Ultrafast Dynamics And Coherent Phonons In Topological Semimetals And Optical Spectroscopy Of A Multiferroic Material

The theoretical and experimental research in topological quantum states and topological quantum materials has witnessed huge developments in past decades,which has been one of the most fertile ground in condensed matter physics.As a new basic concept in physics,topological ordering is applied for the analogue of the phases of the matter,which is similar to symmetry.The understanding of topological ordering is related to many basic problems in physics,such as the quantum phase,quantum phase transition,and the quasiparticles.There are two significant reasons of investigating topological quantum materials.Firstly,the complex symmetries in condensed matter permit the discovery of multiple quasiparticles（elementary excitations）,including but not limiting to the recently found Dirac fermion,Weyl fermion,Majorana fermion,and so forth.Secondly,the topological protected electrons exhibit novel properties,such as quantum Hall effect,anomalous quantum Hall effect,chiral anomaly,which provide possibilities in the future applications in devices.Time resolved ultrafast spectroscopy is one of the most powerful methods in the experimental investigation of condensed matter physics.It is able to investigate the ultrafast processes that happen during the time scale of ps and fs.For instance,the relaxation of Cooper pairs in superconductor,the generation and detection of coherent phonons,the relaxation of spin,charge or spin density wave,etc.There are many advantages of the investigation of topological quantum materials by using time resolved ultrafast spectroscopy,such as surface sensitivity,distinguish of chirality,laser-induced topological properties,multi-freedom of degrees（charge,spin,orbit and lattice）investigation of topological properties.In the first part of this thesis,by using time resolved ultrafast pump probe spectroscopy,we successfully generate and detect a coherent phonon beating among three evenly spaced A_1g optical phonon modes（of frequencies 1.80,1.96,and 2.11 THz,respectively）in topological Dirac semimetal Cd₃As₂.The two side modes contributed to the counter helixes composing Cd vacancies,we name them as helix vacancy phonon mode（HVP mode）.Significantly,the HVP modes experience prominent dynamics waning with temperature increasing,and disappear above 200 K.Our results in the lattice degree of freedom suggest the indispensable role of temperature in investigation the topological properties.Secondly,we further research the ultrafast dynamics during longer delay time range.An acoustic phonon（AP）mode with central frequency f=0.037 THz is unambiguously observed,which we attribute to laser-induced thermal strain.Interestingly,an AP chirping（i.e.,variation of the phonon frequency）in time domain as well as an asymmetry in frequency domain is clearly detected.By comparing our experimental results with theoretical model,we find that the chirping in time domain,instead of Fano resonance,leads to the asymmetry in the frequency domain.In addition,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd₃As₂ application potentials in thermoelectric devices.In the last part,we report the temperature dependent Raman scattering investigation of charge ordering（CO）,spin ordering（SO）and phase transition in a polycrystalline Er_0.1Yb_0.9Fe₂O₄.We unambiguously observed double 2D CO states at high temperature（T>300 K）.The energy gaps between the 3D CO state and the double2D CO states are 170 me V and 193 me V,respectively.We also observed a spin ordering（SO）state（T<210 K）with characteristic energy of 45 me V.Our experiments successfully identified new fine structures of quantum orders in RFe₂O₄ system,which also extends the capability of optical methods in studying other layered quantum materials.