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arXiv · 1705.07071

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

J. Sánchez-Barriga·M. Battiato·M. Krivenkov·E. Golias·A. Varykhalov·A. Romualdi·L. V. Yashina·J. Minár·O. Kornilov·H. Ebert·K. Held·J. Braun
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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.

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J. Sánchez-Barriga, M. Battiato, M. Krivenkov, E. Golias, A. Varykhalov, A. Romualdi, L. V. Yashina, J. Minár, O. Kornilov, H. Ebert, K. Held, J. Braun. 2017-05-19. Sub-picosecond spin dynamics of excited states in the topological insulator Bi$_2$Te$_3$. https://doi.org/10.1103/physrevb.95.125405

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