The diagram shows the conditions that cause the Kohn anomaly to rise in convenient metals (left) and a material is known as a Weyl semimetal (right).  (Image Credit: MIT)


Researchers at MIT and elsewhere have discovered an exotic physical phenomenon called Kohn anomaly for the first time in an unexpected type of material. Their findings could develop a better understanding of certain processes that help to determine why metals and other materials exhibit complex electronic properties that dominate today’s technology. The team presented their findings in the journal Physical Review Letters.


How electrons interact with phonons, which involves vibrations passing through a crystalline material, determines the physical processes that occur in electronic devices. The electron-phonon interactions influence how metals resist electric current, the temperature that unexpectedly transforms some materials into superconductors, and the very low-temperature necessities for quantum computers, are among many other processes.


It has been difficult to observe electron-phonon interactions since they are usually fragile, but the study has discovered a new type of unusual electron-phonon interaction. The team produced a Kohn anomaly, which was believed to only exist in an exotic material known as a topological Weyl semimetal. Their discovery could shed light on crucial aspects of the complex interplay between electrons and phonons.


Kohn anomaly had never been examined before in a topological material with electrical behaviors that are robust against perturbation. The researchers discovered that a Weyl semimetal, a type of topological material, specifically tantalum phosphide, is capable of exhibiting this anomaly. Conventional metals contain a property known as the Fermi surface, which drives the Kohn anomaly’s formation.  In the tantalum phosphide material, the Weyl points act as a driving force.


Since electron-phonon couplings are commonly used everywhere, they can be a significant source of disturbance in physical systems like those used to represent data in quantum computers. It can be challenging to measure the strength of these interactions, which is vital to protect quantum-based technologies, but this new study makes it easier to create the needed measurements.


The researchers used advanced neutron and X-ray scattering probes to measure the interactions, which were done at three national laboratories---Argonne National Laboratory, Oak Ridge National Laboratory, and the National Institute of Standards and Technology. Afterward, they probed the behavior of the tantalum phosphide material.


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


New insights on the electron-phonon couplings could lead to the development of such materials as improved high-temperature superconductors or fault-tolerant quantum computers. This could also be used to probe material properties in search of the ones that aren’t affected at higher temperatures.


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