Kohn anomaly: New phenomenon that could lead to new quantum devices

Exotic states called Kohn anomalies could offer clues to why some materials have the electronic properties they do.

The electron-phonon interactions influence how metals resist electric current, the temperature at which some materials unexpectedly become superconductors, and the low-temperature necessities for quantum computers, among numerous different procedures.

But electron-phonon interactions have been difficult to study in detail as they are usually weak.

Now scientists at MIT and elsewhere have found a new, stronger kind of unusual electron-phonon interaction- Kohn anomaly. The finding could help shed light on essential aspects of the complex interplay between electrons and phonons.

Kohn anomaly had never been observed before in a “topological material,” whose electrical behaviors are robust against perturbation. In this case, a kind of topological material called a Weyl semimetal, specifically tantalum phosphide, was found to be capable of exhibiting this unusual anomaly. Unlike in conventional metals, where a property called the Fermi surface drives the formation of the Kohn anomaly, in this material, the Weyl points serve as the driving force.

Electron-phonon couplings can be a significant source of disturbance in delicate physical systems, such as those used to represent data in quantum computers. Measuring the strength of these interactions could be vital in knowing how to protect such quantum-based technologies.

Previously, it wasn’t easy, but this new study provides a way of making such measurements. Now, Kohn anomaly can be used to quantify how strong the electron-phonon coupling can be.

For measurements, scientists used advanced neutron and X-ray scattering probes at three national laboratories—Argonne National Laboratory, Oak Ridge National Laboratory, and the National Institute of Standards and Technology. They then probed the behavior of the tantalum phosphide material.

Professor Mingda Li at MIT said, “We predicted that there is a Kohn anomaly in the material just based on pure theory. Using their calculations, we could guide the experiments to the point where we want to search for the phenomenon, and we see an excellent agreement between theory and the experiments.”

Martin Greven, a professor of physics at the University of Minnesota who was not involved in this research, says, “this work has impressive breadth and depth, spanning both sophisticated theory and scattering experiments. It breaks new ground in condensed matter physics, in that it establishes a new kind of Kohn anomaly.”

Scientists noted, “A better understanding of the electron-phonon couplings could help lead the way to develop such materials as better high-temperature superconductors or fault-tolerant quantum computers. This new tool could be used to probe material properties in search of those that remain relatively unaffected at higher temperatures.”

MIT graduate student Thanh Nguyen said, “this work helps to demonstrate the sometimes overlooked importance of phonons in the behavior of topological materials. Materials such as these, whose surface electrical properties are different from those of the bulk material, are a hot area of current research.”

MIT graduate student Nina Andrejevic said, “I think this could lead us to understand further processes that would underlie some of these materials that hold a lot of promise for the future.”

Professor of physics and astronomy Pengcheng Dai at Rice University said“Although electron-phonon interaction is long known to exist, the experimental prediction and observation of these interactions are exceedingly rare. These results provide an excellent demonstration of the power of combined theory and experiments as a way to extend our understanding of these exotic materials.”

Journal Reference:
  1. Thanh Nguyen et al. Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.236401

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