Investigation Of Charge Density Waves In Two-dimensional Transition Metal Dichalcogenides 1T-TiSe₂

Charge density wave（CDW）is a macroscopic quantum phenomenon related to the electron instability with periodic modulation of uniformly distributed charge,which causes the changes of electron order and structure order.It accompanied with the opening of electronic band gap and periodic lattice distortion（PLD）.Nanolayer1T-Ti Se₂ is a kind of direct bandgap material,and the second order phase transition occurs at T_c=200 K.In this paper,we grew 1T-Ti Se₂ thin film materials by chemical vapor transport method,then we mainly investigated and discussed the phonon properties and ultrafast dynamic processes and mechanism of CDW phase transition.Our results suggested that electron-phonon coupling played a dominant role in driving CDW phase transition.The study of the light-induced spectral changes of 1T-Ti Se₂provides a new direction for the design of future temperature-dependent 2D devices,which have many practical applications in material,physicochemical and biological sciences.Raman spectroscopy is a common technique to characterize TMDCs.We measured Raman spectrum of 1T-Ti Se₂ in variable temperatures with a self-built Raman system.We also have detected the A_1g-116 cm^-1 associated with strong CDW coupled mode,which belongs to the second resonant Raman peak together with A_1g-201 cm^-1.Meanwhile,the results also showed the mode of E_g-136 cm^-1 redshifted and widened with the increase of temperature.Multiple phonon peaks behaved obvious fluctuations around T_c=200 K,our results well confirmed that the formation of superlattice causes significant changes in the electronic structure near the Fermi surface,which resulted in a second-order transition of transverse atomic displacement.All these phenomena revealed that electrons and phonons were strongly coupled in1T-Ti Se₂.There is a complex coupling of charge carrier and phonons in 1T-Ti Se₂,and the interaction between different particles plays a key role in regulating the properties of the whole system.Due to the short response time of electrons and the long response time of phonons,we used ultrafast spectroscopy to identify the potential mechanism of transient transformation.In this paper,we used a self-built ultrafast pump-probe system to detect the change of relaxation process of 1T-Ti Se₂ with variable temperatures on ultrafast time scale,and we got the T_c from the ultrafast reflectivity spectrum.We also explored the dynamics of carrier relaxation and damped phonon oscillation processes of 1T-Ti Se₂.The relaxation timeτ_ep revealed the electron phonon coupling provided a strong scattering channel to stabilize the CDW phase.We also observed the coherent phonon oscillations of f₁=3.10 THz and f₂=6.08 THz in CDW phase,which highlighted the second-order phase transition characteristics of 1T-Ti Se₂.These results have increased our understanding of the phase transition mechanism of CDW.