In a groundbreaking development, physicists have witnessed a fascinating phenomenon: the transfer of angular momentum between crystal vibrations. This discovery, published in Nature Physics, sheds light on a century-old theoretical prediction and fills a significant gap in our understanding of physics.
The vibrations within a crystal's atomic structure generate a unique form of rotation known as angular momentum. For the first time, researchers have directly observed this rotational energy moving between different vibration modes in real-time.
This achievement is not just a scientific curiosity; it addresses a fundamental question that has lingered since the groundbreaking experiments of Einstein and de Haas over a century ago. In crystals, atoms vibrate like tiny bells, producing phonons. While we've understood the exchange of energy and momentum between phonons, the transfer of rotational momentum has remained elusive.
The team, led by physicists from the Fritz Haber Institute, chose a topological insulator, bismuth selenide, as their test material. By perturbing the crystal with terahertz pulses, they induced a vibrational mode that rotated almost 360 degrees. This rotation, controlled by infrared-active phonons, connected with another mode vibrating at a harmonized frequency, resulting in a stunning observation.
The second phonon mode acquired angular momentum equal and opposite to the first mode's rotation. This change in helicity, while not forbidden by physics, arises from the crystal's unique threefold rotational symmetry. The process, termed rotational phonon, phonon Umklapp scattering, is analogous to linear momentum Umklapp scattering.
What makes this finding even more remarkable is its efficiency. Computational simulations revealed that the transfer of angular momentum from phonon to phonon is over 1000 times more efficient than direct light excitation. This suggests a highly optimized natural process.
The implications of this discovery extend beyond the laboratory. In the field of magnetism, researchers have long sought to understand the transfer of angular momentum during demagnetization. The Einstein-de Haas effect, which relies on phonons to transport spin angular momentum, has left some intermediate mechanisms unclear. The new results provide a crucial piece of this puzzle, confirming the long-standing hypothesis that phonon-phonon angular momentum transfer is permitted via lattice anharmonicity.
Looking ahead, the team envisions a new field, axial nonlinear phononics, where the precise control of axial momentum in phonon modes could lead to applications in ultrafast magnetic switching and topological materials. This development opens up exciting possibilities for future research and technological advancements.
