HOUSTON – (March 23, 2026) – You may not be able to hear it, but all solid materials make a sound. In fact, atoms ⎯ bound in lattices of chemical bonds ⎯ are never silent nor still: Under the placid surface of each and every object in our surroundings, a low hum hovers or a high-energy squeak titters.
As atoms vibrate in their lattices, they do so by either all moving in the same direction, in which case their collective vibration shows up as a low humming sound, or by moving in opposite directions from one another, giving rise to an energetic vibration that registers as a bright squeak or titter.
"These vibrations are crucial for both classical or quantum electronics," said Hanyu Zhu , a corresponding author on a new study published in Physical Review Letters that reports an unusual quantum coherence of these vibrations, or phonons, in cubic boron arsenide, a semiconductor with promising electronic and thermal properties.
The humming sounds, or acoustic phonons, play a key role in heat conduction. When a computer chip warms up, it is acoustic phonons that ferry heat energy away.
The tittering sounds, or optical phonons, govern infrared thermal radiation. They can not only provide another channel for managing excess heat in electronics but also directly transmit information into surrounding space. However, they typically have a shorter lifetime than acoustic phonons because, in most materials, optical phonons transfer energy to acoustic phonons through friction.
"Quantum mechanics dictates that this process must involve an integer number of particles, meaning at least one in and two out," said Zhu, who is the William Marsh Rice Chair and associate professor of materials science and nanoengineering.
Physicists refer to this process as three-phonon scattering. However, the energy transfer from optical phonons to acoustic phonons can also take a different, much less probable pathway which involves splitting into three particles ⎯ a process counterintuitively called four-phonon scattering.
"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 said. "This means that optical phonons in boron arsenide are especially long-lived."
The team of researchers, including Zhifeng Ren's group at the University of Houston and Rui He's group at Texas Tech University, produced high-quality crystals with only boron-11 isotopes. They then used two techniques, high-resolution Raman and infrared spectroscopy, to study phonon scattering pathways at both room temperature and cryogenic temperatures.
"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.
The analysis of coherence temperature dependence confirmed that in boron arsenide, four-phonon scattering is dominant over three-phonon scattering. The findings further suggest that the remaining boron-10 isotope is the main culprit of coherence loss at the quantum ground state.
"Our sample contains some puddles of structural defects, but surprisingly and gladly, they do not affect the coherence of optical phonons at all," said Sanjna Sukumaran, a study co-author who is a doctoral student in the Zhu lab.
"Without isotope impurity, we can extend the lifetime by another 10 times," Zhu said. "These findings encourage further efforts of isotope engineering in boron arsenide, which offers a promising semiconductor platform for quantum phononics."
Tong Lin, a Rice doctoral alumna who worked under Zhu's supervision, is the first author on the study.
The research was supported by the Welch Foundation (C-2128), the Air Force Office of Scientific Research (FA9550-24-1-0135), the U.S. Department of Energy (DE-SC0020334), the U.S. National Science Foundation (2300640, 2425439) and Qorvo Inc. The content in this press release is solely the responsibility of the authors and does not necessarily represent the official views of funding entities.
