Spontaneous Valley Spirals in Magnetically Encapsulated Twisted Bilayer Graphene

TL;DR

This paper explores how twisted bilayer graphene encapsulated between ferromagnetic insulators develops flat bands and valley-related correlated states, revealing new physics driven by the interplay of twist, magnetism, and spin-orbit effects.

Contribution

It introduces a novel platform using magnetic heterostructures to induce and control valley-correlated states in twisted bilayer graphene.

Findings

Formation of flat bands due to twist and magnetic proximity effects.

Emergence of a spontaneous valley-mixed state with geometric frustration.

Electric bias controls valley anisotropy and correlated states.

Abstract

Van der Waals heterostructures provide a rich platform for emergent physics due to their tunable hybridization of electronic orbital- and spin-degrees of freedom. Here, we show that a heterostructure formed by twisted bilayer graphene sandwiched between ferromagnetic insulators develops flat bands stemming from the interplay between twist, exchange proximity and spin-orbit coupling. We demonstrate that in this flat-band regime, the spin degree of freedom is hybridized, giving rise to an effective triangular superlattice with valley as a degenerate pseudospin degree of freedom. Incorporating electronic interactions at half-filling leads to a spontaneous valley-mixed state, i.e., a correlated state in the valley sector with geometric frustration of the valley spinor. We show that an electric interlayer bias generates an artificial valley-orbit coupling in the effective model, controlling both the valley anisotropy and the microscopic details of the correlated state, with both phenomena understood in terms of a valley-Heisenberg model with easy-plane anisotropic exchange. Our results put forward twisted graphene encapsulated between magnetic van der Waals heterostructures as platforms to explore purely valley-correlated states in graphene.