Hysteretic Quantum Hall Ferromagnetism in electronically decoupled twisted bilayer graphene

Twisted bilayer graphene (TBLG) at higher twist angles far from the magic angle TBLG forms a bilayer two-dimensional electron/hole gas system with sub-nm layer separation. The momentum mismatch between the Dirac cones of the individual layers leads to a breakdown of interlayer coherence, removing the layer degree of freedom from the spin and valley degrees of freedom. This results in an eight-fold degeneracy in the Landau levels (LL). Here, we discuss the broken symmetry quantum Hall (QH) states originating from the SU(4) isospin space, encompassing spin and valley degrees of freedom in the individual graphene layers. The longitudinal magnetoresistance (R_xx) minima and corresponding QH plateaus were observed at total filling factors (ν_tot) in multiples of 4, with certain missing values of ν_tot attributed to partial fillings of LLs. Analysis of Shubnikov-de Haas (SdH) oscillations and their corresponding fast Fourier transform (FFT) reveals two beating frequencies, indicative of charge carrier imbalance between the layers. Additionally, finite hysteresis in dual-sweep measurements of R_xx and R_xy is observed, which is associated with the predicted formation of Kekulé structure distortions of valley skyrmion textures near charged defects. These textures give rise to quantum Hall ferromagnetism (QHF) in the system.These findings underscore the intricate interplay of interlayer coupling, symmetry breaking, and many-body interactions in TBLG, advancing the understanding of its electronic properties and quantum phenomena under high magnetic fields.