Ultrafast Carrier And Phonon Dynamics In The Type-? Dirac Semimetal NiTe₂

In recent years,topological semimetals(TSMs)have attracted significant interest in the condensed matter physics.A series of TSMs have been theoretically predicted and experimentally confirmed,such as Dirac semimetal,Weyl semimetal,and nodal-line semimetal.Most of them,e.g.Cd₃As₂,Na₃Bi,Ta As,have typical conical dispersion near the Fermi level in the electronic structure,and belong to the so-called type-I TSMs.However,there also exists another type of quantum matter called type-II TSMs.In contrast to the type-I,the Lorentz symmetry is broken in the type-II TSMs,which are characterized by the strongly tilted Dirac cones in the band structure.The novel fermionic and bosonic excitations arising from the band topology of type-II TSMs can lead to a variety of interesting physical properties,such as the direction-dependent chiral anomaly,unusual superconductivity,exotic quantum oscillations,and giant nonlinear optical responses.This thesis first introduces the design and setup of a time-resolved ultrafast optical pump-probe experimental platform,and proposes a new type of optical delay line.Secondly,preparation of the high-quality NiTe₂single crystal has been discussed.The ultrafast optical pump-probe technology has been used to systematically investigate the type-II Dirac semimetal NiTe₂for the first time.By measuring the transient reflectivity change of this material upon femtosecond laser excitation,we can obtain the ultrafast dynamics of carriers and coherent phonons at different temperatures in the time domain.The related findings for this material are shown below:(1)There are two processes in the relaxation of light-excited non-equilibrium carriers(electrons and holes),which are the electron-phonon scattering process lasting about 100 femtoseconds(femtosecond,fs)and the phonon assisted electron-hole recombination process lasting several picoseconds(ps).In the previous process,the temperature-dependent relaxation can be well understood and described using the two-temperature model,via which we have obtained the electron-phonon coupling constant,Debye temperature and other related physical parameters of this material.In the latter process,its relaxation time is abnormal at the transition temperature T^*?60K.Based on the electronic band structure of this material and the electronic transport measurements,such exotic behavior is most likely related to the sudden change of the band structure near the Fermi surface at T^*,which is associated with the Lifshiftz transition.Using the phonon-assisted electron-hole recombination model,we have obtained the phonon energy involved in the recombination process below and above T^*.Because the energy band structures of the three-dimensional topological Dirac semimetals are very similar,for those systems with the Fermi level near the Dirac point,the discovery might be universal.(2)Due to the impulsive stimulated Raman scattering or displacive excitation mechanism,several coherent optical phonon modes were found in this material,with frequencies of?4.3 THz(?₁/2?),?4.5 THz(?₂/2?),and?2.7 THz(?₃/2?).The intensity of?₃mode is very small,and is strongly depends on the polarization of the excitation light,which is characteristic of the E_gphonon mode.By contrast,?₁and?₂modes with similar frequencies are phonons from the A_1gmode.Based on the temperature-dependent measurements,the relationship between the frequency and the relaxation time of the?₁mode and?₂mode as a function of temperature have been accurately obtained.The temperature-dependent change of?₂mode can be well described by the classic anharmonic decay model.However,the?₁mode has a peculiar temperature renormalization effect,which means its temperature-dependent frequency and lifetime cannot be described by the anharmonic decay model,especially above T^*.By comparison,the energy of this phonon mode is nearly the same as that of a certain phonon mode involved in the phonon-assisted electron-hole recombination process mentioned above.Therefore,the unusual phenomenon associated with the?₁phonon mode also originates from the sudden change of the electronic structure near the Fermi surface.This work mainly reveals the related properties of the non-equilibrium carrier and coherent phonon dynamics in NiTe₂.These properties can not only elucidate the evolution of electronic structure topology near the Fermi surface of NiTe₂with temperature change,but also provide information for further understand the type-II Dirac semimetal.Simultaneously,some non-equilibrium microscopic mechanisms have been unraveled for developing future optoelectronic devices based on such materials.