Competing Zero-Field Chern Insulators in Superconducting Twisted Bilayer Graphene

Abstract

The discovery of magic angle twisted bilayer graphene (MATBG) has unveiled a rich variety of superconducting, magnetic and topologically nontrivial phases. The existence of all these phases in one material, and their tunability, has opened new pathways for the creation of unusual gate tunable junctions. However, the required conditions for their creation -gate induced transitions between phases in zero magnetic field -have so far not been achieved. Here, we report on the first experimental demonstration of a device that is both a zero-field Chern insulator and a superconductor.The Chern insulator occurs near moiré cell filling factor = +1 in a hBN non-aligned MATBG device and manifests itself via an anomalous Hall effect. The insulator has Chern number C= ±1 and a relatively high Curie temperature of Tc ≈ 4.5 K. Gate tuning away from this state exposes strong superconducting phases with critical temperatures of up to Tc ≈ 3.5 K. In a perpendicular magnetic field above B > 0.5 T we observe a transition of the = +1 Chern insulator from Chern number C = ±1 to C = 3, characterized by a quantized Hall plateau with Ryx = h/3e 2 . These observations show that interaction-induced symmetry breaking in MATBG leads to zero-field ground states that include almost degenerate and closely competing Chern insulators, and that states with larger Chern numbers couple most strongly to the B-field. By providing the first demonstration of a system that allows gate-induced transitions between magnetic and superconducting phases, our observations mark a major milestone in the creation of a new generation of quantum electronics.Recently discovered quantum phases in the flat-bands of θm~1.1° magic angle twisted bilayer graphene (MATBG) include correlated insulators 1-5 (CI), superconductors 2,6-16 (SC), and interaction induced correlated Chern insulators [17][18][19][35][36][37][38]. The CCIs can occur with different Chern numbers, and have U(4) valley/spin ferromagnetism in the bulk and topologically protected states at device edges. The search for the exact nature of these exotic phases [20][21][22][23] and the competition 10,[24][25][26][27][28][29] between them requires a complete understanding of the role of electronic interactions in the symmetry breaking of the non-interacting 4-fold spin and valley degenerate bands. The existence of multiple correlated phases in one materials platform opens up new possibilities for the creation of complex gate tunable junctions 30,31 . Among these the most interesting are junctions between superconducting and topological magnetic phases. The clean, gate defined homojunctions of these phases could pave new avenues for the creation of topological and spin-triplet superconductivity, as well as non-abelian particles, such as parafermions and Majorana fermions 32 . However, the necessary requirements for such junctions,

Anomalous Hall effect at half filling in twisted bilayer graphene

“…Magic-angle twisted bilayer graphene (tBLG) has been studied extensively owing to its wealth of symmetry-broken phases, correlated Chern insulators, orbital magnetism, and superconductivity [1][2][3][4][5][6][7][8]. In particular, the anomalous Hall effect (AHE) has been observed at odd integer filling factors (ν = 1 and 3) in a small number of tBLG devices [5,6,9], indicating the emergence of a zero-field orbital magnetic state with spontaneously broken time-reversal symmetry [10][11][12]. However, the AHE is typically not anticipated at half filling (ν = 2) owing to competing intervalley coherent states [10,[13][14][15][16], as well as spin-polarized and valley Hall states that are favored by an intervalley Hund's coupling [10,[16][17][18].…”

Moiré Engineering of Spin-Orbit Coupling in Twisted Platinum Diselenide

“…A vast amount of theoretical work predicts correlation effects for an even larger subspace of the possible twisted graphitic moiré systems [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. In addition, in twisted bilayer graphene, control over topological properties has already been demonstrated [49][50][51][52][53][54][55][56][57][58] showing correlated Chern insulating phases.…”

Moiré lattice effects on the orbital magnetic response of twisted bilayer graphene and Condon instability

“…The existence and competition between different ferromagnetic and Chern insulating phases depends crucially on the alignment to the substrate [14], the presence of a magnetic field or strain [15][16][17][18]. Intrinsic Chern orbital phases have also be observed in TBG [19][20][21]. The nature of the ground state emerges from an intricate competition governed by Coulomb interaction, kinetic energy and topology, with many symmetry-breaking states of nearby energy [22][23][24][25][26][27][28][29][30][31][32].…”