Intervalley coupling by quantum dot confinement potentials in monolayer transition metal dichalcogenides

Liu, Gui-Bin; Pang, Hongliang; Yao, Yugui; Wang, Yao

doi:10.1088/1367-2630/16/10/105011

Cited by 78 publications

(98 citation statements)

References 58 publications

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“…In the valence band, intervalley scattering is forbidden in both cases. Analogous selection rules for the intervalley coupling due to confinement potentials in 2D TMD based quantum dots have previously been noted [15].Our findings can be readily verified with scanning tunneling spectroscopy (STS) which has provided valuable insight to the electronic properties of 2D TMDs [16][17][18][19][20]. In particular, Fourier transform STS (FT-STS) is a powerful method for investigating atomic defects and their scattering properties in 2D materials [21,22].…”

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confidence: 77%

Symmetry-forbidden intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides

et al. 2017

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Intervalley scattering by atomic defects in monolayer transition metal dichalcogenides (TMDs; M X2) presents a serious obstacle for applications exploiting their unique valley-contrasting properties. Here, we show that the symmetry of the atomic defects can give rise to an unconventional protection mechanism against intervalley scattering in monolayer TMDs. The predicted defectdependent selection rules for intervalley scattering can be verified via Fourier transform scanning tunneling spectroscopy (FT-STS), and provide a unique identification of, e.g., atomic vacancy defects (M vs X). Our findings put the absence of the intervalley FT-STS peak in recent experiments in a different perspective.Introduction.-Two-dimensional (2D) monolayers of transition metal dichalcogenides (TMDs; M X 2 ) are promising candidates for spin-and valleytronics applications [1]. Their hallmarks include unique valleycontrasting properties and strong spin-valley coupling [1, 2] exemplified by, e.g., valley-selective optical pumping [3][4][5], a valley-dependent Zeeman effect [6][7][8][9], and the valley Hall effect [10]. Such means to control the valley degree of freedom are instrumental for valleytronics applications.Another prerequisite for a successful realization of valleytronics is a sufficiently long valley lifetime [11,12]; atomic defects are a common limiting factor which can provide the required momentum for intervalley scattering due to their short-range nature. However, as illustrated in Fig. 1(a), the spin-orbit (SO) induced spin-valley coupling in the K, K valleys of 2D TMDs partially protects the valley degree of freedom against relaxation via intervalley scattering by nonmagnetic defects [2]. Due to the small spin-orbit splitting in the conduction band valleys [13,14], only the valence-band valleys fully benefit from this protection. Identification of additional protection mechanisms in the conduction band would hence be advantageous for valleytronics in 2D TMDs.In this work, we demonstrate that besides the spinvalley coupling, the symmetry and position of atomic defects give rise to unconventional selection rules for intervalley quasiparticle scattering in 2D TMDs. As illustrated in Fig. 1(b), we find that for defects with threefold rotational symmetry (C 3 ), e.g., atomic vacancies, intervalley K ↔ K scattering in the conduction band is forbidden for defects centered on the X site while it is allowed for M centered defects. In the valence band, intervalley scattering is forbidden in both cases. Analogous selection rules for the intervalley coupling due to confinement potentials in 2D TMD based quantum dots have previously been noted [15].Our findings can be readily verified with scanning tunneling spectroscopy (STS) which has provided valuable insight to the electronic properties of 2D TMDs [16][17][18][19][20]. In particular, Fourier transform STS (FT-STS) is a powerful method for investigating atomic defects and their scattering properties in 2D materials [21,22]. The measured STS map is a probe of the local density ...

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confidence: 77%

Symmetry-forbidden intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides

et al. 2017

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“…Our scheme for valley pseudospin manipulation should also extend to the recently discovered quantum-confined states of excitons in TMDC layers [29][30][31][32][33] . For these localized excitonic states, the valley pseudospin degree of freedom is expected to be well preserved 34 , but their lifetimes are far longer than for the free excitons examined in this work. In addition, full control of valley pseudospin could be achieved by means of stimulated Raman adiabatic passage in a real or pseudomagnetic field.…”

