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 an approximate SU(4) symmetry of the Landau functional, we show that the structure of the spin/isospin order parameter changes with increasing filling via a cascade of transitions. We compute the feedback from different spin/isospin 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.
|Original language||English (US)|
|Journal||npj Quantum Materials|
|State||Published - Dec 2022|
Bibliographical noteFunding Information:
We thank E. Berg, A. Cherman, Z. Dong, R. Fernandes, F. Guinea, J. Hoffman, S. Ilani, P. Jarillo-Herrero, E. König, C. Lewandowski, L. Levitov, Y. Oreg, H. Polshyn, G. Tarnopolsky, O. Vafek, A. Vishwanath, A. Yazdani, A. Young, and E. Zeldov for fruitful discussions. We are indebted to S. Ilani, K. Nuckolls, A. Yazdani, and U. Zondiner for sharing their data with us. The work by D.V.C and A.V.C. was supported by U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0014402. Y.W. was supported by NSF under award number DMR-2045781. D.V.C. gratefully acknowledges support from Doctoral Dissertation and Larkin Fellowships at the University of Minnesota.
© 2022, The Author(s).