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C. Cacho, A. Crepaldi, M. Battiato, J. Braun, F. Cilento, M. Zacchigna, M.C. Richter, O. Heckmann, E. Springate, Y. Liu, S.S. Dhesi, H. Berger, Ph. Bugnon, K. Held, M. Grioni, H. Ebert, K. Hricovini, J. Minár, F. Parmigiani:
"Momentum-Resolved Spin Dynamics of Bulk and Surface Excited States in the Topological Insulator Bi2Se3";
Physical Review Letters, 114 (2015), S. 097401-1 - 097401-6.

The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi2Se3. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic ab initio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.

http://dx.doi.org/10.1103/PhysRevLett.114.097401