Abstract
We investigate magnetic instabilities in charge-neutral twisted bilayer graphene close to so-called ``magic angles’’ using a combination of real-space Hartree-Fock and dynamical mean-field theories. In view of the large size of the unit cell close to magic angles, we examine a previously proposed rescaling that permits to mimic the same underlying flat minibands at larger twist angles. We find that localized magnetic states emerge for values of the Coulomb interaction UU that are significantly smaller than what would be required to render an isolated layer antiferromagnetic. However, this effect is overestimated in the rescaled system, hinting at a complex interplay of flatness of the minibands close to the Fermi level and the spatial extent of the corresponding localized states. Our findings shed new light on perspectives for experimental realization of magnetic states in charge-neutral twisted bilayer graphene.
Highlights
Since the experimental discovery of graphene [1], two-dimensional materials have been at the focus of intensive research in condensed-matter physics, among others because they bear great promise for technological applications, see, e.g., Refs. [2, 3]
As an alternative to mean-field theory (MFT), one could determine the instabilities of the paramagnetic state with a random-phase approximation (RPA) analysis [37], and we present results from such an RPA analysis in appendix A
We have investigated the onset of magnetism in charge-neutral “magic-angle” twisted bilayer graphene with numerical real-space static and dynamical mean-field approaches
Summary
Since the experimental discovery of graphene [1], two-dimensional materials have been at the focus of intensive research in condensed-matter physics, among others because they bear great promise for technological applications, see, e.g., Refs. [2, 3]. A twist appeared in the field when superconducting and correlated insulating states were discovered in experiments on bilayer graphene where one layer is rotated with respect to the other by a so-called “magic” angle [6,7], see Fig. 1(a) for an illustration of such a “twisted” honeycomb bilayer, Ref. We reexamine the one-band Hubbard model for twisted bilayer graphene (TBG) and demonstrate that magnetism occurs in the charge-neutral (half-filled) system at low values of the on-site Coulomb interaction U, placing magnetic states, including an antiferromagnetic one, among the competitors for the instabilities in charge-neutral magic-angle twisted bilayer graphene.
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