widely studied single-band Hubbard–Holstein model [3] with multi-orbital models featuring different types of phonon couplings. The main result is that phonon modes coupled to the local charge tend to compete with Hubbard repulsion, whereas other modes—such as Jahn–Teller phonons—can coexist with electronic correlations, leading to a rich interplay with Hund’s exchange interaction.
As a consequence, qualitatively different physical scenarios emerge: in some cases, strongly correlated phases are largely unaffected by phonons, while in others phononic effects are enhanced. This can give rise, for instance, to phases where Mott physics coexists with bipolaronic features [4], or to correlation-enhanced phonon-mediated superconductivity [1].
Finally, I will discuss how these scenarios manifest in different classes of materials with diverse properties, including superconductors (such as fullerides and magic-angle twisted bilayer graphene) and excitonic insulators.
This work is based on collaborations with several researchers, including the authors of the cited studies.
[1] M. Capone, M. Fabrizio, C. Castellani, and E. Tosatti, Rev. Mod. Phys. 81, 943 (2009); Y. Nomura et al. Science Advances 1, e1500568 (2015).
[2] M. S. Liang et al., arXiv:2604.04631
[3] M. Capone, C. Castellani, and M. Grilli, Adv. Cond. Mat. Phys. 2010, 920860 (2010)
[4] A. Scazzola, A. Amaricci and M. Capone, Phys. Rev. B 107, 085131 (2023)
[5] S. Giuli et al. 2026