Abstract

Important advancements have recently been made in understanding of the role of Fermi surface topological transitions, van Hove singularities (VHs), and flat bands in correlated systems. Lifshitz transitions and VHs are known to have far-reaching consequences for strongly correlated systems. More recently, theoretical work has shown that even stronger singularities can be stabilized through higher-order VHs (HOVHs), raising the question of how such features drive electronic instabilities. 

The physics of VHs is important in a wide range of correlated electron materials, including ruthenates, heavy fermion compounds, bilayer graphene, and other moiré lattices where long-standing puzzles can be understood in the framework of higher-order VHSs (HOVHSs). Recent studies report new materials exhibiting unusual physics as a result of VHs and the emergence of flat bands, including twisted bilayer graphene, kagomé metals, and superconductors. When combined with strong correlations, flat bands can give rise to entirely new phenomena.

This Mini-colloquium adresses how interaction effects are governed by HOVHSs and flat bands. Topics include (i) spectroscopies with ultra-high energy resolution to resolve VHs on the relevant energy scales, (ii) new experimental approaches to tune their energy and character, (iii) theoretical approaches combining ab-initio modelling with many-body techniques to determine the leading electronic instabilities, and (iv) synthesis of new compounds designed to realize the predicted ground states.

REFERENCES

(i) Checkelsky et al, Flat bands, strange metal and the Kondo effect, Nature Reviews Materials 9, 509 (2024)
(ii) Chandrasekharan et al, On the engineering of higher-order Van Hove singularities in two dimensions, Nat. Commun. 15, 9512 (2024)
(iii) Zhuravlev and Irkhin, Kondo effect in the presence of Van Hove singularities: A numerical renormalization group study, Phys. Rev. B 84, 245111 (2011)
(iv) Classen and Betouras, High-order Van Hove singularities and their connection to flat bands, Annu. Rev. Condens. Matter Phys. 16, 229-51 (2025)

Organizers

Name	Affiliation
Joseph Betouras	Loughborough University, UK
Peter Wahl	University of St-Andrews, UK
Gertrud Zwicknagl	Technische Universität Braunschweig, Germany
Silke Buehler-Paschen	Technische Universität Wien, Austria