Ultrafast Carrier Dynamics In The Topological Material ZrTe₅

Topological state(or material)is one of frontiers in the condensed matter physics and material science for nearly 15 years.Recently,ZrTe5 has attracted intense interest from the researchers due to its association with the topological phases of matter,which are theoretically and experimentally demonstrated to be the topological insulator or the Dirac semimetal.These nontrivial topological properties were mainly investigated by the equilibrium or quasi-equilibrium experimental techniques,which have revealed a few exotic electronic responses such as the chiral magnetic effect,anomalous Hall effect,and 3D quantum Hall effect.It is believed that these effects more or less are associated with the anomalous phase transition around temperature T*,where the resistivity presents a peak.The topological states and the phase transition should include essential information of the electronic structure,carrier-lattice interactions and so on.Clearly,the equilibrium or quasi-equilbrium experiment is still far from revealing the properties mentioned above.In this thesis,the topological material ZrTe5 has been systematically investigated by the ultrafast optical spectroscopy.We have measured the temperature evolution of the transient reflectivity upon excitation of the femtosecond optical pulse.As a result,we have obtained the ultrafast dynamics of excited carriers,and conducted a comprehensive analysis of the results in terms of the carrier relaxation and coherent collective excitations.(1)In the analysis of the carrier relaxation process,we found that:(a)There are two relaxation dynamics in the entire temperature range,i.e.the electron-phonon scattering process with a time scale of sub-picoseconds and the phonon-assisted electron-hole recombination process with a time scale of-5-15 picoseconds;(b)Below the transition temperature T*,there is an additional relaxation process with a time scale of?1.2-2.5 picoseconds,which may represent the electron-phonon relaxtion of the excited non-equilibrium electron in the topological surface state.In addition,we also calculated the corresponding electron-phonon coupling constant associated with the electron-phonon scattering processes.(2)In the analysis of the coherent collective excitations,we found that:(a)The oscillations with frequencies of?1.2 THz,?3.5 THz,?3.6 THz,?4.3 THz correspond to the Ag optical phonon mode,and the oscillations with frequencies of?2.1 THz,?2.8 THz correspond to the B2g optical phonon mode;(b)Below T*,we discovered additional two collective modes,with frequencies of?0.07 THz and?0.43 THz,respectively.Based on the wavelength and temperature dependent experiments,we conclude that they can be attributed to the amplitude mode of charge density wave.We further derived that the modulation period of these two charge density waves are very large in the real space.The discovery of these new physical properties has given us a deeper knowledge of this material,especially the understanding of the three-dimensional quantum Hall effect.These results also will play an important role in the future development of ultrafast opto-electronic devices based on this material.