A team of researchers from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Fritz Haber Institute of the Max Planck Society, and additional collaborators in Berlin, Dresden, Jülich, and Eindhoven have experimentally demonstrated and coherently controlled the transfer of angular momentum between lattice vibrations, providing the first direct observation of how this conserved quantity propagates through a crystal lattice.
In solids, the exchange of energy and linear momentum between phonons via anharmonic coupling is well established. However, tracking angular momentum transfer between lattice modes has remained elusive, despite its central role in magnetization dynamics and spin relaxation phenomena such as the Einstein–de Haas effect. The present work closes this gap by directly resolving how quantized crystal angular momentum is redistributed between coupled vibrational modes.
The experiments rely on intense terahertz excitation to selectively drive a lattice vibration into a circular (chiral) motion, thereby endowing the phonon mode with angular momentum. A second ultrafast probe pulse is then used to monitor how this rotational motion evolves and couples into another mode via anharmonic interactions. Crucially, the team accesses the inverse process of anharmonic decay, enabling coherent control over the transfer pathway.
A key observation is that angular momentum transfer between the two modes is governed by a rotational phonon–phonon Umklapp process. As a result, the total angular momentum is conserved modulo the discrete rotational symmetry of the crystal lattice. This leads to a counterintuitive outcome: when angular momentum is transferred, the receiving mode can rotate in the opposite direction while oscillating at twice the frequency of the initial excitation. This behavior is described as a “1 + 1 = −1” effect, reflecting how two quanta of angular momentum combine into a state with reversed rotational sense under the symmetry constraints of the lattice.
The phenomenon was observed in bismuth selenide, where the symmetry of the crystal enforces equivalence between certain rotational states with opposite handedness. The measured frequency doubling and reversal of rotation provide a direct quantum mechanical signature of angular momentum conservation in a periodic solid, analogous to well-known Umklapp processes for linear momentum.
“I find it extraordinarily elegant how the laws of physics are directly dictated by the symmetries of nature,” says Olga Minakova, doctoral researcher at the Fritz Haber Institute of the Max Planck Society and central experimental physicist of the study.
Sebastian Maehrlein, head of department at the Institute of Radiation Physics at HZDR, professor at TU Dresden, and leader of the study, adds: “For me, these are exceptionally exciting results. We have discovered something fundamentally new that will hopefully make its way into the textbooks.”
Beyond establishing the fundamental analogy between linear and angular momentum conservation in solids, the work introduces axial nonlinear phononics as a new route for controlling material properties on ultrafast timescales. By selectively steering angular momentum between lattice modes, this approach could enable new strategies for manipulating magnetization and other collective degrees of freedom in quantum materials.