Laser-based Time-and Angle-Resolved Photoemission Spectroscopy Of Topological Semimetals

Weyl semimetal is a new topological quantum phase that hosts three-dimensional Weyl Fermions originally predicted in high-energy physics,and exhibits interesting phenonmena,e.g.negative magnetoresistance effect.The Weyl node in the bulk shows Dirac linearly dispersion while open Fermi arcs on the surface connecting a pair of Weyl nodes with opposite chiralities.It has been predicted that a new type of Weyl fermions(type-II Weyl fermions),which are distinguished from those in high energy physics,can also exist in solids called type-II Weyl semimetals.In this thesis,we investigate the electronic structure and dynamics of a candidate type-II Weyl semimetal—Td phase molybdenum telluride(Td-MoTe2).Using high resolution synchrotron based and ultraviolet laser based angle resolved photomisison spectroscopy(ARPES),we reveal the electronic structure and Fermi surface of Td-MoTe2.Furthermore,by performing ultrafast time resolved-ARPES(TR-ARPES),we are able to access the unoccupied band structure and capture the electronic relaxation dynamics near the Weyl node.The main achievement are summaried as follows:(1)Construction of ultrafast TR-ARPES system.We use femtosecond laser and nonlinear fourth harmonics genearation to produce ultraviolet pulse laser with a photon energy of 6.3 eV.The good performance at low kinetic energy demonstrates that our laser-based ARPES experimental setup has high momentum resolution.By combining ultrafast pump-probe technique,we further develop a TR-ARPES system which allows to measure the unoccupied states and capture the nonequilibrium relaxation process with energy,momentum and time resolution.TR-ARPES is a powerful probe to invesgate the electronic dynamics in the momentum space.(2)We provide direct spectroscopic evidences of topological Fermi arcs in Td-MoTe2 with bulk-sensitive laser ARPES and surface-sensitive UV ARPES.The quasiparticle interference pattern from STM measurements provides additional support for the Fermi arcs by revealing the scattering vectors.A comprehensive and complementary comparison between the ARPES,STM and first-principle calculations unambiguously confirms the topological fermi arcs on the surface,which establishes Td-MoTe2 as a typeII Weyl semimetal.(3)By performing TR-ARPES on Td-MoTe2,we can directly access the unoccupied states of band structure and identify the Weyl node to be at 70 meV above the Fermi energy.We further capture the relaxation dynamics along the high sysmetry direction in the momentum space and reveal a sub-picoseocnd response behavior.The quasiparticle dynamics near the Weyl node shows pronounced energy dependence and pump fluence dependence.By extracting the effective temperature from the population distribution,we observe two relaxation timescales,a fast one of ~430 fs and a slow one of ~4.1 ps,which are attributed to ultrafast intraband electronic cooling and interband electron-hole recombination respectively.In summary,Our ARPES and TR-ARPES results on type-II Weyl semimetal TdMoTe2 directly confirms the type-II Weyl semimetal phase and provide important knowledge for understanding electronic relaxation dynamics in topological related materials.