The emergence of two-dimensional materials has fundamentally altered the landscape of condensed matter physics, and a recent breakthrough at the City College of New York (CCNY) is now pushing the boundaries of how light and magnetism interact at the atomic scale. Researchers at CCNY’s Laboratory for Nano and Micro Photonics (LaNMP), led by physicist Vinod M. Menon, have charted a comprehensive framework for a fast-growing area of quantum science centered on materials only a few atoms thick. These systems, known as van der Waals magnetic semiconductors, represent a paradigm shift where light, electric charge, and magnetism are no longer independent variables but are instead intricately coupled within the same crystalline structure.
The research, recently synthesized in a high-impact review published in Nature Materials under the title "Excitons in van der Waals magnetic materials," details how these layered structures allow for unprecedented control over quantum states. By manipulating the interaction between excitons—electrically neutral quasiparticles formed by light—and magnons—collective excitations of electron spins—the team is laying the groundwork for a new generation of optoelectronic and quantum devices. This development could eventually lead to technologies that process information using light and electron spin simultaneously, offering a path toward faster, more energy-efficient computing and secure quantum communications.
The Evolution of 2D Materials and Magnetic Order
To understand the significance of the CCNY research, it is necessary to examine the chronological progression of two-dimensional (2D) material science. The field began in earnest in 2004 with the isolation of graphene, a single layer of carbon atoms. While graphene revolutionized material science due to its exceptional conductivity and strength, it lacked intrinsic magnetic properties. For over a decade, the scientific community believed that long-range magnetic order could not exist in two dimensions at finite temperatures, a concept rooted in the Mermin-Wagner theorem.
However, a landmark shift occurred in 2017 when researchers successfully identified intrinsic magnetism in 2D crystals such as chromium triiodide (CrI3) and chromium germanium telluride (CrGeTe3). This discovery proved that magnetic order could indeed be maintained in atomically thin layers, provided there was enough magnetic anisotropy to counteract thermal fluctuations. Following this discovery, the focus of the global research community shifted toward "van der Waals" materials—crystals composed of layers held together by weak van der Waals forces, similar to the way sheets of paper sit in a stack.
The unique advantage of these materials is that they can be exfoliated into single layers and restacked into "heterostructures" with atomic precision. The work conducted by Vinod M. Menon and his colleagues focuses on a specific subset of these materials: magnetic semiconductors. Unlike traditional magnets that are metallic and opaque, these materials are semiconducting and interact strongly with light, creating a unique laboratory for studying the intersection of photonics and magnetism.
The Physics of Excitons and Magnons
At the heart of the CCNY study is the interaction between two distinct types of excitations: excitons and magnons. An exciton is created when a photon (a particle of light) hits a semiconductor, providing enough energy to kick an electron into a higher energy state. This leaves behind a "hole"—a vacancy with a positive charge. Because of the electrostatic attraction between the negative electron and the positive hole, they remain bound together as a single, neutral particle called an exciton. In 2D materials, because the layer is so thin, the electron and hole are squeezed together, making the exciton exceptionally stable even at room temperature.
Magnons, on the other hand, are the "quasiparticles" of magnetism. Rather than a single electron flipping its spin, a magnon represents a collective wave of spin offsets traveling through a magnetic lattice. Think of a line of fans in a stadium doing "the wave"; no individual fan moves around the stadium, but the disturbance travels the entire length of the stands.
In traditional materials, excitons and magnons rarely "talk" to each other. They exist in different energy regimes and operate through different physical mechanisms. However, in van der Waals magnetic semiconductors, the electronic orbitals that give rise to magnetism are the same ones that participate in the formation of excitons. "In these materials, light and magnetism no longer operate as separate channels," explained Pratap Chandra Adak, a postdoctoral researcher in Menon’s group and the lead author of the Review. "An exciton is not just a passive light-driven excitation sitting on top of the magnetism. It can sense the spin order and magnons, and under the right conditions, even help control the magnetic state itself."
Case Studies in 2D Magnetic Platforms
The Review published in Nature Materials highlights three specific material platforms that have become the workhorses of this new field: chromium triiodide (CrI3), nickel phosphorus trisulfide (NiPS3), and chromium sulfur bromide (CrSBr).
Chromium triiodide (CrI3) was the first material to demonstrate that magnetism could be controlled by an external electric field in a 2D system. Researchers found that the optical response of CrI3 changes dramatically depending on whether the magnetic layers are aligned in the same direction (ferromagnetic) or in opposite directions (antiferromagnetic). This allows scientists to "read" the magnetic state of a device simply by shining light on it and measuring the polarization of the reflected beam.
Nickel phosphorus trisulfide (NiPS3) has gained attention for its "zigzag" antiferromagnetic order. In this material, the coupling between excitons and the magnetic lattice is so strong that it creates "spin-correlated" excitons. These particles are incredibly narrow in terms of their energy spectrum, making them ideal for high-precision optical sensing and potential use in quantum information processing.
