Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene

Abstract

Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators 1 . When twisted close to a magic relative orientation angle near !°, bilayer graphene has flat moiré superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics 2-5 , notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moiré band filling factors # = %, ±!, ±(, ±), and superconductivity below critical temperatures as high as ~ 3 K close to -2 filling. We also observe three new superconducting domes at much lower temperatures close to the # = % and # = ±! insulating states. Interestingly, at # = ±! we find states with non-zero Chern numbers. For # = −! the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moiré flat bands, including near charge neutrality.

Symmetry breaking and skyrmionic transport in twisted bilayer graphene

“…In tBLG, C 2v T symmetry (with T being time-reversal) needs to be spontaneously broken in order to generate a mean-field gap at charge neutrality. Self-consistent Hartree-Fock studies have found that this indeed happens for certain interaction strengths and twist angles [3,26,31]. It was found that the C 2v T symmetry breaking self-consistent Hartree-Fock solutions are very susceptible to C 3v breaking strain [31], an observation which agrees with the STM and transport experiments [32].…”

“…A series of recent experimental breakthroughs has uncovered surprising and fascinating correlated electron phenomena in two-dimensional van der Waals Moiré materials. Transport experiments on twisted bilayer graphene [1][2][3], ABC trilayer graphene on hexagonal Boron-Nitride (hBN) [4,5], and twisted double bilayer graphene [6][7][8] show evidence of insulating states around charge neutrality at electron fillings for which no singleparticle band-gap is expected. To make the story even more interesting, superconducting domes flanking some of these insulating states were observed [2,3,6,8,9].…”

Flat Chern Band from Twisted Bilayer MnBi2Te4

“…Experimentally realizing such states is, however, challenging because flat topological electronic bands are generally required for electron-electron interactions to manifest.Recently, it is shown that Moiré superlattices in twisted or lattice mismatched two-dimensional (2D) materials can give rise to flat topological bands. A prime example is twisted bilayer graphene (tBLG) [32][33][34][35], where the lowest two bands carry a fragile topology [36][37][38][39][40] and become flat near the magic twist angle θ ≈ 1.1 • . 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.…”

General continuum model for twisted bilayer graphene and arbitrary smooth deformations

“…Using now Eqs. (6,7,8) in Eq. (4) and keeping terms to linear order in displacement gradients, we obtain finally…”

“…The experimental discovery of correlated insulators and superconductivity in magic angle Twisted Bilayer Graphene (TBG) [2,3] has galvanized experimental research on these structures [4][5][6][7][8][9][10]. The bedrock for theory in these systems is the seminal work by Bistritzer and Macdonald [1] (BM), who developed a continuum model (CM) for TBG, and by solving it, predicted the presence of magic angles at which the bands closest to the Fermi energy become anomalously flat.…”