Twisted Bilayer CrSBr Demonstrates Magneto-Moiré Excitons and Novel Spin Textures

The emergence of moiré superlattices in layered materials has dramatically advanced the study of electronic and excitonic systems, and now researchers are extending this concept to magnetism. Qiuyang Li, Anton Shubnic, and Nishkarsh Agarwal, along with colleagues at various institutions, report the observation of ‘magneto-moiré excitons’ in twisted bilayer CrSBr, a discovery that links magnetism and light in a fundamentally new way. This research demonstrates how nanoscale magnetic textures, created by twisting the layers of the material, directly influence the energy of excitons, quasiparticles that respond to light. The team’s findings reveal a pathway to engineer excitonic energy landscapes using magnetism, potentially leading to innovative magneto-optical devices for sensing, transduction, and controlling magnetic properties with light.

Moiré Superlattices and Correlated Insulating States

Moiré superlattices in van der Waals materials create periodic electrostatic potentials, revolutionizing the study of electronic and excitonic systems. This research investigates the emergence of correlated insulating states and unconventional superconductivity in twisted bilayer graphene, focusing on how strong electron-electron interactions and unique band structures drive these phenomena. The team employs high-resolution scanning tunneling microscopy and spectroscopy, alongside theoretical modelling, to probe local electronic structure and many-body correlations. Specifically, researchers investigate how the electronic landscape evolves with varying twist angles, identifying correlated insulating phases characterized by localized states and suppressed charge dynamics. They demonstrate the emergence of superconductivity near specific twist angles, revealing a non-monotonic dependence on carrier density and sensitivity to external magnetic fields. This work establishes a direct link between the moiré potential landscape, localized electronic states, and correlated quantum phenomena, providing crucial insights into the design and control of novel quantum materials.

Twisted CrSBr Layers Form Moiré Superlattices

This study pioneered a method for creating and characterizing moiré superlattices in twisted bilayer chromium sulfide bromide (CrSBr), revealing a pathway to control excitonic energy landscapes with magnetism. Researchers meticulously stacked two monolayers of CrSBr to form twisted bilayers with varying twist angles, carefully measuring the resulting structures using dark-field transmission electron microscopy. Detailed analysis of selected area electron diffraction patterns and real-space images confirmed the formation of moiré superlattices, with a locally averaged twist angle of 0.77° ±0.09° observed in one sample.

These measurements were consistently demonstrated on samples with small twist angles. The team then employed magneto-reflection contrast spectroscopy at 5 Kelvin, applying out-of-plane magnetic fields up to 2.4 Tesla to investigate the optical properties of these twisted bilayers. By comparing spectra of monolayers, natural bilayers, and twisted bilayers with different twist angles, scientists observed unique behavior in samples with twist angles less than 2°. These samples exhibited a redshift of the exciton peak, moving towards the energy observed in the monolayer, before blueshifting at angles greater than 2°.

This behavior suggests decreasing, yet finite, interlayer coupling within the twisted bilayers at small twist angles. Further analysis revealed significant narrowing of exciton peaks in the 0.4° and 1.1° twisted bilayers, indicating a saturation effect. This detailed spectroscopic characterization provides crucial insight into the interplay between moiré magnetism and excitonic properties, demonstrating the potential to engineer nanoscale excitonic energy landscapes through magnetic control.

Twisting CrSBr Bilayers Controls Magnetism and Optics

Research into twisted bilayer Chromium Sulfide Bromide (CrSBr), a 2D magnetic material, demonstrates how twisting affects magnetic properties, optical characteristics, and stacking configurations. The goal is to engineer these properties through precise control of the twist angle. Twisting induces the formation of stacking domains, which significantly impact magnetic and optical properties. The research identifies multiple magnetic phase transitions sensitive to twist angle and stacking configuration, and demonstrates that optical properties can be tuned by controlling twist angle and stacking order.

Twisting can lead to non-collinear spin textures, where magnetic moments are not aligned, and moiré patterns play a crucial role in determining magnetic domain structure and stacking domain formation. The study involved sample fabrication using mechanical exfoliation and stacking, optical microscopy to characterize morphology and identify stacking domains, spectroscopic techniques to study optical properties, scanning angle diffraction and transmission electron microscopy to determine crystal structure and stacking order, and first-principles calculations to understand electronic structure, magnetic properties, and stacking energies. Micromagnetic simulations modeled magnetic domain structure and dynamics, Raman spectroscopy characterized vibrational modes and stacking order, and angle-resolved photoemission spectroscopy studied the electronic band structure. This research demonstrates the potential of twisting 2D magnetic materials like CrSBr to engineer novel magnetic and optical properties, opening possibilities for advanced spintronic devices and other applications.

Twist Angle Controls Exciton Energy Landscapes

This research demonstrates the observation of magneto-moiré excitons in twisted bilayer CrSBr, establishing a direct link between excitonic resonances and quasi-one-dimensional spin textures that emerge within moiré superlattices. By creating a twisted bilayer structure, scientists observed how nanoscale magnetic patterns imprint themselves onto the optical spectrum, effectively shifting the energy of excitons through a periodic magnetic exchange field. These findings reveal that moiré magnetism can engineer nanoscale excitonic energy landscapes, offering a new means to manipulate light and magnetism at the nanoscale. The team’s work identifies a critical twist angle of approximately 2 degrees, below which the interplay between interlayer exchange interactions and domain wall formation energy drives the emergence of complex magnetic textures. Importantly, the structural integrity of CrSBr preserves the moiré lattice even at these small angles, enabling the formation of stable spin domains. This platform allows for the direct optical readout of nanoscale spin textures, bypassing the need for invasive probes and opening avenues for exploring emergent quantum phases.