Spin–flip processes and radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers

Slobodeniuk, A. O.; Basko, D. M.

doi:10.1088/2053-1583/3/3/035009

Cited by 85 publications

(108 citation statements)

References 80 publications

(229 reference statements)

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“…A direct consequence of this spin-ordering is the formation of an intravalley (i.e. zero momentum) dark exciton with a lower energy than the bright exciton [28][29][30][31][32][33]. The bright to dark exciton transition reduces the PL quantum yield in W-based 1L-TMDs for decreasing temperatures, as recently shown [33].…”

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confidence: 95%

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS₂

Wang

Molina-Sánchez

Altmann

et al. 2019

2019 Conference on Lasers and Electro-Optics Europe &Amp; European Quantum Electronics Conference (CLEO/Europe-EQEC)

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In monolayer Transition Metal Dichalcogenides (TMDs) the valence and conduction bands are spin split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark exciton, consisting of an electron and a hole with opposite spin orientation, has lower energy than the A exciton. A possible mechanism leading to the transition from bright to dark excitons involves the scattering of the electrons from the upper to the lower conduction band state in K. Here we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics of electrons in the conduction band of single-layer WS2. This process occurs on a sub-ps time scale and it is significantly dependent on the temperature, indicative of a phonon-assisted relaxation. These experimental results are supported by time-dependent ab-initio calculations which show that the intra-valley spin-flip scattering occurs on significantly longer time scales only exactly at the K point. In a realistic situation the occupation of states away from the minimum of the conduction band leads to a dramatic reduction of the scattering time.Transition metal dichalcogenides (TMDs) are promising for opto-electronics[1-4], valleytronics[5-9] and quantum information processing [10]. Monolayers (1L) of TMDs are direct bandgap semiconductors[11] whose optical properties are dominated by excitons with binding energies of up to several hundred meVs [12][13][14][15][16][17][18]. The valence/conduction (VB/CB) band extrema lie at the non-equivalent K and K' points on the edge of the Brillouin zone [19,20]. Their spin-degeneracy is lifted by strong spin-orbit (SO) interaction [21]. For the VB the energy splitting ∆ v ranges between 150 and 400meV [22], while for the CB ∆ c it is one/two orders of magnitude lower (∼1-30meV) [23,24]. Together with a broken inversion symmetry, this results in spin-polarized bands and valley-dependent dipole-allowed interband optical transitions, as first detected by helicity resolved photoluminescence (PL) measurements [25][26][27].The large VB splitting gives rise to two distinct interband transitions (called A and B) strongly renormalized by excitonic effects [16], dominating the optical response in the visible range [22]. Recent theoretical calculations predicted that, while the spin orientation of the upper and lower CB states is antiparallel in K/K', in W-based 1L-TMDs, the upper CB state has the same spin orientation as the upper VB. A direct consequence of this spin-ordering is the formation of an intravalley (i.e. zero momentum) dark exciton with a lower energy than the bright exciton [28][29][30][31][32][33]. The bright to dark exciton transition reduces the PL quantum yield in W-based 1L-TMDs for decreasing temperatures, as recently shown [33]. The formation of dark excitons requires scattering processes such as intravalley relaxation. We stress that other excitonic comple...

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confidence: 95%

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS₂

Wang

Molina-Sánchez

Altmann

et al. 2019

2019 Conference on Lasers and Electro-Optics Europe &Amp; European Quantum Electronics Conference (CLEO/Europe-EQEC)

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“…This larger value is consistent with a larger exchange interaction due to the expected smaller dielectric screening. It was recently predicted that the short-range exchange interaction between the electron and the hole should also lift the double degeneracy of dark neutral excitons 35,36 . However, an experimental evidence of such a fine structure splitting of dark exciton in TMD is still lacking.…”

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confidence: 99%

Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

Robert¹,

Amand²,

Cadiz³

et al. 2017

Phys. Rev. B

179

237

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The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the "spin-forbidden" dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magneto-photoluminescence experiments reveal a zero field splitting δ=0.6 ± 0.1 meV between two dark exciton states. The low energy state being strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state is partially coupled to light with z polarization («grey» exciton). The first determination of the dark neutral exciton lifetime τ D in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure τ D~1 10 ± 10 ps for the grey exciton state, i.e. two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12 K.

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“…The neutral and charged exciton lines (X and CX) are accompanied by a lower energy multi-peak PL band. It has been argued that the lines forming this band originate, perhaps partially, from recombination of dark exciton states via different brightening mechanisms [37,49,50]. A rather outstanding fingerprint of these additional lines is that they exhibit large g-factor values.…”

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Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe ₂ , WSe ₂ and WS ₂ Monolayers

et al. 2018

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We present a comprehensive optical study of the excitonic Zeeman effects in transition metal dichalcogenide monolayers, which are discussed comparatively for selected materials: MoSe2, WSe2 and WS2. We introduce a simple semi-phenomenological description of the magnetic field evolution of individual electronic states in fundamental sub-bands by considering three additive components: valley, spin and orbital terms. We corroborate the validity of the proposed description by inspecting the Zeeman-like splitting of neutral and charged excitonic resonances in absorption-type spectra. The values of all three terms are estimated based on the experimental data, demonstrating the significance of the valley term for a consistent description of magnetic field evolution of optical resonances, particularly those corresponding to charged states. The established model is further exploited for discussion of magneto-luminescence data. We propose an interpretation of the observed large g-factor values of low energy emission lines, due to so-called bound/localized excitons in tungsten based compounds, based on the brightening mechanisms of dark excitonic states.

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Spin–flip processes and radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers

Cited by 85 publications

References 80 publications

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS₂

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS₂

Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe ₂ , WSe ₂ and WS ₂ Monolayers

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Spin–flip processes and radiative decay of dark intravalley excitons in transition metal dichalcogenide monolayers

Cited by 85 publications

References 80 publications

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS2

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS2

Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe 2 , WSe 2 and WS 2 Monolayers

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Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS₂

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS₂

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe ₂ , WSe ₂ and WS ₂ Monolayers