Chromium sulfur bromide (CrSBr) is perhaps the most promising for practical applications. Unlike many other 2D magnets that only function at temperatures near absolute zero, CrSBr maintains its magnetic properties at higher temperatures and is remarkably stable in air. It acts as a "waveguide," meaning it can trap and direct light within its own atomic structure. This allows excitons and magnons to interact over long distances, a prerequisite for building integrated circuits that use both light and spin.
Analytical Implications for Quantum Technology
The ability to unite optical signals with magnetic activity occurring at gigahertz (GHz) and terahertz (THz) frequencies has profound implications for the future of technology. One of the most significant potential applications identified by the CCNY team is the development of quantum transducers.
In the current landscape of quantum computing, there is a "frequency gap." Superconducting qubits—the most common type used by companies like IBM and Google—operate at microwave frequencies. However, to transmit quantum information over long distances (such as through fiber optic cables), the information must be converted to optical frequencies. Quantum transducers based on van der Waals magnets could fill this role, using magnons as the intermediary to bridge the gap between microwave and light signals.
Furthermore, the research discusses the potential for "all-optical logic." In modern computers, logic gates are controlled by electric currents, which generate heat and limit processing speed. In a system where light and magnetism are coupled, one could potentially use a laser pulse to flip a magnetic bit or use a magnetic field to switch a light signal on or off. This would enable the creation of magneto-photonic memory and data readout systems that are orders of magnitude faster and more efficient than current silicon-based technology.
The CCNY researchers also explore the concept of "exciton polaritons." These are hybrid particles that are part-light and part-matter. By confining light between two mirrors (a microcavity) and placing a 2D magnetic material inside, researchers can force the light to interact repeatedly with the material’s excitons. This creates a polariton that possesses the speed of light and the magnetic sensitivity of the material. This technology could lead to "polaritonic lasers" that require much less power to operate than traditional lasers.
Collaborative Efforts and Institutional Support
The work presented in the Nature Materials review is the result of a global collaborative effort, reflecting the multidisciplinary nature of quantum science. Co-authors include Florian Dirnberger of the Technical University of Munich, Swagata Acharya of the National Laboratory of the Rockies, Akashdeep Kamra of Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau, and Xiaodong Xu of the University of Washington.
This collaboration highlights how theoretical physics, material synthesis, and advanced spectroscopy must converge to solve the challenges of 2D magnetism. The project received significant financial backing from the Defense Advanced Research Projects Agency (DARPA) and the Gordon and Betty Moore Foundation. DARPA’s involvement underscores the strategic importance of this research, as the development of radiation-hard, high-speed, and low-power quantum components is a priority for national security and global technological leadership.
Professor Vinod Menon, the senior author of the Review, noted the rapid pace of the field’s development. "Over the past few years, this field has moved from detecting magnetism in atomically thin crystals to actively exploring how magnetic order can control light-matter interactions," Menon said. "The goal of this article is to bring those developments into a coherent framework and identify where the field can go next."
Challenges and the Path Forward
Despite the optimistic outlook, the CCNY researchers are candid about the significant scientific challenges that remain. A primary hurdle is the lack of comprehensive theoretical models. While researchers can observe the interaction between excitons, electron spins, and lattice vibrations (phonons), predicting how these four distinct elements behave simultaneously requires a level of computational power and theoretical complexity that is still being developed.
Additionally, the majority of these materials currently require cryogenic cooling to maintain their magnetic properties. For widespread commercial adoption, scientists must find or engineer van der Waals materials that are ferromagnetic at room temperature. The search for "high-temperature" 2D magnets is currently one of the most competitive areas of material science.
Future research directions identified in the Review include the study of "moiré magnetic excitons." When two layers of 2D materials are stacked with a slight twist or a difference in lattice constant, they create a moiré pattern—a larger-scale periodic interference pattern. This pattern can act as a series of "traps" for excitons and magnons, allowing researchers to create arrays of quantum dots or single-photon emitters that are controlled by magnetism.
Other areas of exploration include the optical control of spin textures, such as skyrmions (tiny magnetic vortices), and the development of magnetic exciton polariton condensation. The latter would involve creating a "superfluid" of light-matter particles that could flow without resistance, potentially leading to a new class of lossless information carriers.
Conclusion
The work coming out of the Menon Laboratory at the City College of New York serves as a roadmap for the next decade of quantum material research. By demonstrating that light and magnetism can be woven together in atomically thin layers, the CCNY team has opened the door to a future where the boundaries between optics, electronics, and spintronics disappear. While the field is still in its nascent stages, the transition from mere observation to active control of these quantum interactions suggests that the next generation of computing and communication technology may very well be built one atom at a time.
