Newly Discovered 'Exotic' Phenomenon Paves Way For Quantum Devices

The findings hold a lot of promise for helping us understand materials that could be the building blocks for quantum computers of the future.
Chris Young

An exotic physical phenomenon known as a Kohn anomaly has been found for the first time in an unexpected type of material by researchers at MIT.

The scientists say the findings could provide new insights into certain fundamental processes that determine the complex electronic interactions under the hood of many of our electronic devices.


In turn, this could lead to "the development of materials with new thermal or electronic properties [that are] so new, we need time to think about what they can do," says Brent Fultz, a professor of materials science and applied physics at Caltech, who was not involved in the study.

Studying electronic interactions

The physical processes that take place inside many electronic devices are determined by interactions between electrons and phonons — which are, essentially, a wave of vibrations passing through a crystalline material. 

Amongst the processes these interactions affect are the way materials resist electric current, as well as the temperature at which some materials become superconductors and the very low-temperature requirements for quantum computers.

The problem is that electron-phonon interactions have been incredibly difficult to study in detail due to the fact that they are very weak and hard to read.

In a bid to study these interactions with more clarity, a team of researchers from MIT induced a Kohn anomaly, which was previously thought to exist only in metals, in an exotic material called a topological Weyl semimetal.

The finding could help to shed light on the complex interactions between electrons and photons, the team says, aiding in the search for new materials for advanced computers.

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A marriage of theory and observation

The study, which is based on both theoretical predictions and experimental observation, was published this week in the journal Physical Review Letters. 

Kohn anomalies were first discovered in the 1950s by physicist Walter Kohn. They reflect a sudden change, sometimes referred to as a wiggle or kink, in the graph that describes a physical parameter called the electron response function.

This sudden discontinuity reflects a change in the capability of electrons for shielding phonons. This can cause instabilities in the propagation of electrons through the material and can lead to a number of new electronic properties.

To measure these precise interactions, the team 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 — to examine 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 explained in a press release. Using the calculations, “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.”

Newly Discovered 'Exotic' Phenomenon Paves Way For Quantum Devices
A diagram depicting the different conditions that give rise to a Kohn anomaly in ordinary metals (left), versus a material called a Weyl semimetal (right), SourceMIT

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.”

A better understanding of electron-phonon couplings, via this novel observation of the Kohn anomaly, could help lead the way to develop materials, such as better high-temperature superconductors or fault-tolerant quantum computers, the research team says.

“I think this could lead us to further understand processes that would underlie some of these materials that hold a lot of promise for the future,” says Andrejevic, who along with Research Scientist Fei Han was a co-lead author on the paper.

By extending our understanding of these exotic materials, we may see as-yet-unknown advances in fields such as quantum computing, which promises to revolutionize the way we interact with the digital world.

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