Sub-picosecond spin dynamics of excited states in the topological insulator Bi${}_2$Te${}_3$

Abstract

Using time-, spin- and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi${}_2$Te${}_3$ following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in about $\sim$2 ps. One state is located below $\sim$0.4 eV just above the bulk continuum, the other one at $\sim$0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within $\sim$0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically-locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with fs resolution to fully understand the sub-ps dynamics of transient electrons on the surface of topological insulators.

10 pages, 5 figures

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• Ultrafast Evolution of Bulk, Surface and Surface Resonance States in Photoexcited Bi${}_2$Te${}_3$
• Van der Waals engineering of ultrafast carrier dynamics in magnetic heterostructures
• Investigation of structural and magneto-transport properties of PdTe2 single crystals
• Ultrafast dynamics of photocurrents in surface states of 3D topological insulators
• Spectroscopic evidence for a new type of surface resonance at noble metal surfaces
• Light-field dressing of transient photo-excited states above $E_F$
• Numerical Solver for the out-of-equilibrium time dependent Boltzmann Collision operator: Application to 2D materials
• Control of intervalley scattering in Bi${}_2$Te${}_3$ via temperature-dependent band renormalization