Valley Carrier Dynamics in Monolayer Molybdenum Disulfide from Helicity-Resolved Ultrafast Pump–Probe Spectroscopy

Wang, Qinsheng; Ge, Shaofeng; Xiao, Li; Qiu, Jun; Ji, Yanxin; Feng, Ji; Sun, Dong

doi:10.1021/nn405419h

Cited by 231 publications

(249 citation statements)

References 38 publications

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“…This value is in reasonable agreement with the value of τ s 7 ps estimated from PL experiments and with recent pump-probe measurements. 14,15 Similarly to the circular polarization degree whose decay is governed by τ zz , the linear polarization decay for the neutral A-exciton is governed by the in-plane pseudospin relaxation times τ xx and τ yy . As follows from Eq.…”

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confidence: 99%

Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

Glazov¹,

et al. 2014

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We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as MoS2. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and k ·p perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.PACS numbers: 71.70.Gm,72.25.Rb,78.66.Li Introduction. Monolayers (MLs) of transition metal dichalcogenides, in particular, MoS 2 form a class of novel two-dimensional materials with interesting electronic and optical properties. The direct band gap in these systems is realized at the edges of the Brillouin zone at points K + and K − .1 Strikingly, each of the valleys can be excited by the radiation of given helicity only.2-4 Recent experiments have indeed revealed substantial optical orientation in ML MoS 2 related to selective excitation of the valleys by circularly polarized light.5-8 Strong spin-orbit coupling in this material lifts the spin degeneracy of electron and hole states even at K + and K − points of the Brillouin zone resulting in relatively slow spin relaxation of individual charge carriers, which requires their intervalley transfer.9-13 However recent time-resolved measurements revealed surprisingly short, in the picosecond range, transfer times between valleys.14,15 This could be due to excitonic effects which are strong in transition metal dichalcogenides 16,17 Although individual carrier spin flips are energetically forbidden, spin relaxation of electron-hole pairs can be fast enough owing to the exchange interaction between an electron and a hole forming an exciton, 18,19 in close analogy to exciton dephasing in quantum wells.

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confidence: 99%

Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

Glazov¹,

et al. 2014

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“…* e-mail: xdcui@hku.hk On the experimental side, however, the valley/spin lifetime of free carriers remains some ambiguity. A spin resolved photocurrent measurements estimated the valley/spin lifetime in the range of 10 0 ∼ 10 2 nanoseconds in monolayer WS 2 , while optical pump-probe spectroscopy and time-resolved photoluminescence (PL) experiments gave a very short valley lifetime of several picoseconds [16][17][18][19][20][21][22][23] with a few exceptions where long valley lifetime of bound excitons were reported. [24][25][26] The huge discrepancy lies in that the excitonic effect is prevalent in optical responses of monolayer TMDs.…”

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confidence: 99%

“…The energy shift is much larger compared to the quasi-2D systems like GaAs quantum wells. [16,43,44] To further study the valley dynamics in monolayer WSe 2 , we set the pumping energy to be 1.737 eV, which is resonant with the A exciton, and tune the probing energy in the range between 1.968 and 2.505 eV. The transient differential reflection at zero time delay are plotted as a function of the probing energy shown in FIG.…”

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Long valley relaxation time of free carriers in monolayer WSe2

Yan

Yang

et al. 2017

Phys. Rev. B

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Monolayer transition metal dichalcogenids (TMDs) feature valley degree of freedom, giant spinorbit coupling and spin-valley locking. These exotic natures stimulate efforts of exploring the potential applications in conceptual spintronics, valleytronics and quantum computing. Among all the exotic directions, a long lifetime of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominates the optical response due to the enhanced Coulomb interaction in two dimensions. The valley lifetime of free carriers remains in ambiguity. In this work, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselinide. The valley lifetime of free carriers is found around 2 ns at 70 K, about 3 orders of magnitude longer than the excitons of about 2 ps. The extended valley lifetime of free carriers evidences that exchange interaction dominates the valley relaxation in optical excitation. The pump-probe spectroscopy also reveals the exciton binding energy of 0.60 eV in monolayer WSe2.

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“…2 A number of exciton decay mechanisms have been suggested as possible underlying processes in previously published studies [31][32][33][34][35][36][37][38][39][40][41][42][43] . For instance, some experiments 36,40 show that the fast decay dynamics (< 50 ps) are dependent on the exciton density and exciton-exciton annihilation is the dominating decay channel, while other studies 33,34,41 show no significant variation in the decay dynamics with the excitation density and that trapping by surface defect states is the responsible process. Other mechanisms such as carrier-phonon scattering, inter/intra-valley scattering, biexciton formation, trion formation, electron-hole recombination and exciton Auger scattering have also been considered 50 .…”

Section: Resultsmentioning

confidence: 99%

Valley Trion Dynamics in Monolayer MoSe2

Titze

Gao

Almeida

et al. 2016

Frontiers in Optics 2016

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Charged excitons called trions play an important role in the fundamental valley dynamics in the newly emerging 2D semiconductor materials. We used ultrafast pump-probe spectroscopy to study the valley trion dynamics in a MoSe 2 monolayer grown by using chemical vapor deposition.The dynamics display an ultrafast trion formation followed by a non-exponential decay. The measurements at different pump fluences show that the trion decay dynamics become slower as the excitation density increases. The observed trion dynamics and the associated density dependence are a result of the trapping by two defect states as being the dominating decay mechanism. The simulation based on a set of rate equations reproduces the experimental data for different pump fluences. Our results reveal the important trion dynamics and identify the trapping by defect states as the primary trion decay mechanism in monolayer MoSe 2 under the excitation densities used in our experiment.1 arXiv:1604.04190v1 [cond-mat.mes-hall]

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Valley Carrier Dynamics in Monolayer Molybdenum Disulfide from Helicity-Resolved Ultrafast Pump–Probe Spectroscopy

Cited by 231 publications

References 38 publications

Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

Long valley relaxation time of free carriers in monolayer WSe2

Valley Trion Dynamics in Monolayer MoSe2

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