Direct Wavefunction Detection of Lattice Vibrations by Inelastic Neutron Scattering

In close analogy to electronic semimetals, a variety of Weyl, Dirac points and nodal lines are predicted for phonons recently by ab initio calculations. However, very few experimental confirmations have been made, and most of them focused on bulk dispersions only. In this talk, we will present the results of our Inelastic Neutron Scattering (INS) experiments on MnSi and CoSi crystals. The measured phononic band structures and coherent cross sections are in remarkable agreement with our DFPT calculations, which enabled us to explore multiple types of unconventional Weyl and Dirac points in the Brillioun zone. Furthermore, being aware of neutron scattering's capability of wavefunction detection and polarization determination, we derived an explicit relation between the topological invariant of a Weyl node and the INS intensities nearby. Apart from protected surface states and novel transport phenomena, our work suggests that topological nodes also leave some observable traces in spectroscopic patterns.