Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), España.
«Strong electronic correlations in moiré materials»
The discovery of insulating and superconducting states in twisted bilayer graphene in
2018 opened a new era in the study of quantum materials. When two similar lattices
are stacked a moiré pattern is formed giving rise to a much larger unit cell which
controls the electronic properties of the system. Under certain circumstances, these 2D
heterostructures present flat bands and become very sensitive to the effects of the
electronic interactions. These heterostructures can be fabricated starting from different
compounds, primarily van der Waals materials, such as graphene or transition metal
dichalcogenides. A plethora of novel effects is being observed in these moiré materials,
some of them had not been detected in any other material before.
During the talks I will first introduce some of the effects that the repulsion between the
electrons produces in the electronic properties, with main focus on the localization
tendencies produced by Mott physics and their consequences in the electronic
spectrum. I will briefly mention strongly correlated materials, like high-Tc
superconductors or heavy fermion materials, in which these properties have been
studied for several decades. I will then introduce the novel heterostructures and moiré
materials which have been fabricated and studied in the last years. I will introduce
some of the fascinating quantum states that they display and the theoretical
framework used to study them. Finally, I will focus on Magic Angle Twisted Bilayer
Graphene. I will introduce the theoretical models and its description as a heavy
fermion system with topological flat bands and discuss the novel properties which
emerge from the interplay between the electronic correlations and the topology of the
flat bands.
References:
- Patrik Fazekas: “Lecture Notes on Electron Correlation and Magnetism”. Series in Modern Condensed Matter Physics: Volume 5, World Scientific, 1999.
- Dynamical mean field theory of strongly correlated fermion systems and the limit of infinite dimensions. A. Georges et al, Rev. Mod. Phys. 68, 13 (1996).
- Superconductivity and strong correlations in moiré flat bands. Balents et al, Nature Physics 16, 725 (2020).
- Semiconductor moiré materials. K.F. Mak, J. Shan, Nature Nano 17, 686 (2022).
- Interactions in the 8-orbital model for twisted bilayer graphene. M.J. Calderón and E. Bascones, Phys. Rev. B 102, 155149 (2020).
- Magic angle twisted bilayer graphene as a topological heavy fermion problem. Song and Bernevig, Phys. Rev. Lett. 129, 047601 (2022).
- Heavy Fermions and cascades without symmetry breaking in twisted bilayer graphene. A. Datta, M.J.Calderón, A. Camjayi and E. Bascones, Nature Comm. 14, 5036 (2023).
- Cascades in transport and optical conductivity of Twisted Bilayer Graphene. MJ Calderón, A. Camjayi, A. Datta and E. Bascones. Phys. Rev. B 112, L041126 (2025).