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.
We use self-consistent Hartree-Fock calculations performed in the full π-band Hilbert space to assess the nature of the recently discovered correlated insulator states in magic-angle twisted bilayer graphene (TBG). In single spin/valley-flavor models we find two closely competing gapped states, one of which breaks the system's valley projected C2T symmetry and produces moiré bands with finite Chern numbers. Broken spin/valley flavor symmetries then enable gapped states to form not only at neutrality but also at total moiré band filling n = ±p/4 with integer p = −3, . . . , 3. We predict that the magic-angle TBG insulating states at n = ±1/4 and n = ±3/4 can exhibit a quantized anomalous Hall effect.
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,
Solitons, localized at the interface between a linear magneto-optic half-space and a nonlinear optical medium, are investigated with the aid of an approximate model validated by direct numerical simulations. The interface is located in a planar waveguide, and is characterized not only by the magneto-optic properties but also by a linear refractive index discontinuity. The analysis is based upon a global envelope equation and uses waveguide and magneto-optic parameters that are averaged over the whole waveguide structure. A variational analysis shows that, even though surface solitons can be localized in deeply stable stationary states, they can also be generated in parts of the parameter space that facilitate movement between stable and unstable regions. This leads to suggestions for isolator and multi-switch operation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.