  Condensed matter systems have now become a fertile ground to discover
emerging topological quasi-particles with symmetry protected modes. While many
studies have focused on Fermionic excitations, the same conceptual framework
can also be applied to bosons yielding new types of topological states.
Motivated by the recent theoretical prediction of double-Weyl phonons in
transition metal monosilicides [Phys. Rev. Lett. 120, 016401 (2018)], we
directly measured the phonon dispersion in parity-breaking FeSi using inelastic
x-ray scattering. By comparing the experimental data with theoretical
calculations, we make the first observation of double-Weyl points in FeSi,
which will be an ideal material to explore emerging Bosonic excitations and its
topologically non-trivial properties.

Materials Science