Ultrafast Electron Dynamics and Circular Dichroism in Dependence of the Stoichiometry of Topological Insulators

Topological Insulators (TIs) are promising materials for spintronics applications because they exhibit spin polarized metallic states – the Dirac-Cone – at their surface at room temperature although being insulating in the bulk. Angle resolved photoemission spectroscopy (ARPES) is an appropriate and powerful tool to investigate the band structure of TIs. We analyze bismuth chalkogenide ternary compounds Bi2SexTe3−x with x = 0, 0.2, 0.35, 2.5 and 3 with equilibrium ARPES measurements as well as time-resolved ARPES by the pump-probe technique with a temporal resolution of 70 fs and circular-dicroism-ARPES with circular polarized light. A MATLAB routine for data handling and analysis was developed obtaining the imaginary part of the self energy standing for quasiparticle lifetimes. Bulk electronic properties like band gap, effective masses, electron-phonon interaction window or electron-electron coupling constants depend on layer-by-layer substitution of Te by Se and the Dirac-Cone shows an entangled evolution. Hints of 2nd surface states appear although this states are not directly visible. The circular dichroic signal of the upper Dirac-Cone in the Te-rich samples displays the same sign which need further investigation. This asymmetry could lead to a clear dichroic signal and a related spin polarization in the bulk conduction band with long lifetime. Because of its optical pump induction it is interesting for opto-spintronics applications.