Charge order and broken rotational symmetry in magic-angle twisted bilayer graphene

Abstract

The discovery of correlated electronic phases, including Mott-like insulators and superconductivity, in twisted bilayer graphene (TBLG) near the magic angle 1-4 , and the intriguing similarity of their phenomenology to that of the high-temperature superconductors, has spurred a surge of research to uncover the underlying physical mechanism 5-9 . Local spectroscopy, which is capable of accessing the symmetry and spatial distribution of the spectral function, can provide essential clues towards unraveling this puzzle. Here we use scanning tunneling microscopy (STM) and spectroscopy (STS) in magic angle TBLG to visualize the local density of states (DOS) and charge distribution. Doping the sample to partially fill the flat band, where low temperature transport measurements revealed the emergence of correlated electronic phases, we find a pseudogap phase accompanied by a global stripe charge-order whose similarity to high-temperature superconductors 10-16 provides new evidence of a deeper link underlying the phenomenology of these systems.

Prominent Cooper pairing away from the Fermi level and its spectroscopic signature in twisted bilayer graphene

“…The counterpart for electron doping (n > 0) exhibits a lower T c as has also been measured [11]. Near half-filling (n (0) ) where insulating states are observed [9,11,35,36], we find T c = 0. This is not surprising since particle-hole pairing is not included in our model.…”

Electronic band structure and pinning of Fermi energy to Van Hove singularities in twisted bilayer graphene: A self-consistent approach

“…[6] and the experimental evidence of fillinginduced symmetry-broken phases [7] emphasize the role of the carrier density in driving the electronic interactions. Note, finally, that the peaks due to van Hove singularities in the density of states can be observed directly by scanning tunneling spectroscopy [8,9,38] As shown in Ref. [39], the long-range Coulomb interactions in TBG away from CN may be comparable or larger than the bandwidth, giving rise to an inhomogeneous Hartree potential which substantially modifies the shape of the electronic bands.…”

“…Even more recent experiments have revealed unexpected features of magic-angle TBG close to CN, such as the presence large spectral gaps at fractional filling [6], the appearance of alternating superconducting and insulating phases on an increase of the occupation of the conduction bands [7] as well as evidence of charge ordering and broken rotational symmetry [8,9].…”

Flat Chern Band from Twisted Bilayer MnBi2Te4

“…In addition, flat valley Chern bands can be realized in tBLG with aligned hBN substrate [41][42][43], twisted double bilayer graphene [44][45][46], ABC trilayer graphene on hBN [47][48][49] and twisted bilayer transition metal dichalcogenides [50,51], etc. The small bandwidths make electron-electron interactions important [52][53][54][55][56][57][58][59], and further lead to intriguing interacting phases in experiments including superconductivity, correlated insulator and QAH effect.So far, all of the experimental Moiré systems are timereversal (TR) invariant at the single particle level, thus the total Chern number always equals to zero. Therefore, even with flat bands, it is difficult to achieve TR breaking interacting topological states such as the FCI in these systems.…”