Skyrmion stripes in twisted double bilayer graphene

TL;DR

This paper explores the complex interplay of topology and electron correlations in twisted double bilayer graphene, revealing novel skyrmion stripe phases with broken symmetries at fractional fillings.

Contribution

It uncovers the emergence of skyrmion stripe phases with broken $C_3$ symmetry in TDBG at fractional fillings, a novel state not previously identified.

Findings

Partially valley polarized ground states at fractional fillings.

Presence of textured spin order indicative of skyrmions.

Spontaneous breaking of $C_3$ symmetry in the stripe phase.

Abstract

Two dimensional moir\'e systems have recently emerged as a platform in which the interplay between topology and strong correlations of electrons play out in non-trivial ways. Among these systems, twisted double bilayer graphene (TDBG) is of particular interest as its topological properties may be tuned via both twist angle and applied perpendicular electric field. In this system, energy gaps are observed at half filling of particular bands, which can be associated with correlated spin polarized states. In this work, we investigate the fate of these states as the system is doped away from this filling. We demonstrate that, for a broad range of fractional fillings, the resulting ground state is partially valley polarized, and supports multiple broken symmetries, including a textured spin order indicative of skyrmions, with a novel $\textit{stripe}$ ordering that spontaneously breaks $C_3$ symmetry. Experimental signatures of this state are discussed.