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A novel theory-driven framework could assist scientists in examining the characteristics of new semiconductors for the forthcoming generation of microelectronic devices, or in discovering materials that enhance the efficiency of quantum computers.
Efforts to create new or improved materials usually involve examining properties that can be consistently measured with current laboratory equipment, but this only represents a small portion of the characteristics that scientists could theoretically investigate. Certain properties remain almost “invisible” because they are exceedingly difficult to measure directly with available techniques.
Consider the interaction of electrons and phonons — this characteristic is vital for a material’s electrical, thermal, optical, and superconducting attributes, yet directly measuring it with current methods is famously difficult.
Now, researchers at MIT have introduced a theoretically grounded method that could transform this difficulty into a chance. Their approach reinterprets neutron scattering, an often-ignored interference phenomenon, as a potential direct probe of the strength of electron-phonon coupling.
The procedure generates two interaction effects within the material. The researchers illustrate that by intentionally structuring their experiment to utilize the interference between the two interactions, they can quantify the strength of a material’s electron-phonon interaction.
The researchers’ theory-guided methodology could influence the design of future experiments, paving the way for measuring new quantities that were previously unattainable.
“Instead of stumbling upon new spectroscopic techniques by sheer chance, we can apply theory to rationalize and guide the design of our experiments and our physical apparatus,” states Mingda Li, the Class of 1947 Career Development Professor and an associate professor of nuclear science and engineering, and senior author of a paper on this experimental approach.
Li is joined in this publication by co-lead authors Chuliang Fu, an MIT postdoctoral researcher; Phum Siriviboon and Artittaya Boonkird, both graduate students at MIT; along with others from MIT, the National Institute of Standards and Technology, the University of California at Riverside, Michigan State University, and Oak Ridge National Laboratory. The research is published this week in Materials Today Physics.
Examining interference
Neutron scattering is a robust measurement technique that involves directing a beam of neutrons at a material and analyzing how the neutrons are scattered post-collision. This method is particularly effective for assessing a material’s atomic structure and magnetic characteristics.
When neutrons strike the material sample, they engage with it through two distinct mechanisms, producing a nuclear interaction and a magnetic interaction. These interactions can interfere with one another.
“The scientific field has recognized this interference phenomenon for quite some time, but researchers often perceive it as a hindrance that can obscure measurement signals. Hence, it has not garnered much focused investigation,” Fu remarks.
The team and their collaborators took a conceptual “leap of faith” and resolved to investigate this frequently overlooked interference effect more comprehensively.
They turned the conventional materials research methodology upside down by commencing with a multifaceted theoretical assessment. They examined the implications of the nuclear interaction and magnetic interaction interfering within a material.
Their assessment indicated that this interference pattern is directly proportional to the strength of the electron-phonon interaction in the material.
“This renders the interference effect a probe we can utilize to uncover this interaction,” elucidates Siriviboon.
Interactions between electrons and phonons influence a variety of material properties. They impact thermal conductivity, affect light absorption and emission, and can even result in superconductivity.
However, the complexity of these interactions makes them difficult to measure directly using existing experimental methods. Consequently, researchers often depend on less accurate, indirect approaches to gauge electron-phonon interactions.
Nevertheless, capitalizing on this interference effect allows for the direct measurement of the electron-phonon interaction, presenting a significant advantage over alternative techniques.
“The ability to directly measure the electron-phonon interaction unlocks numerous new opportunities,” states Boonkird.
Reexamining materials research
Armed with their theoretical findings, the researchers constructed an experimental framework to validate their approach.
As the available equipment proved insufficiently powerful for this kind of neutron scattering experiment, they were only capable of capturing a faint signal of the electron-phonon interaction — yet the results sufficiently supported their theory.
“These findings underscore the need for a new facility where the equipment could be 100 to 1,000 times more capable, enabling scientists to accurately detect the signal and assess the interaction,” adds Landry.
With upgraded neutron scattering facilities, like those proposed for the forthcoming Second Target Station at Oak Ridge National Laboratory, this experimental methodology could serve as an effective technique for measuring numerous essential material properties.
For example, by assisting scientists in identifying and utilizing superior semiconductors, this approach could facilitate more energy-efficient appliances, quicker wireless communication devices, and more dependable medical instruments like pacemakers and MRI machines.
Ultimately, the team perceives this work as a wider message concerning the necessity to reevaluate the materials research process.
“Utilizing theoretical insights to predesign experimental setups can aid us in redefining the properties we can measure,” remarks Fu.
To this end, the team and their collaborators are currently investigating other types of interactions they might utilize to explore additional material properties.
“This is a highly intriguing paper,” says Jon Taylor, director of the neutron scattering division at Oak Ridge National Laboratory, who was not a participant in this research. “It would be fascinating to develop a neutron scattering method that is directly responsive to charge lattice interactions or more broadly electronic effects beyond just magnetic moments. Given that such an effect is anticipated to be rather small, facilities like STS could significantly aid in uncovering that fundamental understanding of the interaction and routinely leverage such effects for research.”
This research is partially funded by the U.S. Department of Energy and the National Science Foundation.
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