Moiréless correlations in ABCA graphene

Abstract

Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene––a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3–5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.

Spin-fluctuation-induced pairing in twisted bilayer graphene

“…Introduction. Twisted two-dimensional materials have become an extremely active research area, with twisted bilayer [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], trilayer [18] and double bilayer graphene [19][20][21][22][23][24][25], transition metal dichalcogenides homo-and heterobilayers [26][27][28][29] as well as other materials [30][31][32] at the frontier of condensed matter research. These systems are fascinating due to a high degree of band-structure and correlation engineering that can be achieved, putting unprecedented topological and exotic correlated states within experimental reach [33].…”

Ultra-Strong Spin-Orbit Coupling and Topological Moiré Engineering in Twisted ZrS2 Bilayers

“…Twisted van der Waals heterostructures have recently emerged as an intriguing and highly tunable platform to realize unconventional electronic phases in two dimensions [1][2][3][4] and more. Spurred by the discovery of Mott insulation and superconductivity in twisted bilayer graphene [5,6], remarkable progress in fabrication and twist-angle control has lead to observations of correlated insulating states or superconductivity in a variety of materials, including trilayer and double-bilayer graphene, homo-and hetero-bilayers of twisted transition metal dichalcogenides [7][8][9][10][11][12][13][14][15][16][17][18], and heterostructures at a twist on hexagonal boron nitride substrates [19,20]. At its heart, this rich phenomenology stems from electronic interference effects due to the moiré superlattice, which can selectively quench kinetic energy scales to realize almost dispersionless bands, permitting a twist angle controlled realization of regimes dominated by strong electronic interactions.…”

Stacking and gate tunable topological flat bands, gaps and anisotropic strip patterns in twisted trilayer graphene

“…Unlike the systems with a single moire twist interface like tBG, tDBG, or tMBG, in twisted trilayer graphene with finite successive interlayer twist angles we have two interfaces giving rise to double moire patterns. When these moire patterns are mutually incommensurate they give rise to supermoire patterns [36][37][38], also called moire of moire patterns [24,39,40] that can multiply the features in the electronic structure, while strongest double moire interference happen for commensurate patterns, exemplified by the large secondary band gaps in graphene encapsulated by hexagonal boron nitride [38]. * jeiljung@uos.ac.kr Commensurate double moire trilayer graphene with a middle layer twist [22, 23, 25-27, 33, 35, 41-44], called here simply twisted trilayer graphene (tTG), has emerged as a system of renewed interest thanks to the observation of moire flat band superconductivity with a critical temperature higher than tBG.…”