Spin Polarization And Ultrafast Dynamics In Transition Metal Chalcogenides And Topological Insulators

In the past ten years,some new materials such as topological insulators and transition metal dichalcogenides(TMDC),have attracted the attention of many researchers.Topological insulators have brought many researches on their transport properties due to their special Dirac-cone like surface state with no bandgap and spin-momentum locking.TMDC are layered materials bonded by van der Waals forces.Although they have been researched for a long time,only in recent years the single-layer materials have been prepared.Moreover,the single layer one generally demonstrates obvious differences from the bulk,such as the transition from indirect-bandgap semiconductor to direct bandgap.TMDC show a variety of properties,from semiconductors to conductors and even superconductors in terms of conductivity.At the same time,angle-resolved photoemission spectroscopy(ARPES)is an ideal method for studying these materials,not only because it can directly detect the electronic band structure.For topological insulators,its surface state structure can be better characterized due to the surface sensitivity of ARPES.For TMDC,because the van der Waals force between layers is weak,the band structure has no obvious dispersion along the k_z direction.Therefore,it is generally not necessary to change the photon energy to obtain the band structure along the k_z direction in the ARPES measurement.ARPES combined with pump-probe technique(TR-ARPES)is also widely used in the study of these materials,including charge density wave,valley polarization and band renormalization in TMDC,and the relaxation process of electrons in surface and bulk states in topological insulators.The chapter 1 of this thesis mainly reviews some particular properties and growth methods of TMDC and topological insulators.The chapter 2 introduces the principle and data analysis method of ARPES and spin-ARPES.Chapter 3 introduces the TR-ARPES system based on extreme ultraviolet(EUV)that the author mainly participated in building,including the structure and principle of each part of the system.TR-ARPES uses pump light to excite electrons and probe light to detect the electronic band of the excited material,which can obtain information about the relaxation process of electrons in the non-equilibrium states.Chapter 4 introduces the work of detecting the hidden spin polarization in 2H-Mo Te₂ using spin-ARPES.It is generally believed that there is no net spin polarization in non-magnetic materials with centrosymmetric structure.The single-layer of 2H-Mo Te₂ is asymmetric but the two-layered unit cell is inversion symmetric.With the virtue of the shallow detection depth of spin-ARPES,only the electrons in the topmost layer is detected.The out-of-plane spin polarizations were detected at K and K?,which are equal in magnitude and opposite in direction,satisfying the time reversal symmetry.Combined with density functional theory calculations,it is proved that what is detected experimentally is the intrinsic hidden spin polarization in the material,rather than the result of the breaking of the symmetry at the surface of material.At the same time,we use TR-ARPES to characterize the K point of 2H-Mo Te₂ and find that the relaxation of electrons in the conduction band conforms to the law of double-exponential decay,and the size of bandgap at the K point is estimated to be 1.15 e V.In Chapter 5,we use TR-ARPES to systematically study the dynamic process of light-excited electrons in high-quality(Bi_1-xCr_x)₂Se₃ with different Cr doping concentrations.The results show that with the increase of Cr doping concentration,the lifetime of excited electrons decreases,and the electron temperature and chemical potential decrease faster,indicating a process of impurity band assisted carrier relaxation.According to the Shockley-Reed-Hall recombination theory,the relationship between the carriers'lifetime and the density of the impurity band is derived.The density functional theory is used to calculate the density of impurity band of samples with different doping concentrations,and the experimental data is well fitted with the derived formula,which demonstrates that the impurity band is the main factor for accelerating the relaxation of excited carriers.Chapter 6 summarizes the work and looks forward to the future research.