posted on 2020-03-31, 17:07authored byManfred Ersfeld, Frank Volmer, Lars Rathmann, Luca Kotewitz, Maximilian Heithoff, Mark Lohmann, Bowen Yang, Kenji Watanabe, Takashi Taniguchi, Ludwig Bartels, Jing Shi, Christoph Stampfer, Bernd Beschoten
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-level-dependent 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).