Anisotropic ultrafast dynamics in BiSbTe$_{\mathrm{2}}$S topological insulator investigated by time-resolved photoemission spectroscopy

Topological insulating phases in 3-dimensional bulk materials are characterized by the presence of a Dirac-like dispersive surface state -- with a specific momentum-locked spin structure - localized within the bulk insulating band gap [1,2] Here we will present time-resolved photoemission (TR-ARPES) experimental results from a new topological insulator, BiSbTe$_{\mathrm{2}}$S. BiSbTe$_{\mathrm{2}}$S exhibits superior chemical stability, as evidenced by the lack of any measurable energy shift of the Dirac point over time. The TR-ARPES signal (1.55-eV pump and 6.2-eV probe) reveals a direct optical population/depopulation of the Dirac states followed by slow recombination processes on a ps-timescale with a marked dependence of the relaxation time on crystallographic orientation. In addition, we also observe an ultrafast pump-induced modification of the equilibrium Dirac state energy dispersion. These effects can be ascribed to an anisotropic pump-induced modification of the phonon population, which in turn leads to an anisotropic electron-phonon assisted scattering of the hot electrons populating the unoccupied Dirac states. [1] Y. Xia et al. Nat. Phys. \textbf{5}, 398 (2009) [2] Z.-H. Zhu et al. Phys. Rev. Lett. \textbf{112}, 076802 (2014)