5 research outputs found
Valley lifetimes of conduction band electrons in monolayer WSe
One of the main tasks in the investigation of 2-dimensional transition metal
dichalcogenides is the determination of valley lifetimes. In this work, we
combine time-resolved Kerr rotation with electrical transport measurements to
explore the gate-dependent valley lifetimes of free conduction band electrons
of monolayer WSe. When tuning the Fermi energy into the conduction band we
observe a strong decrease of the respective valley lifetimes which is
consistent with both spin-orbit and electron-phonon scattering. We explain the
formation of a valley polarization by the scattering of optically excited
valley polarized bright trions into dark states by intervalley scattering.
Furthermore, we show that the conventional time-resolved Kerr rotation
measurement scheme has to be modified to account for photo-induced gate
screening effects. Disregarding this adaptation can lead to erroneous
conclusions drawn from gate-dependent optical measurements and can completely
mask the true gate-dependent valley dynamics.Comment: 5 pages, 3 figure
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Unveiling Valley Lifetimes of Free Charge Carriers in Monolayer WSe 2
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)