Negative Refraction of Weyl Phonons at Twin Quartz Interfaces.

In Nature, α-quartz crystals frequently form contact twins, which are two adjacent crystals with the same chemical structure but different crystallographic orientation, sharing a common lattice plane. As α-quartz crystallizes in a chiral space group, such twinning can occur between enantiomorphs with the same handedness or with opposite handedness. Here, we use first-principles methods to investigate the effect of twinning and chirality on the bulk and surface phonon spectra, as well as on the topological properties of phonons in α-quartz.