Ultrafast Optical Spectroscopy Study On Dirac Semimetal And Quantum Spin-liquid Candidate

Ultrafast time-resolved spectroscopy is a powerful and direct experimental technique to detect microscopic dynamic processes（e.g.spin,electron,and lattice etc）.It is also played an important role in the study of ultrafast dynamic in Dirac semimetals,quantum spin liquid materials and superconductors.In this thesis,we studied the quasiparticle dynamics of the Dirac semimetal PdTe₂,PtSe₂ and the candidate materials of quantum spin liquidα-RuCl₃ by using the femtosecond laser pump-probe technology.The main conclusions are summarized as follows:1.Temperature-dependent ultrafast carrier and phonon dynamics of Dirac semimetal single-crystal PdTe₂ were studied using ultrafast optical pump-probe spectroscopy.Quantitative analysis revealed that a fast relaxation process（τ_f）occurring at a subpicosecond time scale originating from electron–phonon thermalization.This rapid relaxation was followed by a slower relaxation process（τ_s）on a time scale of～7-9.5 ps which originated from phonon-assisted electron–hole recombination.Two significant vibrational modes can be resolved at all measured temperatures.These two modes corresponded to in-plane（E_g）,and out-of-plane(A_1g)of Te atoms vibration mode Te atoms motion.Combined temperature-dependent ultrafast carrier and phonon dynamics,we can know that pure dephasing played an important role in the relaxation processes.Analysis of the electron–phonon coupling constant suggested that theA_1gmode contributes greatly to the superconductivity,and high-frequency phonons are also involved forming of Cooper pairs.Present results provided experimental proof to get insight into the understanding of complex superconductivity of PdTe₂.2.The temperature-dependent ultrafast pump-probe experiment was performed to reveal ultrafast carrier dynamics of 2D PtSe₂ films with different thickness.The obtained transient reflectivity of 2D PtSe₂ shows typical temperature-and thickness-dependent.The relaxation time of the PtSe₂-12 with 12 monolayers increase with temperature rise,while the relaxation time of the PtSe₂-24 with 24 monolayers was almost constant at different temperature.By fitting the amplitude curve with temperature according to the RT equation,an energy gap of 44 me V was obtained for the PtSe₂-12 film sample below 130 K.An energy gap of 7 me V was also obtained for the PtSe₂-24 film sample below 130 K.The results provide evidence for understanding the layer-dependent bandgap characteristics and metal-like properties of two-dimensional PtSe₂ materials.3.Ultrafast pump-probe spectroscopy was used to investigate the temperature-dependent transient reflectivity of quantum spin liquidα-RuCl₃.The results demonstrated that the ultrafast carrier dynamic process ofα-RuCl₃ consists of a fast component induced by thermal excitation and an intermediate component related to in-plane structure at high temperature（130 K-250 K）.Near 168 K,the relaxation time of the slow component jumps as the crystal structure starts to change from ABC stacking to ABAB stacking.Abruptly changes of the intermediate component arise again at60 K which can be attributed to the crystal structure completely changes to a monoclinic crystal system with ABAB stacking.Below 120 K,α-RuCl₃transitions from the conventional paramagnetic state to the Kitaev paramagnetic state,which shows that a relaxation process（slow process）related to Z₂ gauge field is added.In addition,near the Neel temperature（8K）,the magnetic phase transition leads to an abrupt change in the amplitude and relaxation time of all three relaxation processes.The results of ultrafast spectroscopy experiments provide strong evidence to study the magnetic and structural phase transition ofα-RuCl₃,which helps to further demonstrate thatα-RuCl₃ is a Kitaev quantum spin liquid material.