Light-Induced Ferromagnetism in Moiré Superlattices

Abstract: Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would open the possibility to access and control complex electronic phase diagrams on demand. Recently, moiré superlattices formed by two-dimensional materials have emerged as a promising platform for quantum engineering such phenomena. The power of the moiré system lies in the high tunability of its physical parameters by tweaking layer twist angle, electrical field, moiré carrier filling, and interlayer coupling. Here, we report that optical excitation can drastically tune the spin-spin interactions between moiré trapped carriers, resulting in ferromagnetic order in WS2/WSe2 moiré superlattices over a small range of doping at elevated temperatures. Near the filling factor v = -1/3 (i.e., one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism (RMCD) signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photo-excited excitons mediate exchange coupling between moiré trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré trapped holes, and thus magnetic order can arise even in the dilute hole regime under optical excitation. This discovery adds a new and dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter.

• The paper introduces optical excitation as a novel method to induce ferromagnetism in WS₂/WSe₂ moiré superlattices, as evidenced by pronounced hysteresis in RMCD measurements.
• The study employs dual-gated, dry-transferred samples with reflective magnetic circular dichroism, enabling precise control over carrier concentration and vertical electric fields.
• The findings offer a new approach for dynamically tuning many-body interactions in quantum materials, with implications for spintronics and quantum computing.

Light-Induced Ferromagnetism in Moiré Superlattices

Introduction

The paper "Light-Induced Ferromagnetism in Moiré Superlattices" (2203.07161) presents a comprehensive study on the optical manipulation of spin-spin interactions within moiré superlattices, specifically focusing on WS$_2$/WSe$_2$ heterobilayers. The research capitalizes on the tunability of moiré superlattices formed by two-dimensional TMDs, which offer a controllable platform for exploring strongly correlated electronic phenomena. The study elucidates an innovative method where optical excitation serves as a mechanism to induce ferromagnetic order, effectively adding a new dynamical parameter to manipulate the many-body Hamiltonian in quantum materials.

Methodology and Experimental Setup

The authors prepared dual-gated WS$_2$/WSe$_2$ samples to independently control the carrier concentration and the vertical electric field. The samples were fabricated using a dry-transfer method and were probed using piezoresponse force microscopy to ensure precise moiré pattern characterization. Reflective magnetic circular dichroism (RMCD) measurements were conducted to investigate the magnetic properties of the superlattices under varying doping levels and optical excitation conditions. This technique enabled the detection of hysteresis loops, a signature of ferromagnetic order, in the RMCD signal at specific moiré miniband fillings.

Key Findings

The research unveils that optical excitation in moiré superlattices can drastically enhance the exchange interaction between moiré trapped carriers, resulting in the emergence of ferromagnetism at elevated temperatures. Near a filling factor of $v = -1/3$, corresponding to one hole per three moiré unit cells, increasing the excitation power at the exciton resonance leads to a pronounced hysteresis loop in the RMCD signal. This loop is indicative of ferromagnetic order and persists down to charge neutrality, demonstrating that ferromagnetic order can be optically induced in the dilute hole regime.

A particularly intriguing aspect of the findings is the exciton-mediated interaction mechanism, which allows for long-range exchange coupling between moiré trapped holes. This mechanism provides an additional tuning knob for the moiré many-body Hamiltonian, potentially facilitating the exploration of a wide range of correlated electronic states on demand.

Implications and Future Directions

This paper significantly contributes to our understanding of the interplay between light and spin interactions in moiré superlattices. The introduction of a method for optically inducing and controlling ferromagnetic order could drive advances in quantum computing and spintronics, where control over magnetic properties is crucial. Additionally, the ability to dynamically tune the electronic phase diagrams of moiré quantum matter could herald the development of novel devices capable of operating in previously inaccessible parametric regimes.

Future research could further explore the impact of optical orientation of exciton spins on magnetic state formation, and investigate transient phenomena such as optically controlled topological phase transitions. Moreover, expanding this approach to include other superlattice geometries and material systems could widen the scope of potential applications in quantum technologies.

Conclusion

The study provides a critical advancement in the field of moiré quantum materials by demonstrating that optical excitation can be used to induce and manipulate ferromagnetic order in WS$_2$/WSe$_2$ moiré superlattices. This discovery opens new avenues for exploring optically-driven many-body phenomena in correlated electron systems and may have profound implications for the design of tunable quantum devices.