Unconventional sequence of correlated Chern insulators in magic-angle twisted bilayer graphene

Abstract

The interplay between strong electron-electron interactions and band topology can lead to novel electronic states that spontaneously break symmetries. The discovery of flat bands in magic-angle twisted bilayer graphene (MATBG) [1][2][3] with nontrivial topology [4][5][6][7] has provided a unique platform in which to search for new symmetry-broken phases. Recent scanning tunneling microscopy 8,9 and transport experiments [10][11][12][13] have revealed a sequence of topological insulating phases in MATBG with Chern numbers C=±3, ±2, ±1 near moiré band filling factors 𝜈 = ±1, ±2, ±3, corresponding to a simple pattern of flavor-symmetrybreaking Chern insulators. Here, we report high-resolution local compressibility measurements of MATBG with a scanning single electron transistor that reveal a new sequence of incompressible states with unexpected Chern numbers observed down to zero magnetic field. We find that the Chern numbers for eight of the observed incompressible states are incompatible with the simple picture in which the C=±1 bands are sequentially filled. We show that the emergence of these unusual incompressible phases can be understood as a consequence of broken translation symmetry that doubles the moiré unit cell and splits each C=±1 band into a C=±1 band and a C=0 band. Our findings

Field-tuned and zero-field fractional Chern insulators in magic angle graphene

“…More importantly, at non-zero integer fillings, the dispersion is modified due to a Hartree term arising from the filled bands, which gives rise to a significant bandwidth. For instance, at ν = 3, the electron dispersion posses a strong dip at the Γ point with a total bandwidth of ∼ 30 meV [40,42,[46][47][48][49], which is comparable to or even exceeds the interaction scale. One goal of this work is to investigate how this dispersion of TBG affects the possibility of FCI phases.…”

From electrons to baby skyrmions in Chern ferromagnets: A topological mechanism for spin-polaron formation in twisted bilayer graphene

“…H (k) is characterized by a dip at the Γ point and thus have a qualitatively similar shape to the Fock term F (k) up to overall scaling. Thus for ν > 0, the two terms add, leading to a large bandwidth while for ν < 0 they subtract leading to a reduced bandwidth [31,47,69,70]. The minimum bandwidth is realized for ν ≈ −1 to −1.5 depending on the value of κ as shown by the dashed line in Fig.…”

“…is the Hartree potential [31,45,47,[68][69][70]. This form of the dispersion directly allows us to compare our results to TBG since the quasiparticle dispersion…”

“…approximately describes the dispersion of electron (hole) bands for a correlated insulator at integer filling ν (−ν) [31,44,45,47,69,70]. Although the expression ( 15) is only valid for integer ν, we will choose to take ν to be a continuous variable which allows us to continuously tune the band dispersion.…”

Moiré Engineering of Spin-Orbit Coupling in Twisted Platinum Diselenide

“…A vast amount of theoretical work predicts correlation effects for an even larger subspace of the possible twisted graphitic moiré systems [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. In addition, in twisted bilayer graphene, control over topological properties has already been demonstrated [49][50][51][52][53][54][55][56][57][58] showing correlated Chern insulating phases.…”