Abstract
Twistronics is currently one of the most active research fields in condensed matter physics, following the discovery of correlated insulating and superconducting phases in twisted bilayer graphene (tBLG). Here, we present a magnonic analogue of tBLG. We study magnons in twisted ferromagnetic bilayers (tFBL) with collinear magnetic order, including exchange and weak Dzyaloshinskii-Moriya interactions (DMI). For negligible DMI, tFBL presents discrete magnon magic angles and flat moiré minibands analogous to tBLG. The DMI, however, changes the picture and renders the system much more exotic. The DMI in tFBL induces a rich topological magnon band structure for any twist angle. The twist angle turns to a control knob for the magnon valley Hall and Nernst conductivities. Gapped flat bands appear in a continuum of magic angles in tFBL with DMI. In the lower limit of the continuum, the band structure reconstructs to form several topological flat bands. The luxury of twist-angle control over band gaps, topological properties, number of flat bands, and valley Hall and Nernst conductivities renders tFBL a novel device from fundamental and applied perspectives.
Highlights
A interesting class of bilayer graphene is the twisted bilayer graphene, presenting moiré Bloch bands as a result of the twist. tBLG was found to present fascinating electronic and optical properties, giving rise to novel physics that is completely absent in AB stacked bilayer g raphene[26,27,28,29,30,31,32,33]
Unlike tBLG, its magnetic twin with Dzyaloshinskii-Moriya interactions (DMI) presents a continuum of magic angles and numerous
We start with a ferromagnetic honeycomb monolayer (Fig. 1a) with nearest neighbor exchange and nearest neighbors DMI
Summary
Two-dimensional (2D) materials with intrinsic magnetism has recently been r ealized[1,2], opening new horizons in 2D material research[3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. Unlike tBLG, its magnetic twin with DMI presents a continuum of magic angles and numerous The magnon bands valley Berry curvatures and valley Chern numbers are sensitive to the twist angle and the DMI strength. The valley thermal magnon Hall and Nernst conductivities induced by the multiple topological flat bands show a complex and exotic response to the twist angle.
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