Experimental evidence of chiral phonons in 2D materials

Chirality reveals symmetry breaking of the fundamental interaction of elementary particles. In condensed matter, the chirality of electrons governs many unconventional transport phenomena such as the quantum Hall effect. However, chiral phonons exhibiting intrinsic collective atomic rotation, as opposed to superpostion of degenerate linear oscillation, have never been observed without external magnetic fields. Here, we report an experimental evidence of phonons with intrinsic chirality in 2D materials. The broken inversion symmetry of the lattice lifts the degeneracy of clockwise and counter-clockwise phonon modes at the corners of the Brillouin zone. We identified the chirality of phonons by the selection rules of hole-phonon interactions based on pseudo-angular momentum (PAM) conservation. We measured the linear momentum, PAM and energy of the phonon, and the results agree well with the theory. The discovery of phonon chirality is important for electron-phonon coupling in solids, phonon-driven topological states and energy-efficient information processing.