Ultrafast Spectroscopy Of Strong Correlations And Topological Materials

Searching novel functional materials is one of the frontiers of the condensed matter physics and the material science.Among of all the new functional materials discovered,the strongly correlated systems and topological materials are believed to have great poten-tial in the future device applications.Macroscopic physical properties of the materials,measured by the conventional experimental techniques,are usually determined by the microscopic degrees of freedom,e.g.electron,spin,and lattice etc.Therefore,in or-der to understand and control the physical properties of the material,we should have the capability to capture and manipulate these degrees of freedom.However,because their dynamics are extremely fast,generally with a timescale of less than one nanosecond that is far beyond the detecting limit of conventional experimental techniques,new prob-ing and controlling methods are urgently required.Ultrafast optical spectroscopy based on the femtosecond laser systems provides the possibility to study these ultrafast non-equilibrium behaviors,such as quasiparticle dynamics,coherent lattice vibrations,spin dynamics,electron-boson couplings,evolution of the symmetry breaking,and interactions between different order parameters.Thus,for the strongly correlated systems and topo-logical materials,in order to understand the fundamental physics behind and develop new functional devices,it is quite essential for us to investigate their ultrafast non-equilibrium properties using the ultrafast spectroscopy technique.In this dissertation,the principle of ultrafast optical spectroscopy is first introduced.Then,a few conventional analytical methods for revealing the quasiparticle dynamics and spin dynamics are briefly described.Utilizing these experimental and theoretical techniques,our work on three typical mate-rial systems:Lal1.9Ce0.1CuO4±?,Bi2Se3 and SmB6 are discussed in detail.These materials belong to the strongly correlated system,the topological insulator,and the potential topo-logical Kondo insulator,respectively.Our studies provide the quantitative evidence of the antiferromagnetic spin fluctuations acting as the imperative pairing glue in the electron-doped cuprates,and demonstrate a novel technique to coherently manipulate spins of the surface states in the topological insulators.In the end,conclusions and perspectives are presented.