Study On Chiral Phonons In Graphyne And Twisted Bilayer Graphene

The theoretical prediction and experimental discovery of chiral phonons overturn the traditional cognition that phonons do not have angular momentum.At present,chiral phonons have attracted wide attention in many fields and have very important application value.Chiral phonons can exist widely in lattices that break the symmetry of spatial inversion or temporal inversion,such as monolayer tungsten diselenide,monolayer graphene,and allotrome of graphene.This paper focuses on the similarities and differences of phonon chirality between monolayer graphene and twisted bilayer graphene,the allotrope of graphene-graphyne.In this paper,we have studied the chiral phonons in α-,β-and γ-graphyne by first principles.Unlike the chiral phonons in the traditional graphene-like structure,we found that the chiral phonons have the largest phonon polarization values at the non-highly symmetric points,and a quasi-valley pattern appears.Interestingly,the chiral phonons in these graphite-alkynes are hybridization-dependent.Among them,the branch is mainly composed of intermediate frequency phonon sp2 hybrid carbon atoms contribution;And sp hybrid carbon atoms in other branches of phonon frequency dominated.In addition,due to the uneven distribution of force constants around the K(K’)valley,differences in the rate at which phonon polarization weakens in different directions at the K(K’)valley have been observed in β-and γ-graphyne.In addition,we investigated chiral phonons in twisted bilayer graphene by first principles system.For the AB stacking of graphene,the position of chiral phonons is similar to that of graphene.With the change of angle,the increase of atomic number,and the opening of old degenerate points,new degenerate points will appear,making the existence of chiral phonons more diverse and complex than that of monolayer graphene.In addition,after studying the effect of vertical stress on twisted bilayer graphene,we found that adjusting the vertical stress by changing the layer spacing does not produce new chiral phonons,but only has a certain effect on the frequency of existing chiral phonons.The research in this paper can not only deepen the understanding of chiral phonons,expand the research of phonon chirality,but also hopefully expand the practical application of chiral phonons,and provide theoretical guidance for the design of phonological devices.