Optical Switching of Moiré Chern Ferromagnet

Optical manipulation of quantum matter offers a non-contact, high-precision and fast control. Fractional Chern ferromagnet states in moiré superlattices are promising for topological quantum computing, but an effective optical control protocol has remained elusive. Here, we demonstrate robust optical switching of integer and fractional Chern ferromagnets in twisted MoTe2 bilayers using circularly polarized light. Highly efficient optical manipulation of spin orientations in the topological ferromagnet regime is realized at zero field using a pump light power as low as 28 nanowatts per square micrometer. Utilizing this optically induced transition, we also demonstrate magnetic bistate cycling and spatially resolved writing of ferromagnetic domain walls. This work establishes a reliable and efficient optical control scheme for moiré Chern ferromagnets, paving the way for dissipationless spintronics and quantized Chern junction devices.

💡 Research Summary

This work demonstrates a highly efficient, low‑power optical protocol for switching both integer and fractional Chern ferromagnet states in small‑angle twisted MoTe₂ bilayers. By encapsulating 3.5° and 3.65° twisted 1L/1L MoTe₂ in a graphite/h‑BN dual‑gate stack, the authors independently tune the charge filling factor ν and the out‑of‑plane electric displacement field D/ε₀. Using circularly polarized light (CPL) as a pump and reflective magnetic circular dichroism (RMCD) as a probe, they monitor the magnetization direction and magnitude in real time. The strong spin‑orbit coupling in MoTe₂ locks spin to the valley degree of freedom, so σ⁺ CPL couples to the K valley and σ⁻ CPL to the K′ valley. When the pump photon energy resonates with the absorption peak of hole‑doped MoTe₂, electron‑hole pairs are generated, and spin‑polarized holes accumulate in the opposite valley. Once this optically injected valley polarization exceeds a critical density, exchange interactions produce an effective Zeeman‑like field that overcomes the intrinsic valley splitting and reverses the ferromagnetic order. Consequently, the helicity of the CPL deterministically sets the magnetization direction at zero external magnetic field. Remarkably, a pump power density as low as 28 nW·µm⁻² achieves near‑100 % switching efficiency for the ν = ‑1 integer Chern ferromagnet and the ν = ‑2/3 fractional Chern insulator, with critical powers of ~20 nW·µm⁻² and ~17 nW·µm⁻² respectively. The switching efficiency follows the absorption spectrum of the material and declines monotonically as the pump polarization deviates from pure circular, confirming that the mechanism relies on spin angular momentum transfer rather than the inverse Faraday effect, which requires orders‑of‑magnitude higher intensities. The authors also demonstrate robust bidirectional switching over many cycles and spatially resolved writing of ferromagnetic domain walls, enabling programmable Chern junctions and topological memory concepts. This study establishes a practical, non‑volatile, and ultrafast optical control scheme for moiré Chern ferromagnets, opening pathways toward dissipationless spintronic devices, quantized Chern junction electronics, and the manipulation of exotic quasiparticles such as non‑Abelian anyons for fault‑tolerant topological quantum computation.