HOUSTON – The world around us isn’t as still as it seems. Beneath the surface of every solid object, atoms are in constant motion, vibrating within their chemical bonds. These vibrations, often imperceptible, are now the focus of groundbreaking research at Rice University, the University of Houston, and Texas Tech University. Scientists have achieved a record-breaking level of quantum coherence in these vibrations – known as phonons – within a material called boron arsenide, potentially opening doors to advancements in quantum electronics and heat management. This research, published in the journal Physical Review Letters, centers on understanding how these atomic-level movements can be harnessed for technological innovation.
The significance of these vibrations extends beyond a purely theoretical curiosity. Phonons play a critical role in how materials conduct heat and interact with light. “These vibrations are crucial for both classical or quantum electronics,” explains Hanyu Zhu, a corresponding author on the study and the William Marsh Rice Chair and associate professor of materials science and nanoengineering at Rice University. Understanding and controlling them is key to developing more efficient and powerful technologies, particularly as we strive to overcome the limitations of traditional silicon-based electronics. The field of quantum phononics, which explores the quantum properties of these vibrations, is rapidly gaining momentum.
Unlocking Coherence in Boron Arsenide
The team’s work focuses on cubic boron arsenide, a semiconductor material that has shown promise in both electronic and thermal applications. Atoms within a material vibrate in different ways. Acoustic phonons, described as a “humming” sound, are responsible for conducting heat. Optical phonons, a “tittering” sound, govern how materials radiate heat and can potentially transmit information. Typically, optical phonons lose energy quickly due to friction as they transfer energy to acoustic phonons – a process known as three-phonon scattering. However, boron arsenide presents a unique scenario.
“In boron arsenide, an optical phonon contains more energy than any possible combination of two outgoing acoustic phonons, so the friction against one optical phonon by two acoustic phonons does not occur,” Zhu explained. So optical phonons in boron arsenide can persist for a significantly longer time, retaining their energy and coherence. The researchers discovered that a less common process, four-phonon scattering, dominates in boron arsenide, further contributing to this extended coherence. This is a crucial finding, as longer coherence times are essential for utilizing phonons in quantum technologies.
Isotope Engineering and Record-Breaking Results
To achieve these results, the researchers produced high-quality crystals of boron arsenide composed almost entirely of boron-11 isotopes. Isotopes are variations of an element with different numbers of neutrons. By using a single isotope, they minimized disruptions to the phonon vibrations. They then employed high-resolution Raman and infrared spectroscopy to study how phonons scatter within the material at both room temperature and extremely cold, cryogenic temperatures.
The results were striking. “We found record-high coherence for phonons at low temperatures, when the vibration completed nearly a thousand cycles before fading, compared to less than a hundred in typical materials,” Zhu said. This represents a significant leap forward in phonon coherence, opening up possibilities for manipulating and utilizing these vibrations in new ways. The team also determined that the remaining trace amounts of boron-10 isotopes are the primary cause of coherence loss at the lowest temperatures.
Implications for Future Technologies
The implications of this research are far-reaching. Longer phonon coherence times could lead to more efficient heat dissipation in electronic devices, preventing overheating and improving performance. This is particularly vital as computer chips become increasingly powerful and densely packed. The ability to control phonons could pave the way for new types of quantum devices that utilize phonons to store and process information. The U.S. Department of Energy is actively investing in research related to quantum materials, recognizing their potential to revolutionize various fields.
Sanjna Sukumaran, a doctoral student in Zhu’s lab and a co-author of the study, noted that even with some structural defects in the crystal samples, the coherence of the optical phonons remained remarkably unaffected. “Our sample contains some puddles of structural defects, but surprisingly and gladly, they do not affect the coherence of optical phonons at all,” she said. Zhu believes that further refining the material by eliminating boron-10 isotopes could extend the phonon lifetime by another tenfold. “Without isotope impurity, we can extend the lifetime by another 10 times,” he stated. This highlights the potential of “isotope engineering” – carefully controlling the isotopic composition of materials – as a powerful tool for materials science.
The research was supported by funding from the Welch Foundation, the Air Force Office of Scientific Research, the U.S. Department of Energy, the U.S. National Science Foundation, and Qorvo Inc. Tong Lin, a Rice doctoral alumna, served as the first author on the study.
Looking ahead, the team plans to continue exploring isotope engineering techniques to further enhance phonon coherence in boron arsenide. The ultimate goal is to create a semiconductor platform that can fully exploit the potential of quantum phononics, leading to a new generation of electronic and thermal technologies. The next step involves creating even purer samples of boron arsenide, minimizing the presence of boron-10, and observing the resulting increase in phonon lifetime.
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