A researcher at the Department of Physics at Tohoku University has uncovered a surprising quantum phenomenon hidden inside ordinary crystals: the strength of interactions between electrons and lattice vibrations – known as phonons – is not continuous, but quantized. Even more remarkably, this strength is universally linked to one of physics’ most iconic numbers: the fine-structure constant.

What makes this dimensionless number (α ≈ 1/137) so iconic is its ability to explain electromagnetic interactions, independent of the units used. Imagine it like a ratio where one pencil is twice as long as another pencil – this ratio won’t change no matter whether you measure the pencil length in cm, inches, or feet.

The study, led by Masae Takahashi of Tohoku University, reveals that electron-phonon coupling strength is always an integer multiple of a base unit equal to the fine-structure constant multiplied by the Boltzmann constant. In other words, about one part in 137 of the phonon’s energy is transferred during each interaction. Using advanced terahertz spectroscopy, which probes vibrations in the energy range between infrared and microwaves, electron-phonon coupling was measured with unprecedented precision. This breakthrough demonstrates that a fundamental constant governing electromagnetic forces also applies to the microscopic “dialogue” between electrons and crystals.

Why does this happen? Takahashi traced the origin to a process resembling Compton scattering, where electrons collide not directly with phonons but with photons emitted by phonons. This insight explains why the energy transfer scales with α to the first power, rather than α² as in spin-orbit interactions. Overall, this research reveals a universal quantum rule governing how electrons interact with lattice vibrations inside crystals.

“This new finding was exciting, as it’s the first time in quite a while that we can add new information to well-established quantum mechanics,” remarks Takahashi.

By quantifying these interactions and rules, scientists can design materials with tailored properties for faster electronics and more efficient energy technologies. For example, electron-phonon interactions govern the performance of semiconductors, superconductors, and next-generation quantum devices. Terahertz waves can also influence processes such as cell division, implying that this finding may eventually impact future innovations not just for everyday electronics like smartphones and computers, but for life sciences as well.

“This work shows that even the whispers between electrons and crystals follow the universal language of quantum constants,” says Takahashi.