Exciton valley relaxation in a single layer of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi mathvariant="normal">WS</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:math>measured by ultrafast spectroscopy

Mai, Cong; Semenov, Yu. G.; Barrette, Andrew; Yu, Yifei; Jin, Zhenghe; Cao, Linyou; Gündoğdu, Kenan

doi:10.1103/physrevb.90.041414

Cited by 132 publications

(164 citation statements)

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“…Given that the effective bias added on the channel is on the order of V ds − ðE g + E exciton binding Þ ∼ 2.5 eV where we assume the band bending at both contacts is roughly of the electronic band gap at most and the mobility of 0.1-1 cm 2 · v=s of the devices (SI Appendix), the spin-free path indicates the estimated spin-valley lifetime around 10 1 ∼10 2 ns. This estimate is orders of magnitude larger than the valley lifetime estimated from polarization-resolved photoluminescence and pump-probe spectroscopy in which the exciton effect predominates the optical properties and consequently the valley lifetime of excitons instead of free carriers is probed (37,38). Note that the electron-hole exchange interaction provides the major channel for excitons' spinvalley depolarization (39), whereas the exchange interactions are greatly suppressed in oppositely drifting free carriers in a nearly intrinsic state, and consequently the free carriers presumably show significantly longer spin-valley lifetime.…”

Section: Resultsmentioning

confidence: 66%

Manipulating spin-polarized photocurrents in 2D transition metal dichalcogenides

Xie

Cui

2016

Proc. Natl. Acad. Sci. U.S.A.

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Manipulating spin polarization of electrons in nonmagnetic semiconductors by means of electric fields or optical fields is an essential theme of the conceptual nonmagnetic semiconductor-based spintronics. Here we experimentally demonstrate an electric method of detecting spin polarization in monolayer transition metal dichalcogenides (TMDs) generated by circularly polarized optical pumping. The spin-polarized photocurrent is achieved through the valleydependent optical selection rules and the spin-valley locking in monolayer WS 2 , and electrically detected by a lateral spin-valve structure with ferromagnetic contacts. The demonstrated long spinvalley lifetime, the unique valley-contrasted physics, and the spinvalley locking make monolayer WS 2 an unprecedented candidate for semiconductor-based spintronics.monolayer transition metal dichalcogenides | spin-valley coupling | spintronics | spin lifetime | valley lifetime A longtime focus in nonmagnetic semiconductor spintronics research is to explore methods to generate and manipulate spin of electrons by means of electric fields or optical fields instead of magnetic fields, enabling scalable and integrated devices (1). The present efforts follow two distinct paths. One uses spin Hall effect or optical pumping in III-V semiconductors which feature a significant spin-orbit coupling in a form of Dresselhaus and/or Rashba terms (2-4); the other focuses on spin transport [usually generated by spin injection from ferromagnetic (FM) electrodes] in semiconductor structures made of silicon (5), carbon nanotube (6), graphene (7), etc. which have long spin-coherence length due to weak spin-orbit coupling. The emergence of atomic two-dimensional group VI transition metal dichalcogenides (TMDs) MX 2 (M = Mo, W; X = S, Se), featuring nonzero but contrasting Berry curvatures at inequivalent K and K′ (equivalent to −K) valleys and unique spin-valley locking, provides an alternative pathway toward spintronics (8).Valleys refer to the energy extremes around the high symmetry points of the Brillouin zone, either a "valley" in the conduction band or a "hill" in the valence band. Owing to their hexagonal lattices, the family of TMDs has degenerate but inequivalent K(K′) valleys well separated in the first Brillouin zone. This gives electrons an extra valley degree of freedom, in addition to charge and spin. In monolayer TMDs the inversion symmetry breaking of crystal structures gives rise to nonzero but contrasting Berry curvatures at K and K′ valley which are a characteristic of the Bloch bands and could be recognized as a form of orbital magnetic moment of Bloch electrons (9-11). These contrasting Berry curvatures of electrons (holes) at K(K′) valleys lead to contrasting response to certain stimulus (9-19). One example is the valley Hall effect: An electric field would drive the electrons at different valleys (K and K′) toward opposite transverse directions, in a similar way as in spin Hall effect (10,20). A more pronounced manifestation is valley-dependent circular optical selectio...

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Section: Resultsmentioning

confidence: 66%

Manipulating spin-polarized photocurrents in 2D transition metal dichalcogenides

Xie

Cui

2016

Proc. Natl. Acad. Sci. U.S.A.

