A team of engineers and materials scientists at CU Boulder’s Paul M. Rady Department of Mechanical Engineering has uncovered how tiny thermal vibrations—known as phonons—interact inside a single molecule. Just like musical notes, these phonons can interfere with one another, either amplifying or canceling each other out, depending on how the molecule is structured.
Ultra-high vacuum scanning probe setup modified by the Cui Research Group to conduct thermal microscopy experiments. Image Credit: University of Colorado Boulder
Phonon interference at the molecular scale has never been measured at room temperature—until now. This team has developed a new technique that makes it possible to observe these subtle, heat-carrying motions with unprecedented clarity.
Led by Assistant Professor Longji Cui and his group, the research was recently published in Nature Materials. The work was supported by the National Science Foundation and involved collaborators from Spain (Instituto de Ciencia de Materiales de Madrid, Universidad Autónoma de Madrid), Italy (Istituto di Chimica dei Composti Organometallici), and CU Boulder’s Department of Chemistry.
Understanding how phonons behave is essential because they’re the main carriers of energy in insulating materials. Cui and his team believe this discovery could ultimately reshape how we manage heat in next-generation electronics and materials.
Interference is a fundamental phenomenon. If you have the capability to understand interference of heat flow at the smallest level, you can create devices that have never been possible before.
Longji Cui, Assistant Professor, University of Colorado Boulder
Cui is also affiliated with the Materials Science and Engineering Program and the Center for Experiments on Quantum Materials.
The World’s Strongest Set of Ears
Cui explains that molecular phononics—the study of phonons within a single molecule—has long been a largely theoretical field. Observing these molecular vibrations in real time has been out of reach, primarily because the tools needed to “hear” these tiny, heat-carrying motions simply didn’t exist.
That changed with the work of Cui and his team.
They developed a thermal sensor so small it's nearly invisible—smaller than a grain of sand, even tinier than a speck of sawdust. Despite its size, this sensor is incredibly precise. With record-breaking resolution, it can isolate a single molecule and detect phonon vibrations at the smallest measurable scale.
Using these custom-designed thermal probes, the researchers investigated how heat travels through individual molecular junctions. What they found was striking: certain molecular structures can trigger destructive interference, where phonon vibrations collide and cancel each other out, reducing the flow of heat.
Sai Yelishala, a Ph.D. student in Cui’s lab and lead author of the study, explained that this experiment marks the first time destructive phonon interference has been observed at room temperature.
In practical terms, this means the team has found a way to control heat flow at the molecular level—the scale where all materials originate.
Let’s say you have two waves of water in the ocean that are moving towards each other. The waves will eventually crash into each other and create a disturbance in between. That is called destructive interference and that is what we observed in this experiment. Understanding this phenomenon can help us suppress the transport of heat and enhance the performance of materials on an extremely small and unprecedented scale.
Sai Yelishala, Study Lead Author and PhD Student, University of Colorado Boulder
Tiny Molecules, Vast Potential
Building the most sensitive tool yet for detecting phonon behavior is a major feat. But the bigger question remains: what can these tiny vibrations actually do?
Yelishala added, “This is only the beginning for molecular phononics. New-age materials and electronics have a long list of concerns when it comes to heat dissipation. Our research will help us study the chemistry, physical behavior and heat management in molecules so that we can address these concerns.”
Take polymers, for example. These common organic materials tend to have low thermal conductivity and are sensitive to temperature changes—traits that can lead to overheating and material breakdown.
But with insights from phonon interference research, scientists may one day design new molecular architectures that behave more like metals, using constructive phonon interactions to enhance heat transport instead of limiting it.
The potential extends even further. The technique could significantly impact fields like thermoelectricity, which involves converting heat into electrical energy. By better controlling how heat flows through a material, researchers could boost the efficiency of thermoelectric devices and move closer to scalable clean energy solutions.
According to the team, this study is only a first step. They’re already working with CU Boulder chemists on follow-up projects aimed at exploring other phonon behaviors using their advanced thermal probes.
“Phonons travel virtually in all materials. Therefore we can guide advancements in any natural and artificially made materials at the smallest possible level using our ultra-sensitive probes,” concluded Yelishala.
Journal Reference:
Yelishala, S, C., et al. (2025) Phonon interference in single-molecule junctions. Nature Materials. doi.org/10.1038/s41563-025-02195-w