Theoretical Study On The Regulation Of Electrical Conductivity By Chiral Phonons And Vacancies

Contemporarily,with the rapid development of electronic technology,the requirements of electronic device performance are also increasing.Adapting to the development and application of electronic devices,it has become a concerned research topic to find effective methods of tuning the properties of materials.The dissertation is based on the subject of material property tuning,we performed first-principles calculations to investigate the effect of external electric field on chiral phonons in the low-buckled group-IVA monolayers and oxygen vacancies on conductivity of vanadium dioxide.(1)We investigate the vertical electric field response of chiral phonons in the low-buckled group-IVA monolayers.The vertical electric field breaks the spatial inversion symmetry of these structures,and the degeneracy of phonon modes are also broken at high-symmetry±K points of the phonon Brillouin zone.Moreover,the size of the phononic gap is proportional to the strength of the electric field.The gapped phonon modes at±K points possess chiralities with considerable phonon circular polarizations and discrete phonon pseudoangular momenta.In addition,the chiralities of phonons are robust against the variation of the field strength,but reversed by changing the field direction.Electric control of chiral phonons adds a new dimension to the tuning of chiral phonons,which has potential use in the design of phononic and valley devices.(2)Vanadium dioxide has two structures,M₁ phase and R phase,and they are insulator phase and metal phase,respectively.The material undergoes phase transition at 67?,which is accompanied by a transition between insulator and metal,thus can be applied in the design of temperature controlled electronic devices.By introducing oxygen vacancies in the vanadium dioxide film,the cooperative experimental group reduced the transition temperature of vanadium dioxide to room temperature.Based on first-principles calculations,we found that the introduction of oxygen vacancies leads to a decrease in the symmetry of the two phase structure of vanadium dioxide,and the two phase structure become more similar.According to the electronic structures,both M₁phase and R phase with oxygen vacancies exhibit metallic state.The energy difference between the two phases also decreases with the introduction of oxygen vacancies.Therefore,the oxygen vacancy will make the two phases more consistent in structure,electronic properties and energy,resulting in reducing transition temperature of vanadium dioxide.The theoretical study supports the corresponding experimental results.