Collective magnetic excitations in AA- and AB-stacked graphene bilayers

Abstract

We discuss novel transverse plasmon polaritons that are hosted by AA-and AB-stacked bilayer graphene due to perfect nesting. They are composed of oscillating counterflow currents in between the layers, giving a clear interpretation of these collective modes as magnetic excitations carrying magnetic moments parallel to the planes. For AA-stacked bilayer graphene, these modes can reach zero frequency at the neutrality point and we thus predict a symmetry-broken ground state leading to in-plane orbital ferromagnetism. Even though it could be hard to detect them in real solid-state devices, these novel magnetic plasmons should be observable in artificial setups such as optical lattices. Also, our results might be relevant for magic angle twisted bilayer graphene samples, as their electronic properties are mostly determined by confined AA-stacked regions.

Neutral Magic‐Angle Bilayer Graphene: Condon Instability and Chiral Resonances

“…where μ 0 denotes the magnetic permeability. [39] For AA-stacked graphene, this limit is reached due to the logarithmic divergence of the magnetic susceptibility. [39] However, the response of twisted bilayer graphene is generally too weak to reach the instability, i.e., including damping, one obtains D mag ¼ À6.6t e 2 ℏ 2 .…”

“…[39] For AA-stacked graphene, this limit is reached due to the logarithmic divergence of the magnetic susceptibility. [39] However, the response of twisted bilayer graphene is generally too weak to reach the instability, i.e., including damping, one obtains D mag ¼ À6.6t e 2 ℏ 2 . [17,18] Our refined calculations without damping now yield a significantly lower bound for θ¼ 1.1 ∘ with D mag ¼ À36t e 2 ℏ 2 .…”

“…These current densities are induced by an in-plane electric and magnetic field, respectively. [17,39] The chiral response involves the two current densities j 1…”

“…Another topic is related to the Condon instability [35] that has recently been discussed in related systems. [36][37][38][39] We will argue that the Condon instability arises in the continuum model [1,7] of twisted bilayer graphene at the neutrality point in the immediate vicinity of the magic angle.Apart from the above, the large counterflow or magnetic response has also been discussed in several articles. [7,17,40] Nevertheless, the importance of acoustic plasmonic modes has not received sufficient attention, so far.…”

“…Another topic is related to the Condon instability [35] that has recently been discussed in related systems. [36][37][38][39] We will argue that the Condon instability arises in the continuum model [1,7] of twisted bilayer graphene at the neutrality point in the immediate vicinity of the magic angle.…”