Researchers at The University of Hong Kong have identified a quantum phenomenon where Hall conductance exhibits values at half-integer levels, rather than the traditionally quantized resistance. This discovery challenges established understandings of the quantum Hall effect, typically observed in insulating materials, as it occurs within two-dimensional ferromagnetic semimetals possessing finite longitudinal conduction. Shun-Qing Shen and collaborators found that a single massless Dirac cone, rather than previously theorized massive Dirac fermions, is the origin of this unexpected half-integer quantization, prompting a re-evaluation of the parity anomaly. Shen will present the historical development of this topic, from its field-theoretical origin to its modern realization in quantum materials, discussing key theoretical ideas, recent experimental progress, and broader implications for topological phases and quantum transport on June 12.
Half-Quantized Hall Effect in Ferromagnetic Semimetals
A surprising quantum effect has emerged in ferromagnetic materials, quantizing Hall conductance instead of resistance. Shun-Qing Shen and collaborators identified this half-quantized Hall effect within two-dimensional ferromagnetic semimetals, a departure from the conventional integer and fractional quantum Hall effects typically observed in insulating systems. Unlike those established phenomena, this new effect manifests even with finite longitudinal conduction, meaning current continues to flow along the material’s length alongside the Hall current. This challenges the long-held assumption that quantization necessitates an insulating state, opening new avenues for exploring quantum phenomena in metallic systems. The theoretical underpinnings of this discovery required a revision of previously accepted models; researchers initially believed massive Dirac fermions were responsible for generating the half-integer quantized Hall conductance.
Experimental confirmation arrived through studies of magnetic topological-insulator thin films, demonstrating the predicted characteristics of a system with a single massless Dirac cone: half-integer Hall conductance, a minimum longitudinal conductance, and an algebraically decaying Hall current emanating from the material’s boundaries. Shen will present the historical development of this topic, from its field-theoretical origin to its modern realization in quantum materials, discussing key theoretical ideas, recent experimental progress, and the broader implications for topological phases and quantum transport. He will also reflect on how an initially incorrect theoretical picture can stimulate important developments and eventually lead to the discovery of new physics.
Massless Dirac Cones and Parity Anomaly Revisions
Shun-Qing Shen of The University of Hong Kong and his collaborators discovered that the accepted theoretical framework, positing massive Dirac fermions as the origin of this behavior, required reevaluation. Shen will present how this finding fundamentally altered the understanding of the underlying physics and prompted a reassessment of the parity anomaly. The implications extend beyond simply identifying the correct particle; it necessitates a broader reconsideration of how the parity anomaly manifests within condensed-matter systems and opens new avenues for exploring topological phases and quantum transport phenomena. Shen intends to present a historical overview of this evolution, from its origins in quantum field theory to its current realization in advanced materials, demonstrating how even initially incorrect theoretical models can catalyze crucial advancements and ultimately reveal new physics.
However, in , my collaborators and I found that the accepted picture needed to be revised: it is a single massless Dirac cone, rather than massive Dirac fermions, that gives rise to the half-integer quantized Hall conductance.
Experimental Confirmation in Magnetic Topological Insulators
Researchers focused on magnetic topological-insulator thin films, a system where conventional expectations regarding quantum Hall effects do not apply. Unlike the integer and fractional quantum Hall effects, these materials exhibit finite longitudinal conduction alongside the unusual Hall conductance. This departs from the established context of insulating materials typically required for observing these quantum phenomena, forcing a re-evaluation of fundamental assumptions. This confirmation is significant because it not only validates a revised theoretical picture but also expands the known landscape of topological phases and quantum transport mechanisms, suggesting that initial misconceptions can stimulate important developments and lead to the discovery of new physics within materials science. Shen will present the historical development of this topic, from its field-theoretical origin to its modern realization in quantum materials, and discuss the key theoretical ideas and recent experimental progress on June 12. The findings, detailed in publications including B. Shen, Quantum anomalous semimetals, npj Quantum Materials 7, 94 (), open new avenues for exploring and manipulating quantum phenomena in materials beyond traditional insulators.
