Ultrafast Optical Spectroscopy Studies Of The Magnetic Topological Insulator MnBi₂Te₄

The topological insulator(TI),different from the ordinary insulator,belongs to a newly discovered topologically nontrivial electronic state.It is classified by the topo-logical invariant inside the momentum space,where the bulk state and surface state are insulating and conducting,respectively.There exists unique spin texture in the surface state,i.e.spin-momentum locking.Therefore,TI is believed to be useful in the future optical electric devices and spintronic devices.In recent years,there are many TIs being predicted theoretically and verified experimentally,such as Hg Te/Cd Te quantum well,the three-dimensional TI Bi_1-xSb_xand Bi₂Se₃,and the magnetic TI MnBi₂Te₄.Compared to the conventional TI,the magnetic TI is expected to show more exotic phenomena due to its intrinsic antiferromagnetism which induces the gap in the surface state.MnBi₂Te₄is predicted to be a potential candidate in quantum anomalous Hall effect,chiral Majorana fermion and axion insulator.This thesis focuses on using the time-resolved ultrafast optical pump-probe spec-troscopy to investigate the magnetic TI MnBi₂Te₄.Specifically,the transient reflectivity is employed to reveal the carrier and coherent phonon dynamics inside this material.The main findings are:(1)Upon photoexcitation,there are three nonequilibrium decay processes inside the MnBi₂Te₄,including the electron-phonon scattering process with a timescale of 1 ps(picosecond),the phonon assisted electron-hole recombination process with a timescale of~5 ps,and the long decay process with a timescale of nanoseconds.The last process only emerges below the antiferromagnetic transition temperature,although the others are also clearly influenced by the antiferromagnetic order.(2)There appear three phonon modes in MnBi₂Te₄within our detection limit.The strongest phonon participates the electron-hole recombination process,and clearly devi-ates from the anharmonic phonon model since its energy experiences an obvious anomaly near the antiferromagnetic ordering temperature.These discoveries are important for the analysis of magnetic ordering in TIs.This thesis fills the gap,where no nonequilibrium dynamics of magnetic TIs was reported previously,and reveals the importance of the magnetic ordering for understanding the carrier and phonon dynamics.The related findings are also essential for the future research of MnBi₂Te₄such as searching the fundamental quantum anomalous Hall effect and realizing the applicable electronic devices based on this material system.