Magnetism and Topology in Twisted Graphene Heterostructures
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Magnetism and Topology in Twisted Graphene Heterostructures

Abstract

Graphene, a crystalline atomic monolayer of carbon atoms, is a model system as the regularity and cleanliness of its lattice enables precise descriptions of its properties. Theoretical models predict that the behavior of electrons in a heterostructure consisting of two stacked graphene monolayers with a slight rotational misalignment in their lattices is dictated by interactions and topology. We investigate a twisted bilayer sample with both electrical resistivity measurements and nanoSQUID on tip microscopy. The latter, a novel magnetic imaging method developed as part of this dissertation, is uniquely well matched to studying the dilute magnetic signals expected in twisted graphene heterostructures.

We observe the emergence of a quantized anomalous Hall effect in twisted bilayer graphene aligned to hexagonal boron nitride with Hall resistance is quantized to within 0.1\% of the von Klitzing constant h/e^2 at zero magnetic field. In contrast to magnetically doped (Bi,Sb)_2Te_3 quantum anomalous Hall variants, intrinsic strong correlations polarize the electrons into a single valley resolved miniband with Chern number C=1 arising from inversion symmetry breaking and the formation of a moir e: the system does not host band inversion or spin orbit coupling. The measured transport energy gap $\Delta/k_B\approx 27$ K, the largest observed to date, is almost four times the Curie temperature for magnetic ordering $T_C\approx 7.5$ K. We find that electrical currents as small as 1 nA can be used to controllably switch the magnetic order between states of opposite polarization, forming an electrically rewritable magnetic memory. Magnetic imaging reveals a magnetization primarily orbital in nature dominated by chiral edge state contributions from the topological gap of the quantum anomalous hall phase. Mapping the spatial evolution of field-driven magnetic reversal, we find a series of reproducible micron scale domains, pinned to structural disorder, whose boundaries host chiral edge states.

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