The Study Of Manipulation And Origin Of The Charge Density Wave In Transition Metal Dichalcogenides VX₂ And TaX₂(X=Se,Te)

Transition metal dichalcogenides(TMDCs)VX2 and TaX2(X=Se,Te)show abundant charge density wave(CDW)orders and structural phase transition,and have attracted much attention in two-dimensional materials in recent years.The experimental researches on them have been extensively carried out,but the mechanism of the CDW order and structure phase transition is still controversial.For the application in low-dimensional electronic devices,it is important to find some effective methods to manipulate and clarify the microscopic mechanism of the CDW order,and to obtain some new long-range quantum orders such as superconductivity.Using first-principles calculations,we systematically investigated the multiple CDW orders in 1T-VSe2,the strain-controlled transition from CDW order to superconductivity,the response of the CDW order in TaSe2 system under doping and strain,the origin of phase transition in TaTe2.The details are as follows:Using first-principles calculations,we found that both Fermi-surface nesting and electron-phonon coupling account for the CDW order in bulk 1T-VSe2,while the momentum-dependent electron-phonon coupling-induced(?)×(?)CDW order is the ground state in the freestanding monolayer 1T-VSe2.We also simulated the effect of applying biaxial strain and carrier doping on the CDW order in monolayer 1T-VSe2.The results show biaxial compressive strain and low concentration of electron doping can turn the ground state into the 4x4 CDW superstructure,while biaxial tensile strain and low-concentration hole doping maintain the ground state.Our research results reasonably explained the origin of the multiple CDW orders in 1T-VSe2 observed experimentally.By calculating the Fermi surface nesting and electron-phonon coupling properties of the monolayer 1T-VTe2,we find that the CDW order is induced by momentum-dependent electron-phonon coupling.The distortion of the V atom,the formation energy of CDW and the response of the phonon dispersion of the high-symmetry phase under strain are simulated.It is found that the biaxial compressive strain can enhance the CDW instability,while the tensile strain applied to the system,the CDW order was suppressed and induced the superconductivity with a critical temperature Tc=5.1 K,which can be explained by coupling between the in-plane atomic vibrations and electronic orbits.The controllable phase transition between the CDW and the superconductivity in monolayer 1T-VTe2 pave a new avenue to exploring the relationship between CDW and the superconductivityWe systematically studied the response of the CDW order in TaSe2 system to the above-mentioned two methods.The results show that for monolayer TaSe2,hole doping makes the imaginary frequency of the undistorted phase disappear,indicating that the CDW order of the system is suppressed and making the high-symmetry phases(both 1T and 1H)be the stable phase.In addition,the superconductivity with a critical temperature of 5.8(7.7)K in 1T-(1H-)phase is induced.Conversely,electron doping enhances the imaginary frequency of undistorted 1T phase and enhances the CDW instability.We also simulated the effect of biaxial strain on the CDW order of the two structures.The results show that the biaxial compressive strain enhances the CDW order,while the biaxial tensile strain suppresses the CDW order,which is related to the distortion mode of Ta atoms of CDW superstructure formation.It is worth noting that the manipulation effect is negligible for monolayer 1T-TaSe2,but for 1H-TaSe2,the biaxial tensile strain can completely suppress the CDW order and induce a superconductivity with critical temperature of 7.2 K.The origin of the structural phase transition in TaTe2 is explored through both theory and experiment.The calculated results show that the bonding between Ta-Ta at room temperature leads to a double zigzag chain(3×1 superstructure).At low temperatures,the distance between Ta1B-Ta1B increases and destroys the 3x1 superstructure,and then forms a the 3×3 superstructure.The experimental Raman results show that there are some abnormal red shifts and new vibration modes appear when the temperature decreases,where the new Raman modes are closely related to CDW formation.Therefore,we think that the structural phase transition in TaTe2 is caused by the competition between two different CDW order(the 3×1 superstructure at room temperature and the 3×3 superstructure at low temperature).This thesis systematically investigates the origin of the CDW order in the above-mentioned system,the switching from the CDW order to superconductivity under the doping and strain,and the relationship between the CDW order and the structural phase transition.Our results show that the electron-phonon coupling effect plays an important role in CDW formation,in addition,doping and strain can effectively manipulate the transition between the CDW phase and the superconducting phase.This thesis clarified the physical origin of CDW order in VX2 and TaX2(X=Se,Te),and theoretically predicted that the manipulation of CDW order under doping and strain is effective,which has certain significance for the application of materials in electronic devices.