First-principles Study Of Chiral Phonons In Two Two-dimensional Heterojunction Systems

In classical physics theory,phonons have always been considered to be linearly polarized with no angular momentum.Until recently,people have discovered theoretically and experimentally that phonons have angular momentum.In magnetic materials,the angular momentum of phonon will correct the gyromagnetic ratio measured by Einstein-de Haas effect.In non-magnetic materials,although the total angular momentum of phonons is zero,the breaking of spatial inversion symmetry leads each phonon mode to carry angular momentum.In other words,phonons are chiral,and this theory has recently been proved by experiments.With the experimental verification of chiral phonons,the study of phonon chirality has attracted attention in many fields owing to its important potential application value.In this paper,we study the nondegenerate chiral phonons in the interlayer heterojunction of graphene/hexagonal boron nitride(G/h-BN)and also investigate chiral boundary mode at the interface of the interlayer heterojunction composed of AB-type boron nitride and BA-type boron nitride.We systematically study the chiral phonons in the classical heterostructure of G/h-BN by first-principles calculations.It is found that the broken inversion symmetry and the interlayer interaction of G/h-BN not only open the phononic gaps but also lift the degeneracy of left-handed and right-handed chiral phonons at the first-Brillouin zone corners(valleys).At valleys,the hybridization makes chiral phonon modes solely contributed from one individual layer.Moreover,we demonstrate that the vertical stress is effective to tune the degenerated phononic gap while keeping the valley-phonon chirality of G/h-BN heterostructure,which is favorable for the Raman or ultrafast infrared spectroscopy measurement.We also analyze the pseudoangular momentum of valley phonon modes,which provide important references for the excitation and measurement of the chiral phonons in the process of electronic intervalley scattering.Collectively,our results on the chiral phonons in the G/h-BN heterostructure system could stimulate more experimental and theoretical studies and promote the future applications on the phonon-chirality-based phononics.In addition,we study the topological protected chiral phonons in quasi-one-dimensional intralayer heterojunction structures with homogenous bond defects by first-principle calculations.It is found that h-BN nanoribbons and AIN nanoribbons(Z-ABBA/BN nanoribbons and Z-ABBA/AlN nanoribbons),due to the existence of homogenous bond defects,topological boundary mode appears.For the phonon band which is protected by the topology,the atoms in the vicinity of the defect are circularly rotated,and the amplitude is decreased from the middle to the two ends,and the atoms at the two ends of the nanoribbons hardly do circular polarization vibration.However,without defects in the nanobelts(Z-ABAB/BN nanoribbons and Z-ABAB/AlN nanoribbons),there is no chiral boundary mode with topological protection.Our results reveal the possibility of exciting and detecting topologically protected chiral phonons in intralayer heterojunction nanoribbon materials.