8 research outputs found
Spin States Protected from Intrinsic Electron-Phonon-Coupling Reaching 100 ns Lifetime at Room Temperature in MoSe
We present time-resolved Kerr rotation measurements, showing spin lifetimes
of over 100 ns at room temperature in monolayer MoSe. 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.Comment: 18 pages, 17 figure
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
Valley-hybridized gate-tunable 1D exciton confinement in MoSe2
Controlling excitons at the nanoscale in semiconductor materials represents a
formidable challenge in the fields of quantum photonics and optoelectronics.
Achieving this control holds great potential for unlocking strong
exciton-exciton interaction regimes, enabling exciton-based logic operations,
exploring exotic quantum phases of matter, facilitating deterministic
positioning and tuning of quantum emitters, and designing advanced
optoelectronic devices. Monolayers of transition metal dichalcogenides (TMDs)
offer inherent two-dimensional confinement and possess significant binding
energies, making them particularly promising candidates for achieving
electric-field-based confinement of excitons without dissociation. While
previous exciton engineering strategies have predominantly focused on local
strain gradients, the recent emergence of electrically confined states in TMDs
has paved the way for novel approaches. Exploiting the valley degree of freedom
associated with these confined states further broadens the prospects for
exciton engineering. Here, we show electric control of light polarization
emitted from one-dimensional (1D) quantum confined states in MoSe2. By
employing non-uniform in-plane electric fields, we demonstrate the in-situ
tuning of the trapping potential and reveal how gate-tunable
valley-hybridization gives rise to linearly polarized emission from these
localized states. Remarkably, the polarization of the localized states can be
entirely engineered through either the spatial geometry of the 1D confinement
potential or the application of an out-of-plane magnetic field
Spin states protected from intrinsic electron-phonon coupling reaching 100 ns lifetime at room temperature in MoSe₂
We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe. 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 photoexcited 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 intervalley 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
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)
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