**Title: First observations of topological phonons in crystalline materials**

Host: Jen Cano

Condensed matter systems have now become a fertile ground to discover emerging topological quasiparticles 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. This idea has, for example, been applied to great effect in macroscopic waveguides. Motivated by the application of these ideas to naturally occurring crystal lattices, we used inelastic x-ray scattering to make the first observation of topological phonons [1]. We demonstrate that new classes of topological crossing can be accessed in this way, such as “Double Weyl” crossings in FeSi and parity-time symmetry protected helical nodal lines in MoB_{2}[2]. Phonon band structures thus provide compelling new playgrounds for exploring topological properties and I will discuss how they differ from the well-studied electronic band structures from this perspective. I will end by speculating how this might be useful in the future.

**References**

[1]H. Miao, T. T. Zhang, L. Wang, D. Meyers, A. H. Said Y. L. Wang, Y. G. Shi, H. M. Weng, Z. Fang, and M. P. M. Dean, Phys. Rev. Lett. 121, 035302 (2018)

[2] T. T. Zhang, H. Miao, Q. Wang, J. Q. Lin, Y. Cao, G. Fabbris, A. H. Said, X. Liu, H. C. Lei, Z. Fang, H. M. Weng, and M. P. M. Dean, submitted (2019)