Twist engineering of the two-dimensional magnetism in double bilayer chromium triiodide homostructures

Abstract

Twist engineering, or the alignment of two-dimensional (2D) crystalline layers with desired orientations, has led to tremendous success in modulating the charge degree of freedom in heteroand homo-structures, in particular, in achieving novel correlated and topological electronic phases in moiré electronic crystals 1,2 . However, although pioneering theoretical efforts have predicted nontrivial magnetism 3,4 and magnons 5 out of twisting 2D magnets, experimental realization of twist engineering spin degree of freedom remains elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide (CrI3) to fabricate twisted double bilayer homostructures with tunable twist angles and demonstrate the successful twist engineering of 2D magnetism in them. Using linear and circular polarization-resolved Raman spectroscopy, we identify magneto-Raman signatures of a new magnetic ground state that is sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3. With careful magnetic field and twist angle dependence, we reveal that, for a very small twist angle (~ 0.5 o ), this emergent magnetism can be well-approximated by a weighted linear superposition of those of 2L and 4L CI3 whereas, for a relatively large twist angle (~ 5 o ), it mostly resembles that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1 o ), its magnetism cannot be simply inferred from the 2L and 4L cases, because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3 and features a dramatic Raman circular dichroism that is absent in natural 2L and 4L ones. Our results demonstrate the possibility of designing and controlling the spin degree of freedom in 2D magnets using twist engineering.

Excitons in mesoscopically reconstructed moiré heterostructures

“…Finally, samples with relatively large twists completely locked in moiré pattern may have received only a little attention in optical spectroscopy due to the dark nature of exciton-polarons. The broad evidence for mesoscopic reconstruction in bilayer graphene [14,16,19,32] hBN [64,65], CrI 3 ferromagnets [66,67] and TMD semiconductors [18, 20-22, 24, 25] suggests that the phenomenon is universal across samples and devices of layered van der Waals heterostructures.…”

Torsional Periodic Lattice Distortions and Diffraction of Twisted 2D Materials

“…Torsional PLDs in twisted 2D materials are a universal phenomenon and not limited to TBG [8][9][10]28]. The relaxation behavior is present in layered transitional metal dichalcogendies (MX 2 ) and trihallides (MH 3 ).…”

“…Subsequent reports showed moiré superlattices of other vdW systems with similar periodic restructuring [8][9][10][11]. Furthermore, this restructuring has a dramatic effect on the band-structure, magnetism, and superconducting properties [6,9,12]. Therefore, understanding twisted 2D materials requires a full description of the atomistic structure down to picoscale displacements.…”

Magnon corner states in twisted bilayer honeycomb magnets

“…Our theory is immediately testable considering rapid experimental progresses in 2D twisted magnets [94][95][96]. Monolayer Chromium triiodide (CrI 3 ) is a 2D ferromagnet with honeycomb lattice [103].…”

“…Meanwhile, researchers have turned their attention to twisted bilayer honeycomb magnets (TBHMs) analogous to twisted bilayer graphene and theoretically study moiré magnons in TBHMs [91][92][93]. Very recently, moiré magnetism has been experimentally reported in twisted bilayer CrI 3 [94][95][96]. Inspired by the recent theoretical and experimental developments in twisted 2D magnets, it is tempting to ask that whether higher-order topological magnon insulators can occur in TBHMs.…”