We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe2. These long lifetimes are accompanied by an intriguing temperature dependence of the Kerr amplitude, which increases with temperature up to 50 K and then abruptly switches sign. Using ab initio simulations we explain the latter behavior in terms of the intrinsic electron-phonon coupling and the activation of transitions to secondary valleys. The phonon-assisted scattering of the photo-excited electron-hole pairs prepares a valley spin polarization within the first few ps after laser excitation. The sign of the total valley magnetization, and thus the Kerr amplitude, switches as a function of temperature, as conduction and valence band states exhibit different phonon-mediated inter-valley scattering rates. However, the electron-phonon scattering on the ps time scale does not provide an explanation for the long spin lifetimes. Hence, we deduce that the initial spin polarization must be transferred into spin states which are protected from the intrinsic electron-phonon coupling, and are most likely resident charge carriers which are not part of the itinerant valence or conduction band states. arXiv:1708.00228v2 [cond-mat.mes-hall]
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.Time-resolved magneto-optics is a well-established optical pump-probe technique to generate and to probe spin coherence in semiconductors. By this method, spin dephasing times T * 2 can easily be determined if their values are comparable to the available pump-probe delays. If T * 2 exceeds the laser repetition time, however, resonant spin amplification (RSA) can equally be used to extract T * 2 . We demonstrate that in ZnO these techniques have several tripping hazards resulting in deceptive values for T * 2 and show how to avoid them. We show that the temperature dependence of the amplitude ratio of two separate spin species can easily be misinterpreted as a strongly temperature-dependent T * 2 of a single spin ensemble, while the two spin species have T * 2 values, which are nearly independent of temperature. Additionally, consecutive pump pulses can significantly diminish the spin polarization, which remains from previous pump pulses. While this barely affects T * 2 values extracted from delay line scans, it results in seemingly shorter T * 2 values in RSA.
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