Ultrafast Optical Spectroscopy Studies On The Antiferromagnetic Topological Semimetal Mn₃Sn

Topological semimetals are new quantum materials with unique optoelectronic prop-erties,which make them ideal platform for novel quantum phenomena in condensed matter physics and material science.Among them,the magnetic Weyl semimetals with time-reversal symmetry breaking is with both topological non-trivial electronic structure and long-range magnetic order.The complex coupling of topological and magnetic proper-ties can generate enormous anomalous Hall effect,magneto-electric quantum oscillation,topological surface state Fermi arc,and chiral anomalous negative magnetoresistance,etc.Recently,some magnetic topological semimetals have been theoretically and ex-perimentally verified,including ferromagnetic topological semimetal Co₃Sn₂S₂,and anti-ferromagnetic topological semimetals Mn₃Sn,Mn₃Ge,etc.They generally exhibit chiral anomaly,huge anomalous Hall effect,and anomalous Nernst effect,etc.Antiferromag-netic topological semimetal Mn₃Sn is also a strongly correlated material,and its magnetic,topological,and strong electron correlation may give rise to more rich physical properties.Therefore,research on magnetic topological semimetals will be helpful for the develop-ment of spintronics and condensed matter physics.In this thesis,the ultrafast dynamics of the antiferromagnetic topological semimetal Mn₃Sn was performed by using ultrafast optical pump-probe techniques.By measuring the ultrafast time evolution of transient reflectivity under optical excitation,the ultrafast carrier and coherent phonon dynamics were obtained.The main findings are:（1）The relaxation processes of non-equilibrium carriers under photoexcitation.The two processes with time scales of～0.3 ps and～30 ps were studied in detail.The former～0.3 ps process is an electron-phonon scattering process,which can be explained by a two-temperature model.Furthermore,the relevant physical parameters such as heat capacity coefficient and electron-phonon coupling constant were obtained by model.The latter process shows a sudden change at a temperature T^*～30 K,which can be explained as follows:below T^*,due to the existence of the Kondo effect,the excess Mn localized d-electron band near the Fermi level hybridizes with the conductive electron band to form a narrow gap,which causes an increase in the quasi-particle relaxation time,and the R-T model can explain the size of the gap（～4.03 me V）.（2）Within the experimental measurement range,we found a coherent optical phonon mode and a coherent acoustic phonon mode.The softening phenomenon of the coherent optical phonon with temperature can be explained by an anharmonic model,and the gen-eration and detection of the coherent acoustic phonon can be described by the strain pulse propagation model.This thesis focuses on the ultrafast dynamics of non-equilibrium carriers and phonons in the antiferromagnetic topological semimetal Mn₃Sn,and the hybridization of the elec-tronic band in the Weyl semimetal system is observed.These findings will facilitate the research and applications of ultrafast optoelectronic devices based on Mn₃Sn materials.