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“…Excitonic states in the MX 2 2D semiconductors (2DSCs) have been studied both theoretically and experimentally [12,13,[16][17][18][19][20] In both absorption and photoluminescence spectra, two strong exciton resonances are observed, commonly labelled A and B. They are associated with electronic transitions involving an electron and a hole (from the upper VB for A states and from the lower one for B states), with parallel spins.…”

Section: Arxiv:150909015v1 [Cond-matmes-hall] 30 Sep 2015mentioning

confidence: 99%

Exciton polaritons in two-dimensional dichalcogenide layers placed in a planar microcavity: Tunable interaction between two Bose-Einstein condensates

et al. 2015

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Exciton-polariton modes arising from interaction between bound excitons in monolayer thin semiconductor sheets and photons in a Fabry-Perot microcavity are considered theoretically. We calculate the dispersion curves, mode lifetimes, Rabi splitting, and Hopfield coefficients of these structures for two nearly 2D semiconductor materials, MoS2 and WS2, and suggest that they are interesting for studying the rich physics associated with the Bose-Einstein condensation of exciton-polaritons. The large exciton binding energy and dipole allowed exciton transitions, in addition to the relatively easily controllable distance between the semiconductor sheets are the advantages of this system in comparison with traditional GaAs or CdTe based semiconductor microcavities. In particular, in order to mimic the rich physical properties of the quantum degenerate mixture of two bosonic species of dilute atomic gases with tunable inter-species interaction , we put forward a structure containing two semiconductor sheets separated by some atomic-scale distance (l) using a nearly 2D dielectric (e.g. h-BN), which offers the possibility of tuning the interaction between two exciton-polariton Bose-Enstein condensates. We show that the dynamics of this novel structure are ruled by two coupled Gross-Pitaevskii equations with the coupling parameter ∼ l −1 .

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“…Neglecting the contributions of integrals involving atomic wave functions at different sites, the matrix elements in Eq. (33) can be simplified to nk|e −i(q+G)r |n k + q = P (q + G)S nk,n k+q (q + G), (34) where P (q + G) and S nk,n k (q + G) are defined above.…”

Section: B Screening Within the Random Phase Approximationmentioning

confidence: 99%

“…Note, however, that recent studies have started to elucidate various aspects of valley relaxation and decoherence in two-dimensional (2D) transition-metal dichalcogenides [32][33][34] and graphene [35,36], as well as carbon-based [19,37] and silicon quantum dots [38,39].…”

Section: Introductionmentioning

confidence: 99%

Valley relaxation in graphene due to charged impurities

Boross

Pályi

2015

Phys. Rev. B

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Monolayer graphene is an example of materials with multivalley electronic structure. In such materials, the valley index is being considered as an information carrier. Consequently, relaxation mechanisms leading to loss of valley information are of interest. Here, we calculate the rate of valley relaxation induced by charged impurities in graphene. A special model of graphene is applied, where the p z orbitals are two-dimensional Gaussian functions, with a spatial extension characterized by an effective Bohr radius a eB . We obtain the valley relaxation rate by solving the Boltzmann equation, for the case of noninteracting electrons, as well as for the case when the impurity potential is screened due to electron-electron interaction. For the latter case, we take into account local-field effects and evaluate the dielectric matrix in the random phase approximation. Our main findings are as follows: (i) The valley relaxation rate is proportional to the electronic density of states at the Fermi energy. (ii) Charged impurities located in the close vicinity of the graphene plane, at distance d 0.3Å, are much more efficient in inducing valley relaxation than those farther away, the effect of the latter being suppressed exponentially with increasing graphene-impurity distance d. (iii) Both in the absence and in the presence of electron-electron interaction, the valley relaxation rate shows pronounced dependence on the effective Bohr radius a eB . The trends are different in the two cases: In the absence (presence) of screening, the valley relaxation rate decreases (increases) for increasing effective Bohr radius. This last result highlights that a quantitative calculation of the valley relaxation rate should incorporate electron-electron interactions as well as an accurate knowledge of the electronic wave functions on the atomic length scale.

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Exciton valley relaxation in a single layer ofWS2measured by ultrafast spectroscopy

Cited by 132 publications

References 49 publications

Manipulating spin-polarized photocurrents in 2D transition metal dichalcogenides

Manipulating spin-polarized photocurrents in 2D transition metal dichalcogenides

Exciton polaritons in two-dimensional dichalcogenide layers placed in a planar microcavity: Tunable interaction between two Bose-Einstein condensates

Valley relaxation in graphene due to charged impurities

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