Ultrafast Spectroscopy Studies Of Carrier Dynamics In WTe₂

Transition-metal dichalcogenides (TMDs) of formula MX2 have been paid more attention for its unique physical properties, for example transport properties, optical properties, spintronics, valleytronics, magnetic properties, type-II Weyl semimetal, su-perconducting and so on. Since the non-saturating magnetoresistance of WTe2 was found, many works have been done to understand the phenomenon. Such an exotic behavior is explained in the framework of Fermi surface compensation, so efforts are made on the electron structure near Fermi surface. In our work, we used the ultrafast spectroscopy to study the carrier dynamics in WTe2 . The main results is summarized below:1. From the polarization dependent pump-probe experiment, we observed two processes. The fast one occurs on lps time scale is caused by electron-phonon ther-malization. The slow one occurs on a time scale about several hundred picosecond. We prove the slow one coming from electron behavior, which means that the slow one is electron-hole recombination rather than thermal effect. Then we used the MOKE experiment to prove that spin polarization is important in ultrafast spectroscopy. Then we propose and prove for the first time that the well defined spin near Fermi surface is important in e-h recombination process, and slows down the process.2. We carried out the broadband pump probe experiment for the first time with WTe2, and gained the spectrum response of processes of electron-phonon thermaliza-tion and e-h recombination from global fitting. And we did the same operation for the polarization broadband pump probe. The responses are useful in the further study.3. We studied the coherent vibrational dynamics in WTe2 by using ultrafast pump-probe spectroscopy. By using polarization-and wavelength-dependent measurements and theoretical calculation, the oscillation is ascribed to the coherent dynamics of the lowest energy Al optical phonons. With increasing temperature, the phonon energy decreases due to an harmonic decay of the optical phonons into acoustic phonons. In-terestingly, a large electronic softening effect in WTe2 has been observed with a sig-nificant drop in the phonon energy when pump power increases due to light-induced lattice distortion.