 Award ID(s):
 1916958
 NSFPAR ID:
 10207501
 Date Published:
 Journal Name:
 SciPost Physics Core
 Volume:
 3
 Issue:
 2
 ISSN:
 26669366
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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Abstract Motivated by recent experimental work on moiré systems in a strong magnetic field, we compute the compressibility as well as the spin correlations and Hofstadter spectrum of spinful electrons on a honeycomb lattice with Hubbard interactions using the determinantal quantum Monte Carlo method. While the interactions in general preserve quantum and anomalous Hall states, emergent features arise corresponding to an antiferromagnetic insulator at halffilling and other incompressible states following the Chern sequence ± (2 N + 1). These odd integer Chern states exhibit strong ferromagnetic correlations and arise spontaneously without any external mechanism for breaking the spinrotation symmetry. Analogs of these magnetic states should be observable in general interacting quantum Hall systems. In addition, the interacting Hofstadter spectrum is qualitatively similar to the experimental data at intermediate values of the onsite interaction.more » « less

null (Ed.)A major recent breakthrough in materials science is the emergence of intrinsic magnetism in twodimensional (2D) crystals, which opens the door to more cuttingedge fields in the 2D family and could eventually lead to novel datastorage and information devices with further miniaturization. Herein we propose an experimentally feasible 2D material, Fe 2 I 2 , which is an intrinsic roomtemperature ferromagnet exhibiting perpendicular magnetic anisotropy (PMA). Using firstprinciples calculations, we demonstrate that singlelayer (SL) Fe 2 I 2 is a spingapless semiconductor with a spinpolarized Dirac cone and linear energy dispersion in one spin channel, exhibiting promising dissipationless transport properties with a Fermi velocity up to 6.39 × 10 5 m s −1 . Our results reveal that both strain and ferroelectric polarization switching could induce an outof to inplane spin reorientation in the 2D Fe 2 I 2 layer, revealing its advantage in assembling spintronic devices. In addition, spin–orbit coupling (SOC) triggers a topologically nontrivial band gap of 301 meV with a nonzero Chern number ( C  = 2), giving rise to a robust quantum anomalous Hall (QAH) state. The 2D crystal also exhibits high carrier mobilites of 0.452 × 10 3 and 0.201 × 10 3 cm 2 V −1 s −1 for the electrons and holes, respectively. The combination of these unique properties renders the 2D Fe 2 I 2 ferromagnet a promising platform for high efficiency multifunctional spintronic applications.more » « less

Abstract Flat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries. Twisted monolayerbilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving firstorder topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.

Electrons in moiré flat band systems can spontaneously break timereversal symmetry, giving rise to a quantized anomalous Hall effect. In this study, we use a superconducting quantum interference device to image stray magnetic fields in twisted bilayer graphene aligned to hexagonal boron nitride. We find a magnetization of several Bohr magnetons per charge carrier, demonstrating that the magnetism is primarily orbital in nature. Our measurements reveal a large change in the magnetization as the chemical potential is swept across the quantum anomalous Hall gap, consistent with the expected contribution of chiral edge states to the magnetization of an orbital Chern insulator. Mapping the spatial evolution of fielddriven magnetic reversal, we find a series of reproducible micrometerscale domains pinned to structural disorder.

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