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).

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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