Topological Phonons in RhSi

The concept of topological states of matter has been extended from electrons to other elementary excitations, such as phonons and magnons, in condensed matter systems, providing fundamentally novel ways of understanding and engineering materials properties. In this work, we study phonon excitations in RhSi by first-principles calculations based on the density functional perturbation theory (DFPT). RhSi has been reported as an ideal material hosting unconventional chiral fermions with large topological fermi arcs [1]. Our calculations for the first time show that RhSi also has topological phonons and the calculated bulk phonon band structure is consistent with the phonon dispersion measured using non-resonant inelastic x-ray scattering. Weyl nodes with opposite chirality are observed at the Brillouin zone (BZ) center and corners. Simulations of a slab structure using a Wannier interpolated tight-binding model show chiral surface phonon states propagating on two opposite surfaces of the slab. As expected, isofrequency phonon arcs are observed from calculated surface phonon density of states, connecting projections of Weyl nodes with opposite chirality. We discuss experimental techniques to probe these topological phonons predicted in RhSi, as well as possible novel electron-phonon properties in this unique material possessing both electron and phonon topology.

[1] G. Chang et al. Phys. Rev. Lett. 119, 206401 (2017).