Symmetry principles constitute the foundation of physics, linking conservation laws to translational and rotational invariance. In crystalline solids, the transfer of energy and linear momentum between lattice vibrations via anharmonic coupling is a well-established concept governing thermal conductivity and equilibrium properties of materials. However, it has remained an open experimental challenge to directly observe how angular momentum is exchanged and conserved among lattice modes, even though angular momentum transfer within a crystal lattice is thought to play an important role in achieving magnetization equilibrium and in spin relaxation phenomena such as the Einstein-de Haas effect and ultrafast demagnetization.

In their Nature Physics paper, the THz Structural Dynamics group demonstrates the transfer of angular momentum between two lattice modes by employing the inverse process of anharmonic phonon decay. Umklapp scattering of rotational phonon-phonon is observed, which enforces the conservation of quantized crystal angular momentum as dictated by the discrete rotational symmetry of the crystal lattice. These findings provide direct experimental confirmation of the fundamental analogy between linear and angular momentum conservation in solids. Moreover, the work establishes axial nonlinear phononics as a promising new handle for the ultrafast control of material properties.

—