Breakthrough in Quantum Anomaly Unveiled by Cornell Researchers Challenges Established Beliefs on Quantum Insulators

Cornell University researchers, employing magnetic imaging techniques, have made a groundbreaking discovery regarding the behavior of electrons within quantum anomalous Hall insulators. Contrary to the conventional understanding, their findings reveal that the flow of electrons occurs within the material’s core rather than being restricted to its edges. This revelation not only sheds light on the intricate dynamics of electrons in quantum anomalous Hall insulators but also promises to resolve a long-standing debate concerning the flow of current in broader quantum Hall insulators. These insights hold significant implications for the development of topological materials crucial for the advancement of next-generation quantum devices.

The findings of this research have been published in the esteemed journal “Nature Materials,” with the lead author being Dr. Matt Ferguson, who currently serves as a postdoctoral researcher at the Max Planck Institute for Chemical Physics of Solids in Germany.

Unveiling the Quantum Hall Effect

This pioneering study, led by Assistant Professor Katja Nowack from the College of Arts and Sciences’ Physics department and the paper’s senior author, traces its origins to the phenomenon known as the quantum Hall effect, initially observed in 1980. This effect manifests when a magnetic field is applied to a specific material, resulting in a remarkable occurrence: the material’s bulk interior transforms into an insulator, while an electric current flows unidirectionally along its outer edge. These currents exhibit quantized resistances, adhering to a value dictated by the fundamental universal constant and reducing to zero.

A quantum anomalous Hall insulator, first identified in 2013, replicates this effect by employing a magnetized material. Quantization still occurs, and longitudinal resistance disappears, enabling electrons to move along the material’s edges without energy dissipation, akin to superconductivity.

Dispelling Conventional Beliefs

However, the prevailing belief that the current exclusively flows along the edges has been challenged by the recent findings. Professor Nowack states, “The picture where the current flows along the edges can really nicely explain how you get that quantization. But it turns out, it’s not the only picture that can explain quantization.” She emphasizes that the complexities of local voltages and currents can be far more intricate than what the edge-centric perspective suggests.

The study concentrated on chromium-doped bismuth antimony telluride, the same compound where the quantum anomalous Hall effect was initially observed a decade ago. The sample was grown by collaborators under the leadership of Professor Nitin Samarth at Pennsylvania State University. To scrutinize the material, Nowack and Ferguson employed a superconducting quantum interference device (SQUID), an exceptionally sensitive magnetic field sensor capable of operating at low temperatures to detect minuscule magnetic fields. The SQUID effectively captured current flows, which generate magnetic fields, and these images were amalgamated to reconstruct current density.

Revealing Discoveries and Future Prospects

The researchers’ observation of electrons flowing within the material’s bulk rather than along its boundaries prompted them to delve into earlier studies. They discovered that in the years following the initial discovery of the quantum Hall effect in 1980, there was substantial debate concerning the location of current flow—a controversy unknown to most contemporary materials scientists.

Professor Nowack expresses her hope that this work will encourage the younger generation working with topological materials to revisit this debate. She emphasizes that we still lack a comprehensive understanding of some fundamental aspects of these materials, especially how current flows within them.

These insights could also prove valuable in the development of more complex devices, such as hybrid technologies that combine superconductors with quantum anomalous Hall insulators to generate even more exotic states of matter.

Professor Nowack concludes by highlighting the allure of topological materials, as their behavior in electrical measurements is governed by general principles, regardless of microscopic details. Nevertheless, comprehending what transpires at the microscopic level is crucial for both fundamental understanding and practical applications. This interplay between general principles and finer nuances renders the study of topological materials captivating and fascinating.

Reference: “Direct visualization of electronic transport in a quantum anomalous Hall insulator” by G. M. Ferguson, Run Xiao, Anthony R. Richardella, David Low, Nitin Samarth and Katja C. Nowack, 3 August 2023, Nature Materials. DOI: 10.1038/s41563-023-01622-0

Co-authors include doctoral student David Low and Penn State researchers Nitin Samarth, Run Xiao, and Anthony Richardella.

The primary support for this research came from the U.S. Department of Energy’s Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. Material growth and sample fabrication were supported by the 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP), funded by the National Science Foundation at Penn State.

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More about Quantum Insulator Discovery

• Nature Materials Journal Article – The original research paper published in Nature Materials.
• Cornell University News – Cornell University’s news article about the research.
• Quantum Hall Effect – Information about the quantum Hall effect, a key concept mentioned in the text.
• Superconducting Quantum Interference Device (SQUID) – Details about the SQUID, the sensitive sensor used in the study.