Xiulin Ruan receives the 2023 Brillouin Medal for four-phonon scattering

Xiulin Ruan has received the 2023 Brillouin Medal, given to honor a specific seminal contribution in the field of phononics. This highlights Ruan's groundbreaking work on four-phonon scattering.
Xiulin Ruan (center) receives the 2023 Brillouin Medal from the board members of the International Phononics Society at their Phononics 2023 Conference in Manchester, UK in June 2023.

Ruan, professor of mechanical engineering at Purdue University, studies a broad range of topics related to heat transport, nanotechnology, and machine learning. In 2012, he and Ph.D. student Tianli Feng (now assistant professor at the University of Utah) started tackling four-phonon scattering, a phenomenon which is fundamental to how a material conducts heat. They spent two years to establish the general formulation of four-phonon scattering, and another two years to develop its computational method and predict the importance of four-phonon scattering on thermal conductivity at high temperature. In 2016, they finally reported these findings in a groundbreaking paper on four-phonon scattering. In 2017, they published an influential further study, where they extended the method to first principles and predicted unexpected strong impact of four-phonon scattering on a high thermal conductivity semiconductor called boron arsenide, even at room temperature.

“Being able to predict four-phonon scattering has been a decades-long challenge,” said Ruan in 2017. "Three-phonon interactions have long been studied, but four-phonon scattering was considered to be too difficult to model, and hence it was ignored. Now we have clearly shown its importance."

At the time, their theory and predictions came as a surprise to the community, and there was a lot of skepticism about the work. However, it was later confirmed by several important experiments, which facilitated the acceptance of their theory. The 2017 publication has now received nearly 400 citations, becoming a top 1% cited paper in physics. It was also named a Highly Cited Paper by Clarivate Web of Science. The four-phonon scattering theory was included in the 2nd edition of a widely used graduate textbook, Nano/Microscale Heat Transfer, published by Springer in 2020.

“People refer to our 2017 paper much more, as it studied boron arsenide, now a star material,” said Ruan. “However, in our minds the 2016 work is more important, because it took the first step to tackle four-phonon scattering and laid down the formulation and computational method. It was the longest and most challenging step in our journey.”

Since then, Ruan has published several other papers on the topic, including demonstrating four-phonon scattering in infrared and Raman spectra for the first time. He has been invited to give more than ten plenary, keynote, tutorial, or invited talks on this topic at ASME, MRS, SPIE, and other international conferences. Funding agencies have recognized the breakthrough by awarding Ruan and Feng significant grants where four-phonon scattering is a key capability; these include multiple awards from DARPA, NSF, and NASA. These theories have also contributed to Ruan's invention of the world's whitest paint, which has garnered huge media attention from around the world, as well as a Guinness World Records™ title.

The Brillouin Medal, awarded every two years, honors a specific seminal contribution in the field of phononics (including phononic crystals, acoustic/elastic metamaterials, nanoscale phonon transport, wave propagation in periodic structures, coupled phenomena involving phonons, topological phononics, and related areas). The 2023 medal was awarded to Ruan and Feng by the International Phononics Society at their Phononics 2023 - 6th International Conference on Phononic Crystals/Metamaterials/Metasurfaces, Phonon Transport, Topological Phononics in June 2023 in Manchester, UK.

While white paint has put his name in the headlines, Ruan is especially proud of the honor given to the fundamental scientific discovery on four-phonon scattering. "This has opened a new research area of higher-order phonon scattering and inspired numerous uses and developments," he said. "Researchers have applied these theories to high thermal conductivity materials, low thermal conductivity materials, high-temperature applications, two-dimensional materials, batteries, nuclear materials, thermoelectrics, infrared and Raman spectra, and thermal radiative transport. I can't wait to see what the future holds for this science."


Source: Xiulin Ruan, ruan@purdue.edu

Writer: Jared Pike, jaredpike@purdue.edu, 765-496-0374