Fractional Chern Ferromagnets in Moiré Systems

• Fractional Chern ferromagnets are states featuring coexisting ferromagnetic and topological order in flat moiré bands with fractional excitations.
• Optical control using circularly polarized light achieves nearly 100% switching efficiency and allows for precise domain wall writing.
• These systems promise advancements in dissipationless spintronics and reconfigurable quantum devices through tunable topological phase transitions.

Fractional Chern ferromagnetism refers to phases where ferromagnetic order coexists with topological order and fractionalized excitations in partially filled topological flat bands. In moiré-engineered materials, such as twisted bilayer MoTe₂, robust optical control of these fractional Chern ferromagnet states is now experimentally achievable. The phenomenon exploits the unique combination of spin-valley locking, strong electron correlations, and the topological properties of the flat moiré bands, enabling high-efficiency, reversible switching of quantum ferromagnetism at zero magnetic field using low-power optical excitation.

1. Optical Manipulation of Moiré Chern Ferromagnets

In twisted bilayer MoTe₂, fractional Chern ferromagnets emerge when fractional fillings of nearly flat topological bands spontaneously break time-reversal symmetry, producing valley and spin polarization. Optical control is implemented using a pump–probe scheme involving circularly polarized light (CPL):

• Initialization (reset): A low-intensity (~28 nW·μm⁻²) CPL pulse (e.g., o– helicity) sets the initial ferromagnetic (FM) state.
• Switching: A pump pulse of opposing helicity (e.g., o+) targets the system at the relevant absorption resonance. The light preferentially excites carriers in one valley, generating a transient population imbalance.
• Relaxation & Readout: After a brief dark interval, the spin polarization is measured by reflective magnetic circular dichroism (RMCD), providing a direct readout of the FM order.

The optical excitation operates at zero external magnetic field and achieves nearly 100% switching efficiency by harnessing robust coupling between CPL and valley-locked spin states in the moiré flat bands.

2. Underlying Physical Mechanism

The switching mechanism is controlled by the spin–valley locking and the optically induced spin-polarized carrier injection:

• Each valley (K/K′) in the moiré Brillouin zone carries a distinct spin orientation due to strong spin–orbit coupling.
• CPL selectivity: CPL of one helicity (o+) couples strongly to the K valley, the other (o–) to K′. Optical pumping populates spin-polarized holes in the targeted valley.
• The injected spin imbalance acts as an effective Zeeman field ($E_Z$), whose scale is controlled by the density of optically injected carriers ($n_\text{pump}$). Once $E_Z$ exceeds the intrinsic valley splitting, the FM order reverses.
• This non-thermal, light-driven process efficiently switches the FM order between bistable configurations corresponding to the two valleys (and spins), without Joule heating or electrical contacts.

Relevant expressions: $$\eta = \frac{M_\text{switched} - M_\text{initial}}{|M_\text{initial}|} \times 100\%, \hspace{1cm} E_Z \propto n_{\text{pump}}$$ where $\eta$ is the switching efficiency, $M$ denotes the layer-resolved net magnetization before/after switching, and $E_Z$ is the field governing valley-polarized FM inversion.

3. Experimental Protocols: Magnetic Bistability and Domain Engineering

Two operational demonstrations are established:

• Magnetic bistate cycling: By alternating the helicity of CPL pulses, the system can be cycled bidirectionally between the two FM states. The RMCD signal, which tracks the net magnetization, exhibits clear hysteresis analogous to a first-order transition, confirming bistable behavior and nearly complete switching at each cycle.
• Domain wall writing: Employing spatially resolved focused CPL, arbitrary ferromagnetic domains with opposite magnetization can be "written" onto the sample. Scanning the pump beam creates well-defined domain walls—interfaces where the Chern number changes sign and topologically protected chiral edge states are expected.

Gate-tunability, via top- and bottom-layer gating, allows precise exploration of filling factor ($v$) regimes—including the fractional Chern insulator range (e.g., $v = -2/3, -3/5$)—and the manipulation of interlayer electric field ($D/\varepsilon_0$) for additional phase control: $$n = \frac{V_\text{tg} C_\text{tg} + V_\text{bg} C_\text{bg}}{e} - n_0$$

$$\frac{D}{\varepsilon_0} = \frac{V_\text{tg} C_\text{tg} - V_\text{bg} C_\text{bg}}{2\varepsilon_0} - \frac{D_0}{\varepsilon_0}$$

where $V_\text{tg/bg}$ are gate voltages, $C_\text{tg/bg}$ are capacitances, $n_0, D_0$ denote offsets.

4. Impact on Spintronics and Quantized Chern Junctions

The achieved optically driven ferromagnetic switching and domain wall engineering have far-reaching implications:

• Dissipationless spintronics: Non-contact, low-power control of FM order enables ultrafast, non-volatile devices where spin and topological order can be manipulated without the need for electrical current, reducing energy dissipation and enhancing device miniaturization and integration.
• Programmable quantized Chern junctions: The domain boundaries correspond to chiral one-dimensional modes carrying quantized current, dictated by the underlying Chern number. These can be engineered and reconfigured optically to design customizable junction networks, laying the foundation for robust topological transport channels.
• Quantum information science: The ability to spatially pattern and dynamically rewrite FM domains in fractional Chern ferromagnets is a key step toward realizing architectures for topological quantum computing based on anyonic edge modes and domain wall manipulation.

5. Distinguishing Features and Future Directions

Notable characteristics of the system include:

• Switching is achieved at pump powers as low as $28~\text{nW}·\mu\text{m}^{-2}$, greatly outperforming conventional optical spin control protocols in energy efficiency.
• Robustness against disorder and gate-induced tuning further enhances device reliability and functionality.
• The moiré flat band platform is highly versatile, allowing exploration of a range of correlated topological phenomena at various fractional fillings.
• Future research will likely focus on integrating this optical control with edge state imaging techniques (Ji et al., 10 Apr 2024), detailed microscopic modeling of the topological order, and the development of topologically protected spintronic and quantum information devices.

6. Summary Table: Key Concepts and Experimental Signatures

Concept	Mechanism	Experimental Outcome
Optical bistability	Helicity cycling of CPL pumps	Repeatable, nearly 100% RMCD switching
Domain wall writing	Localized CPL beam	Programmable ferromagnetic domain boundaries
Topological edge modes	Chiral edge at domain wall	Quantized Chern currents, protected transport
Gate tunability	Top/bottom electric fields, dual-gate control	Manipulation of filling and topological phase

The integration of low-power optical writing, bistate cycling, and domain control establishes a practical and scalable methodology for real-time control of fractional Chern ferromagnetic order in moiré superlattices, with direct relevance for dissipationless spintronics and quantum transport device engineering (Cai et al., 27 Aug 2025).