Symmetry-broken Chern insulators and Rashba-like Landau-level crossings in magic-angle bilayer graphene

Abstract

Flat-bands in magic angle twisted bilayer graphene (MATBG) have recently emerged as a rich platform to explore strong correlations, superconductivity and magnetism. However, the phases of MATBG in magnetic field, and what they reveal about the zero-field phase diagram remain relatively unchartered. Here we use magnetotransport and Hall measurements to reveal a rich sequence of wedge-like regions of quantized Hall conductance with Chern numbers C = ±1, ±2, ±3, ±4 which nucleate from integer fillings of the moiré unit cell 𝜈 = ±3, ±2, ±1, 0 correspondingly. We interpret these phases as spin and valley polarized Chern insulators, equivalent to quantum Hall ferromagnets. The exact sequence and correspondence of Chern numbers and filling factors suggest that these states are driven directly by electronic interactions which specifically break time-reversal symmetry in the system. We further study quantum magneto-oscillation in the yet unexplored higher energy dispersive bands with a Rashba-like dispersion. Analysis of Landau level crossings enables a parameter-free comparison to a newly derived "magic series" of level crossings in magnetic field and provides constraints on the parameters w0 and w1 of the Bistritzer-MacDonald MATBG Hamiltonian. Overall, our data provides direct insights into the complex nature of symmetry breaking in MATBG and allows for quantitative tests of the proposed microscopic scenarios for its electronic phases.

Competing correlated states and abundant orbital magnetism in twisted monolayer-bilayer graphene

“…3 do not follow a simple progression with ν and B, reflecting the rich competition between the gapped single-particle Hofstadter subbands and the tendency towards spontaneous flavor polarization into a subset of these bands (see Methods for additional discussion). This is similar to recent observations in tBLG [27][28][29][30][31][32] , however, here we observe a more complicated sequence of symmetry breaking in which various flavor-polarized and unpolarized ground states closely compete. For example, in Fig.…”

Importance of long-ranged electron-electron interactions for the magnetic phase diagram of twisted bilayer graphene

“…Next, we consider the effect of doping. While at ν = −1 insulating states are not often observed in experiments, some signatures of insulating states have been found at ν = +1 [4,8,14,20]. This is consistent with our Hartree+U results, which yield lower values of U c for ν = +1 than for ν = −1.…”

“…At ν = +3, a strong insulating state is observed in experiments, especially when the tBLG is aligned with the hexagonal boron nitride substrate [15,16]. In contrast, the ν = −3 insulating state is almost never observed [1,4,5,20]. For insulating phases to emerge at these doping levels both valley and spin symmetries must be broken; i.e., the insulating state must be FM [15,16].…”

“…Since the discovery of superconductivity in proximity to correlated insulator states at half (electron or hole) filling of the flat bands [1,2], there has been great interest in the electronic properties of magic-angle twisted bilayer graphene (tBLG) [3]. Additional experiments [4][5][6][7][8][9][10] discovered correlated insulator phases and superconductivity at other doping levels of the flat bands and revealed a wide range of interesting phenomena [11,12] including strange metal behavior [13,14], ferromagnetic order [15,16], superconductivity without correlated insulators [17][18][19], Chern insulators [20][21][22], and nematic order [6,[23][24][25].…”

Realization of topological Mott insulator in a twisted bilayer graphene lattice model

“…T wisted bilayer graphene (TBG) burst on the scene as a tunable two carbon-atom layers thick system realizing a remarkable multitude of interaction-driven macroscopic quantum phenomena [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . Although significant progress has been achieved in understanding the nontrivial topology of the narrow bands, as well as the correlated electron states in the magic-angle TBG , many important questions remain open.…”