Exciton Orbital Magnetism Theory Resolves Discrepancies of Nearly One Order of Magnitude in Quantum Phenomena

The magnetic properties of excitons, bound pairs of electrons and holes within semiconductors, represent a growing frontier in quantum science, yet current theoretical understanding falls short of fully explaining experimental observations. Gurjyot Sethi from University of Louie, alongside Jiawei Ruan and colleagues at University of California at Berkeley and Lawrence Berkeley National Laboratory, now presents a comprehensive quantum theory addressing this gap. This new framework incorporates previously overlooked interactions between electrons and holes, as well as the influence of the material’s quantum geometry, revealing fundamentally new contributions to exciton magnetic behaviour. The resulting calculations demonstrate strong agreement with existing measurements, establishing the critical importance of a complete theoretical treatment for accurately predicting and harnessing the magnetic response of these quantum entities.

Recent research has uncovered novel quantum phenomena and is establishing bilayer graphene as a promising platform for quantum information science. A complete theoretical understanding of the magnetic response of excitons, correlated electron-hole pairs in semiconductors, has remained elusive due to insufficient treatment of electron-hole interactions and topological effects, as evidenced by discrepancies between theory and experiment. Theoretical predictions of the valley g-factor for excitons in biased bilayer graphene previously deviated from experimental observations by nearly an order of magnitude. To address these shortcomings, this work develops a quantum theory of the exciton orbital magnetic moment, revealing conceptually new terms absent in prior theoretical frameworks, including a contribution arising from the quantum geometry of the exciton band structure.

Exciton G-factors in Bilayer Graphene Calculations

This research investigates the origin of the exciton g-factor, a measure of how strongly an exciton responds to a magnetic field, in bilayer graphene. The team employs a sophisticated quantum mechanical approach to calculate this property and understand contributing factors, aiming to provide a theoretical framework that explains experimental observations and guides the design of materials with tailored optical and electronic properties. Key to understanding this work are several concepts: excitons are bound states of an electron and a hole, fundamental excitations in semiconductors and two-dimensional materials; the g-factor characterizes the interaction between an exciton and a magnetic field; the Berry phase is a geometric phase acquired by an electron or hole as it moves through a crystal lattice, related to the curvature of the band structure; and bilayer graphene consists of two layers of graphene, exhibiting different electronic properties than its single-layer counterpart.

Exciton Magnetic Moment, Topology and Interactions Revealed

Scientists have developed a new quantum theory of exciton orbital magnetic moment, revealing previously unknown terms crucial for understanding how these correlated electron-hole pairs respond to magnetic fields. This breakthrough addresses long-standing discrepancies between theoretical predictions and experimental observations, particularly in biased bilayer graphene. The team’s approach delivers a complete quantum mechanical expression for the exciton orbital magnetic moment, utilizing a sophisticated many-body GW plus Bethe-Salpeter equation framework. The research establishes the importance of fully accounting for both electron-hole interactions and topological effects when calculating the magnetic response of excitons.

Previous theories often approximated the exciton orbital magnetic moment as a simple difference between electron and hole magnetic moments, neglecting critical internal dynamics and the influence of the exciton band structure. This new theory explicitly addresses the divergent behavior of the magnetic moment operator, incorporating previously missing terms related to exciton internal dynamics and the quantum geometry of the exciton band structure. Experiments confirm the accuracy of this refined theoretical model, demonstrating excellent agreement with measurements of exciton behavior in various materials. Accurately calculating the exciton orbital magnetic moment requires a rigorous treatment of the position operator in Bloch states, acknowledging inherent non-localities within the crystal structure. This advancement resolves inconsistencies in existing models and provides a foundation for exploring magneto-optical platforms for quantum information science, where excitons function as optically addressable qubits and building blocks for quantum sensing technologies.

Exciton Magnetic Moments and Topological Effects

This research presents a comprehensive quantum theory for understanding the orbital magnetic moment of excitons, correlated electron-hole pairs in semiconductors. The team developed a new formulation, within the Bethe-Salpeter equation framework, that incorporates previously neglected terms arising from the interaction between electrons and holes, as well as topological effects related to their motion. This approach rigorously addresses challenges posed by the extended nature of the electron, hole, and exciton states, providing a more accurate description of how excitons respond to magnetic fields. The results demonstrate excellent agreement between theoretical calculations and experimental measurements of exciton valley g-factors in bilayer boron gallium nitride.

This confirms the importance of fully accounting for interaction and topological effects when modelling exciton magnetic responses, and highlights the distinct roles of band geometry, electron-hole relative motion, and exciton pseudospins. Extending the formalism to include second-order perturbation terms will enable investigation of exciton diamagnetic response and other related phenomena. Future work promises to advance understanding of exciton behaviour in diverse materials, including multi-layer systems, moiré heterostructures, and topological flat band materials, potentially contributing to advancements in quantum technologies and information science.