We demonstrate long trion spin lifetimes in a WSe2 monolayer of up to 150 ns at 5 K. Applying a transverse magnetic field in time-resolved Kerr-rotation measurements reveals a complex composition of the spin signal of up to four distinct components. The Kerr rotation signal can be well described by a model which includes inhomogeneous spin dephasing and by setting the trion spin lifetimes to the measured excitonic recombination times extracted from time-resolved reflectivity measurements. We observe a continuous shift of the Kerr resonance with the probe energy, which can be explained by an adsorbate-induced, inhomogeneous potential landscape of the WSe2 flake. A further indication of extrinsic effects on the spin dynamics is given by a change of both the trion spin lifetime and the distribution of g-factors over time. Finally, we detect a Kerr rotation signal from the trion's higher-energy triplet state when the lower-energy singlet state is optically pumped by circularly polarized light. We explain this by the formation of dark trion states, which are also responsible for the observed long trion spin lifetimes. arXiv:1702.03712v1 [cond-mat.mtrl-sci]

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]

We report on nanosecond long, gate-dependent valley lifetimes of free charge carriers in monolayer WSe2, unambiguously identified by the combination of time-resolved Kerr rotation and electrical transport measurements. While the valley polarization increases when tuning the Fermi level into the conduction or valence band, there is a strong decrease of the respective valley lifetime consistent with both electron-phonon and spin-orbit scattering. The longest lifetimes are seen for spin-polarized bound excitons in the band gap region. We explain our findings via two distinct, Fermi leveldependent scattering channels of optically excited, valley polarized bright trions either via dark or bound states. By electrostatic gating we demonstrate that the transition metal dichalcogenide WSe2 can be tuned to be either an ideal host for long-lived localized spin states or allow for nanosecond valley lifetimes of free charge carriers (> 10 ns).

We demonstrate all-electrical spin generation and subsequent manipulation by two successive electric field pulses in an n-InGaAs heterostructure in a time-resolved experiment at zero external magnetic field. The first electric field pulse along the [11¯0] crystal axis creates a current-induced spin polarization (CISP) which is oriented in the plane of the sample. The subsequent electric field pulse along [110] generates a perpendicular magnetic field pulse leading to a coherent precession of this spin polarization with 2-dimensional electrical control over the final spin orientation. Spin precession is probed by time-resolved Faraday rotation. We determine the build-up time of CISP during the first field pulse and extract the spin dephasing time and internal magnetic field strength during the spin manipulation pulse.

One important goal in the field of 2D materials is the investigation of valley physics in semiconducting transition metal dichalcogenides (TMDs). [1,2] As valley dynamics are governed by a delicate interplay of different electron-electron, electronphonon, and many-body interactions, an overall understanding of valley physics is only possible, and physical models can only be tested when different device properties such as valley lifetimes, exciton lifetimes, spin and momentum scattering times, or phonon and electron dispersion relations are analyzed as a function of Fermi level position-ideally for the same device. To accomplish this, a variety of different measurements are required, most importantly, the combination of both optical and electrical techniques, e.g., only the combination of gate-dependent electrical transport measurements, photoluminescence (PL) spectroscopy, and time-resolved Kerr rotation (TRKR) measurements recently enabled us to identify the dynamics which are responsible for the transfer of a polarization from optically excited bright trions to a valley polarization of free charge carriers in monolayer WSe 2. [3] However, this necessary prerequisite for the investigation of valley physics encounters practical obstacles, as not every measurement system can simultaneously conduct optical and electrical measurements. Even worse is the fact that some measurement techniques are mutually exclusive, e.g., in valley-sensitive TRKR measurements, the pump and probe energies normally have to be set to neutral or charged exciton energies (i.e., energies below the band gap), [3-5] but for this, first PL measurements with above-bandgap excitation are necessary to determine the energetic position of these exciton features. [6,7] Therefore, it is important to compare gate-dependent measurements recorded under different conditions.

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