Chiral Phonons Of Two Different Crystal Lattices

In magnetic materials,through the spin-phonon interaction which breaks time reversal symmetry,nonzero angular momentum of phonons has been theoretically predicted.Thus the gyromagnetic ratio obtained through the Einstein-de Haas effect needs to be corrected by including the phonon contribution in the total angular momentum.The nonzero phonon angular momentum means that the phonon canbe circularly or elliptically polarized.In nonmagnetic hexagonal lattice with broken inversion symmetry,the intrinsic chirality of phonons was predicted theoretically,where the non-degenerate chiral phonon is not the superposition of linear modes.For the chiral valley phonons,which have nonzero phonon Berry curvature,thus the valley phonon Hall effect can be proposed.Very recently,by using transient infrared spectroscopy in monolayer tungsten diselenide,the non-degenerate chiral phonons can be excited by electronic intervally scattering through an optical pump-probe technique.With the verification of chiral phonons in experiments,the research progress of phonon chirality has attracted the attention of many fields,and has extremely important research value.In this thesis,by introducing the center-stacked bilayer triangle lattice model,we observe chiral phonons in its Brillouin-zone corners,and the phonon modes at valleys K(K’)point are right-handed or left-handed.In addition,the phonon chirality remains robust with changing the sublattice mass ratio and interlayer coupling,such robust property is helpful for materials realization.For the valley chiral phonons,we find that they have nonzero phonon Berry curvature,which can act as an effective magnetic field to distort phonon transport under a strain gradient thus a valley phonon Hall effect appears.By first-principles calculations,we do obtained chiral valley phonons both in the real material NaCl lattice along the[111]direction,where the results well fit the theoretical simulation well.We can obtain superlattice model by the periodic atomic mass doping in honeycomb AB lattices.Since the two inequivalent valleys can fold and couple into the same Γ,we can realize the study of phonon chirality at the Brillouin-zone center.For orthohexagonal(?)×(?)honeycomb superlattices,we do observe right-handed or left-handed chiral phonon near the Brillouin-zone center Γ,and circularly polarized phonons which protected by the threefold rotation symmetry are also observed.In addition,we also find that the chiral phonons have nonzero phonon Berry curvature,thus the Hall effect can be proposed.We also discover a useful valley manipulation mechanism at Γ point by adjusting the doping sublattice’s mass.Thus chiral phonons can be engineered in specific bands.Finally,we also discuss the phonon chirality of a deformed(?)×(?)honeycomb superlattice.The chiral elliptically chiral phonons are also observed in the Brillouin-zone center Γ,and the valley manipulation mechanism still exist under the doping sublattice’s mass changing.Through the above two lattice systems,we have systematically study the extremely topological properties of the chiral phonons at the high-symmetry points K(K’)and Γ in the Brillouin-zone,and obtain a series of important conclusion.We firmly believe that such a work can provide a new idea for the study of phonons,and provide theoretical guidance for the follow-up experiments,which will greatly promote phononic applications.