Microstructural And Optically Induced Ultrafast Dynamic Features On TaSe_2-xTe_x Systems

The transition metal dichalcogenide compounds 1T-TaX₂?X=Se?Te?are typical quasi-two-dimensional?2D?charge density wave?CDW?materials with rich physical properties and phase transitions;the superconductivity could be tuned by chemical doping or external stimulation in 1T-TaX₂ compounds.In this thesis,the structural modulation correlated with CDW,local structural distortion,the structural dynamics of equilibrium phase transition,photoinduced nonequilibrium phase transition in 1T-TaSe₂ and the ultrafast structural dynamics in 1T-TaSe_2-xTe_x compounds have been systematically investigated using conventional TEM techniques and ultrafast transmission elctron microscope?UTEM?.The main results can be summaried as following:1.Structural modulations and thermal driven equilibrium phase transition in 1TTaSe₂.The CDW modulation in 1T-TaSe₂ can be written as (8=(4(6*-(7*+ 2?8*?/13.The phase transition of 1T-TaSe₂ has been investgated with atomic resolution using chip-based TEM heating holder of high stability.The experimental results demonstrate that the 1T-TaSe₂ goes through the CCDW?commensurate CDW?phase directly to ICCDW?incommensurate CDW?via increasing temperature;the CCDW phase and ICCDW phase coexist in a narrow temperature range and no nearly commensurate?NC?CDW phase has been found during the heating.2.Direct observation of an optically induced charge density wave phase transition in 1T-TaSe₂.Under photoexcitation,UTEM measurements reveal remarkable changes in both the CDW intensity and orientation,associated with the transformation from a CCDW phase into an ICCDW phase in a time scale of picoseconds.Careful analysis of dynamic change of the CDW states suggests that the photoinduced phase transition in 1T-TaSe₂ contains three transient steps: commensurate?C?phase suppression,incommensurate?IC?phase growth,and a partial CDW reordering from the IC to the C phase??=3 ps?;a time delay?about 2 ps?is observed between the C-phase suppression and the IC-phase growth.Moreover,the transient states reveal a notable “structural isosbestic point” at a wave vector of qiso where the C and IC phases yield their diffracting efficiencies in equal ratio.This observation reveals that the crystal planes parallel to qiso adopt a common dynamic feature in the CDW phases throughout whole nonequilibrium transition.The second-order phase transition characteristics observed in this nonequilibrium phase transition have also been analyzed based on time-resolved structural data.3.Microstructure analysis and it's correlation to superconductivity in 1T-Ta Se2-xTex systems.The structural features and three dimensional nature of CDW state of the layered chalcogenide 1T-TaSe_2-xTe_x are characterized by Cs-corrected transmission electron microscopy measurements;our result clearly reveals that the coupling of the CDW states among layers are evident in all the samples despite the weak van der Waals bonding;notable changes of both average structure and the CDW state arising from Te substitution for Se are clearly demonstrated in samples with x>0.3.The commensurate CDW state characterized by the known star-of-David cluster in the 1T-TaSe₂ crystal becomes visibly unstable with Te substitution and vanishes when x=0.3.1T-TaSe_2-xTe_x?0.3?x?1.0?samples generally adopt a remarkable incommensurate CDW state with monoclinic distortion,which could be fundamentally related to the strong q-dependent electron-phonon coupling induced period-lattice-distortion?PLD?as identified in the TaTe2.Systematic analysis demonstrates that the occurrence of superconductivity is connected with the suppression of the commensurate phase Mott insulating state and the presence of discommensuration is an evident structural feature observed in the superconducting samples.4.Dynamic diffraction effects and coherent breathing oscillations in ultrafast electron diffraction investigated in layered 1T-Ta SeTe.The ultrafast dynamics of twodimensional layered 1T-TaSeTe material have investigated using UTEM and a rich variety of structural phenomena in correlation with the lattice relaxation were extracted from the time-resolved experimental data.Under photoexcitation,remarkable changes in both the CDW and main diffraction spots are clearly observed.Detailed analysis shows that the satellite spots arising from CDW was suppressed in 2.8 ps and the time constant of lattice thermalization associated with electron-phonon coupling is about 4.5 ps.Anisotropic lattice movements due to the different intralayer and interlayer bonding feature have been observed;it is clearly revealed that the intensity of Bragg reflection spots often exhibit remarkable changes arising from the dynamic diffraction effects and anisotropic lattice movement.Our theoretical analysis based on the dynamic diffraction theory clearly demonstrated that these structural features observed in the Bragg peak evolution is attributed to the joint effects of the Debye-Waller effect and change of deviation parameter s induced by anisotropic lattice oscillation for the first time.