Hierarchy of Symmetry Breaking Correlated Phases in Twisted Bilayer Graphene

Abstract

Twisted bilayer graphene (TBG) near the magic twist angle of ∼1.1°exhibits a rich phase diagram.However, the interplay between different phases and their dependence on twist angle is still elusive.Here, we explore the stability of various TBG phases and demonstrate that superconductivity near filling of two electrons per moiré unit cell alongside Fermi surface reconstructions, as well as entropydriven high-temperature phase transitions and linear-in-T resistance occur over a range of twist angles which extends far beyond those exhibiting correlated insulating phases. In the vicinity of the magic angle, we also find a metallic phase that displays a hysteretic anomalous Hall effect and incipient Chern insulating behaviour. Such a metallic phase can be rationalized in terms of the interplay between interaction-driven deformations of TBG bands leading to Berry curvature redistribution and Fermi surface reconstruction. Our results provide an extensive perspective on the hierarchy of correlated phases in TBG as classified by their robustness against deviations from the magic angle or, equivalently, their electronic interaction requirements. TBG is a highly tunable platform for exploring the effects of strong electronic interactions and topological bands [1][2][3][4][5][6][7][8][9] . At the magic angle, i.e., when the strength of the interactions among electrons is maximized relative to their kinetic energy, pronounced signatures of correlated phases emerge 1,2,10 . Away from the magic angle, the effective interaction strength is reduced and the correlated phases are believed to disappear rapidly. However, despite the strong impact of the twist angle on the phase diagram of nearly-magic TBG, this dependence is still experimentally under-explored. Here we report systematic measurements on multiple devices covering a wide range of twist angles between 0.79°and 1.23°(see Supplementary Table I for an overview) and examine the overall impact of twist angle, and thus strength of interactions, on the phase

Heavy quasiparticles and cascades without symmetry breaking in twisted bilayer graphene

“…The incoherent spectral weight is resilient with temperature, becoming broadened and less defined around integer values. This is consistent with experimental findings which show that broadened cascade signatures persist to at least 50 K [17,18].…”

“…The cascades of spectral weight in Fig. 1b and c look remarkably similar to the ones observed in STM experiments [10,18]. This strong similarity imply that the cascades are not the consequence of a symmetry breaking order, prohibited in our calculations.…”

“…The cascade phenomena have been detected up to much higher temperatures than the superconductivity, the correlated insulators and the anomalous Hall effects [17,18], and they are also found in a wider range of angles, suggesting that the cascades constitute the parent state in which these low temperature phenomena emerge.…”

Promotion of superconductivity in magic-angle graphene multilayers

“…Moreover, our findings suggest a scenario in which the symmetrybroken n = ±2 state strongly favors the formation of the superconducting state whereas the cascade corresponding to n = ±3 suppresses it. Notably, this scenario is consistent not only with previous TBG (2,9,12,13,33,34) and TTG (3,4,8) observations but also in part with the recently investigated ABC trilayers (35) and Bernal bilayers with and without (36) a WSe 2 substrate (37) in which superconductivity is observed near or within phases with two out of four flavors being predominantly filled. These commonalities suggest that symmetry-broken states with similar types of polarization underlie superconductivity in all of these graphene-based superconductors.…”

Cascade of transitions in twisted bilayer graphene -- the Van Hove scenario

“…In either scenario, the Luttinger theorem states that the splitting due to spin/isospin orders can lead to the formation of insulating states only at integer fillings. In a recent experimental study [43] the authors argued that the cascade of transitions in TBG is present in a range of twist angles, even when there are no insulating states near integer fillings. These results lend further support to our Van Hove-based scenario of the cascade of phase transitions in TBG.…”