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confidence: 79%

Optical manipulation of valley pseudospin

Ye¹,

Sun²,

Heinz³

2016

Nature Phys

233

223

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The coherent manipulation of spin and pseudospin underlies existing and emerging quantum technologies, including quantum communication and quantum computation 1,2 . Valley polarization, associated with the occupancy of degenerate, but quantum mechanically distinct valleys in momentum space, closely resembles spin polarization and has been proposed as a pseudospin carrier for the future quantum electronics 3,4 . Valley exciton polarization has been created in the transition metal dichalcogenide monolayers using excitation by circularly polarized light and has been detected both optically 5-7 and electrically 8 . In addition, the existence of coherence in the valley pseudospin has been identified experimentally 9 . The manipulation of such valley coherence has, however, remained out of reach. Here we demonstrate all-optical control of the valley coherence by means of the pseudomagnetic field associated with the optical Stark e ect. Using below-bandgap circularly polarized light, we rotate the valley exciton pseudospin in monolayer WSe 2 on the femtosecond timescale. Both the direction and speed of the rotation can be manipulated optically by tuning the dynamic phase of excitons in opposite valleys. This study unveils the possibility of generation, manipulation, and detection of the valley pseudospin by coupling to photons.The broken inversion symmetry in monolayer transition metal dichalcogenide (TMDC) crystals in the semiconducting MX 2 (M = Mo, W; X = S, Se) family gives rise to a nontrivial Berry phase at the K and K valleys in momentum space 10 , where the optically accessible direct bandgap occurs in these materials 11,12 . This Berry phase, together with the angular momentum of the atomic orbitals, leads to different optical selection rules for the two valleys: excitons in the K valley couple to left-circularly polarized photons, while excitons in the K valley couple to right-circularly polarized photons, thus permitting the optical generation of valley polarization through control of the helicity of light [5][6][7] . Moreover, when a TMDC monolayer is excited with linearly polarized light, a coherent superposition state is established between the K and K valley excitons 9 . Such a coherent state can, in principle, be manipulated by lifting the energy degeneracy between valleys through the breaking of time-reversal symmetry in the material. In this regard, d.c. magnetic fields have been applied to achieve a valley splitting of a few meV [13][14][15][16] . Circularly polarized light also breaks time-reversal symmetry, and researchers have used it to create larger valley splittings, corresponding to pseudomagnetic fields up to 60 T (refs 17,18). In addition, this optical approach permits us to access the ultrafast timescale: in this work, we demonstrate the coherent manipulation of the valley pseudospin on the femtosecond timescale.The optical Stark effect has been applied to manipulate the spin/pseudospin in quantum systems, such as atomic quantum gases 19 , quantum dots 20,21 , and III-V quantum wells 22 . H...

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“…An open question is whether the QD like emitters inherit the valley selection rules. 5,6 Concerning spin manipulation, the reduced hyperfine interaction with the nuclear spin bath for states at the K-points (direct gap) as compared to electronic states in III-V semiconductor quantum dots will be an advantage. 4,6,52 The control of the emission properties of these quantum dot like emitters paves the way for further engineering of the light matter interaction in these atomically thin materials.…”

Section: -3mentioning

confidence: 99%

“…In a charge tunable sample, we record discrete jumps on the meV scale as charges are added to the emitter when changing the applied voltage. [4][5][6] The 2D host materials have the advantage of being cost efficient, with highly tunable properties 3,7 and optical access to the electron valley index in momentum space, 8,9 an additional degree of freedom compared to other solid state qubits in III-V quantum dots (QDs) or NV centres in diamond, for example. There are several approaches to achieve 3D quantum confinement, such as patterning TMD MLs, 10 chemically synthesized TMD nano-crystals, [11][12][13][14][15][16] and defect engineering.…”

mentioning

confidence: 99%

Discrete quantum dot like emitters in monolayer MoSe2: Spatial mapping, magneto-optics, and charge tuning

Branny

Wang

Kumar

et al. 2016

Applied Physics Letters

118

110

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Transition metal dichalcogenide monolayers such as MoSe2,MoS2 and WSe2 are direct bandgap semiconductors with original optoelectronic and spin-valley properties. Here we report spectrally sharp, spatially localized emission in monolayer MoSe2. We find this quantum dot like emission in samples exfoliated onto gold substrates and also suspended flakes. Spatial mapping shows a correlation between the location of emitters and the existence of wrinkles (strained regions) in the flake. We tune the emission properties in magnetic and electric fields applied perpendicular to the monolayer plane. We extract an exciton g-factor of the discrete emitters close to -4, as for 2D excitons in this material. In a charge tunable sample we record discrete jumps on the meV scale as charges are added to the emitter when changing the applied voltage.

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Intervalley coupling by quantum dot confinement potentials in monolayer transition metal dichalcogenides

Cited by 78 publications

References 58 publications

Symmetry-forbidden intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides

Symmetry-forbidden intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides

Optical manipulation of valley pseudospin

Discrete quantum dot like emitters in monolayer MoSe2: Spatial mapping, magneto-optics, and charge tuning

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