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In the realm of magnetism, our comprehension has predominantly been anchored in the concept that the magnetic properties we encounter stem from the alignment of electron spins, a process governed by exchange interactions. However, a revolutionary discovery within condensed matter physics has unveiled an alternative pathway to magnetism, challenging the conventional narrative. This new form of magnetism, inspired by the theoretical work of Japanese physicist Yosuke Nagaoka in 1966, diverges from traditional understandings by deriving from the electrons’ quest to minimize their kinetic energy, rather than through exchange interactions.
Nagaoka’s theoretical framework, known as Nagaoka ferromagnetism, envisioned a unique type of magnetism arising within a two-dimensional lattice under specific conditions. The recent experimental breakthrough, documented in the journal Nature, observed phenomena in line with Nagaoka’s predictions within an engineered material only six atoms thick. This marks a significant advancement in the decades-long quest to substantiate Nagaoka ferromagnetism experimentally, heralding new avenues for scientific exploration.
Livio Ciorciaro, a co-author of the study and a doctoral researcher at the Swiss Federal Institute of Technology Zurich’s Institute for Quantum Electronics, expressed excitement over this discovery, highlighting the allure of uncovering previously unknown aspects of the natural world. This enthusiasm is emblematic of the broader scientific community’s reaction to this landmark finding.
The leap from theory to experimental evidence came when researchers demonstrated Nagaoka ferromagnetism in a system containing just three electrons, one of the smallest possible systems for such phenomena. The recent experiments expanded on this by revealing Nagaoka ferromagnetism within an extended system crafted into a moiré lattice, a complex structure created by stacking two nanometer-thin semiconductor sheets.
This unconventional magnetism’s foundation lies in the unique behavior of electrons within these materials. Unlike traditional ferromagnetism, where electrons align their spins in response to an external magnetic field due to exchange interactions, the magnetism observed in the recent study stems from a different mechanism proposed by Nagaoka. By removing an electron from a lattice filled with single electrons, the remaining electrons align their spins to minimize the system’s energy, thereby inducing ferromagnetism.
The experiment, led by Ataç İmamoğlu and his team, utilized moiré materials crafted from layers of molybdenum diselenide and tungsten disulfide, chosen based on simulations suggesting their potential for Nagaoka-style magnetism. The application of weak magnetic fields and the monitoring of electron spin alignment revealed ferromagnetic behavior under conditions not entirely aligned with Nagaoka’s original theory, opening up new questions about the underlying mechanisms.
The discovery that the material exhibited magnetism when the number of electrons exceeded the number of lattice sites by up to 50% points to a novel interaction within the lattice. This interaction, involving the formation of electron pairs known as doublons, leads to localized ferromagnetic regions, diverging from both traditional ferromagnetism and the exact predictions of Nagaoka ferromagnetism.
While this newfound form of magnetism deviates from established theories, it offers a fertile ground for further scientific inquiry. The moiré materials used in the study provide a unique platform to delve into the complex behaviors of electrons, potentially uncovering new principles in condensed matter physics.
Although practical applications may be on the distant horizon, İmamoğlu views this discovery as a pivotal step toward novel ways of understanding and manipulating electron behavior. Collaborations with theoretical physicists are underway to explore the potential of kinetic mechanisms, like those observed in moiré materials, for applications such as superconductivity.
This breakthrough in condensed matter physics serves as a reminder of the endless possibilities that lie in reexamining familiar phenomena from fresh perspectives. As research into kinetic ferromagnetism progresses, the quantum world may yet reveal more surprises, further expanding our understanding of the fundamental forces that shape our universe.