Thermodynamic "valley noise" in monolayer semiconductors: access to intrinsic valley relaxation timescales
Invited
Abstract
The new class of atomically-thin transition-metal dichalcogenide (TMD) semiconductors such as monolayer MoS2 and WSe2 has focused broad attention on the concept of "valleytronics", founded on the idea of encoding information in an electron’s valley degree of freedom. A key parameter is therefore the intrinsic timescale of an electron’s inter-valley relaxation, and recent optical pump-probe studies have shown long (microsecond) valley relaxation of resident carriers in monolayer TMDs [1,2]. However, a significant drawback of all such pump-probe experiments is that they are by design perturbative: the optical pumping that drives the carrier polarization away from equilibrium also inevitably introduces "dark" excitons, whose presence may mask the intrinsic valley relaxation of resident carriers.
Here we present a completely alternative approach for measuring valley dynamics, based on the idea of passively "listening" to the random spontaneous scattering of carriers between K and K' valleys that occurs even in strict thermal equilibrium. We demonstrate that the stochastic valley noise is measurable by optical means and, in accord with the fluctuation-dissipation theorem, encodes the true intrinsic timescales of valley relaxation, free from any pumping, excitation, or other perturbative effects [3]. Using this new fluctuation-based methodology we measure very long valley relaxation dynamics of both electrons and holes in a single electrostatically-gated WSe2 monolayers. Noise spectra reveal long intrinsic valley relaxation with a single sub-microsecond time scale. Moreover, they validate both the relaxation times and the wavelength dependence observed in conventional pump-probe measurements, thereby resolving concerns about the role of dark excitons and trions in studies of long-lived valley relaxation.
[1] J. Kim et al., Science Advances 3, e1700518 (2017).
[2] P. Dey et al., Phys. Rev. Lett. 119, 137401 (2017).
[3] M. Goryca et al., arXiv:1808.01319 (2018).
Here we present a completely alternative approach for measuring valley dynamics, based on the idea of passively "listening" to the random spontaneous scattering of carriers between K and K' valleys that occurs even in strict thermal equilibrium. We demonstrate that the stochastic valley noise is measurable by optical means and, in accord with the fluctuation-dissipation theorem, encodes the true intrinsic timescales of valley relaxation, free from any pumping, excitation, or other perturbative effects [3]. Using this new fluctuation-based methodology we measure very long valley relaxation dynamics of both electrons and holes in a single electrostatically-gated WSe2 monolayers. Noise spectra reveal long intrinsic valley relaxation with a single sub-microsecond time scale. Moreover, they validate both the relaxation times and the wavelength dependence observed in conventional pump-probe measurements, thereby resolving concerns about the role of dark excitons and trions in studies of long-lived valley relaxation.
[1] J. Kim et al., Science Advances 3, e1700518 (2017).
[2] P. Dey et al., Phys. Rev. Lett. 119, 137401 (2017).
[3] M. Goryca et al., arXiv:1808.01319 (2018).
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Presenters
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Mateusz Goryca
Los Alamos National Laboratory
Authors
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Mateusz Goryca
Los Alamos National Laboratory
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Nathan Wilson
Department of Physics, University of Washington
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Prasenjit Dey
Los Alamos National Laboratory
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Xiaodong Xu
University of Washington, Department of Physics, University of Washington, University of Washington, Seattle
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Scott Crooker
Los Alamos National Laboratory