Abstract
We propose multilayer moiré structures in strong external magnetic fields as a novel platform for realizing highly tunable, frustrated Hubbard physics with topological order. Identifying the layer degree of freedom as a pseudospin allows us to retain SU(2) symmetry while controlling ring-exchange processes and concurrently quenching the kinetic energy by large external magnetic fields. This way, a broad class of interacting Hubbard-Hofstadter states and their transitions can be studied. Remarkably, in the limit of strong interactions the system becomes Mott insulating and we find chiral pseudospin-liquid phases which are induced by the magnetic field. We find that this topologically ordered state remains exceptionally stable toward relevant perturbations. We discuss how layer pseudospin can be probed in near-term experiments. As the magnetic flux can be easily tuned in moiré systems, our approach provides a promising route toward the experimental realization and control of topologically ordered phases of matter.
- Received 15 September 2022
- Revised 6 February 2024
- Accepted 8 March 2024
DOI:https://doi.org/10.1103/PhysRevX.14.021013
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Spin liquids are exotic quantum phases in which the electron spins do not settle into any order when cooled—rather, they continue to fluctuate. This in turn leads to unusual properties like fractionalized charges and closely connects these states to fundamental theories of nature, such as electromagnetism. Furthermore, spin liquids have the potential to provide a basis for robust quantum computations. Despite the significance of spin liquids, experimental observations have remained elusive due to the fragility of such states and their being difficult to characterize. In this work, we analyze how carefully designed multilayer structures of 2D materials may overcome some of these obstacles.
Instead of working with intrinsic electronic angular momentum, which can point in two opposite directions, we focus on situations in which electrons can reside in two distinct layers of the material. This “layer index” acts as a pseudospin and allows us to introduce chirality—or a preferred directionality—into the system using external magnetic fields without polarizing the layer pseudospin of the electrons. We find that magnetic fields stabilize a particularly robust chiral spin liquid, which is a bosonic analog of a fractional quantum Hall state (in which electronic excitations carry particular, fractional electric charges). Not only does this spin liquid remain stable across vast parameter regimes, but it also persists in the presence of experimentally relevant perturbations.
The electronic nature of the layer pseudospin implies that many observables are easier to access than their electronic spin counterparts. In addition, our study suggests the existence of various continuous phase transitions in the phase diagram. Investigating these transitions will provide us with a better understanding of how spin-liquid phases destabilize.
