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TT: Fachverband Tiefe Temperaturen
TT 35: Focus Session: Correlations in Moiré Quantum Matter I
TT 35.4: Hauptvortrag
Mittwoch, 29. März 2023, 16:45–17:15, HSZ 03
Cascade of transitions in twisted and non-twisted graphene layers within the van Hove scenario — •Laura Classen1, Dmitry Chichinadze2, Yuxuan Wang3, and Andrey Chubukov2 — 1Max Planck Institute for Solid State Research, Stuttgart, Germany — 2University of Minnesota, Minneapolis, USA — 3University of Florida, Gainesville, USA
Fermions in layered graphene structures are described by both spin and valley degrees of freedom. Due to weak coupling between valleys, this can lead to an approximate SU(4) symmetry made from combined spin and valley transformations, which plays out differently depending on filling and interactions. Motivated by measurements of compressibility and STM spectra in twisted bilayer graphene, we analyze the pattern of symmetry breaking for itinerant fermions near a van Hove singularity. Making use of the approximate SU(4) symmetry of the Landau functional, we show that the structure of the order parameter changes with increasing filling via a cascade of transitions. We compute the feedback from different spin/valley orders on fermions and argue that each order splits the initially 4-fold degenerate van Hove peak in a particular fashion, consistent with the STM data and compressibility measurements, providing a unified interpretation of the cascade of transitions in twisted bilayer graphene. Our results follow from a generic analysis of an SU(4)-symmetric Landau functional and are valid beyond a specific underlying fermionic model. We argue that an analogous van Hove scenario explains the cascade of phase transitions in non-twisted Bernal bilayer and rhombohedral trilayer graphene.