Exciton band structure of monolayer<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>MoS</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

Wu, Fengcheng; Qu, Fanyao; MacDonald, Allan H.

doi:10.1103/physrevb.91.075310

Cited by 309 publications

(410 citation statements)

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“…While the exchange interaction is very important for direct excitons in a TMDC monolayer studied in Ref. 47, the exchange interactions in a spatially separated electron-hole system in a bilayer are suppressed due to the low tunneling probability coming from the shielding of the dipole-dipole interaction by the insulating barrier. Hence, at large interlayer separations the exchange phenomena, caused by the distinction between excitons and bosons, can be neglected for the electron-hole bilayers [73].…”

Section: The Collective Excitations For Spatially Separated Elecmentioning

confidence: 99%

“…47 the exchange effects in exciton-exciton interaction were considered for a monolayer. It would be instructive to compare these limiting cases.…”

Section: The Collective Excitations For Spatially Separated Elecmentioning

confidence: 99%

“…The properties of direct excitons in mono-and few-layer TMDCs on a SiO 2 substrate were experimentally and theoretically investigated, identifying and characterizing not only the ground-state exciton but the full sequence of excited (Rydberg) exciton states [46]. The exciton binding energy for monolayer, few-layer and bulk TMDCs and optical gaps were evaluated using the tight-binding approximation [47], by solving the BSE [48,49], applying an effective mass model, density functional theory and subsequent random phase approximation calculations [50], and by a generalized time-dependent density-matrix functional theory approach [51]. Significant spin-orbit splitting in the valence band leads to the formation of two distinct types of excitons in TMDC layers, labeled A and B [50].…”

Section: Introductionmentioning

confidence: 99%

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High-temperature superfluidity of the two-component Bose gas in a transition metal dichalcogenide bilayer

Berman

Kezerashvili

2016

Phys. Rev. B

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The high-temperature superfluidity of two-dimensional dipolar excitons in two parallel TMDC layers is predicted. We study Bose-Einstein condensation in the two-component system of dipolar A and B excitons. The effective mass, energy spectrum of the collective excitations, the sound velocity and critical temperature are obtained for different TMDC materials. It is shown that in the Bogolubov approximation the sound velocity in the two-component dilute exciton Bose gas is always larger than in any one-component. The difference between the sound velocities for two-component and one-component dilute gases is caused by the fact that the sound velocity for a two-component system depends on the reduced mass of A and B excitons, which is always smaller than the individual mass of A or B exciton. Due to this fact, the critical temperature Tc for superfluidity for the twocomponent exciton system in a TMDC bilayer is about one order of magnitude higher than Tc in any one-component exciton system. We propose to observe the superfluidity of two-dimensional dipolar excitons in two parallel TMDC layers, which causes two opposite superconducting currents in each TMDC layer.

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Section: The Collective Excitations For Spatially Separated Elecmentioning

confidence: 99%

“…47 the exchange effects in exciton-exciton interaction were considered for a monolayer. It would be instructive to compare these limiting cases.…”

Section: The Collective Excitations For Spatially Separated Elecmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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High-temperature superfluidity of the two-component Bose gas in a transition metal dichalcogenide bilayer

Berman

Kezerashvili

2016

Phys. Rev. B

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“…This information allows us to estimate the ratio between T 1 and T 2 , leaving only two independent parameters in the model (Supplementary Section I). (We note that the partial degree of linear polarization has several possible origins, including intervalley scattering induced by defects/impurities, intravalley scattering, and the exchange interaction between excitons [23][24][25] . Within our model, we neglect possible initial depolarization effects in creating the valley excitation, in accord with the near-resonant character of our optical excitation.)…”

mentioning

confidence: 99%

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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“…As we explain below, the moiré pattern produces a periodic potential, mixing momentum states separated by moiré reciprocal lattice vectors and producing satellite optical absorption peaks that are revealing. The exciton energy-momentum dispersion can be measured by tracking the dependence of satellite peak energies on twist angle.The valley pseudospin of an exciton is intrinsically coupled to its center-of-mass motion by the electronhole exchange interaction [27][28][29][30]. This effective spinorbit coupling endows the exciton with a 2π momentumspace Berry phase [31].…”

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confidence: 99%

Topological Exciton Bands in Moiré Heterojunctions

2017

Self Cite

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Moiré patterns are common in van der Waals heterostructures and can be used to apply periodic potentials to elementary excitations. We show that the optical absorption spectrum of transition metal dichalcogenide bilayers is profoundly altered by long period moiré patterns that introduce twist-angle dependent satellite excitonic peaks. Topological exciton bands with non-zero Chern numbers that support chiral excitonic edge states can be engineered by combining three ingredients: i) the valley Berry phase induced by electron-hole exchange interactions, ii) the moiré potential, and iii) the valley Zeeman field.Stacking two-dimensional (2D) materials into van der Waals heterostructures opens up new strategies for materials property engineering. One increasingly important example is the possibility of using the relative orientation (twist) angle between two 2D crystals to tune electronic properties. For small twist angles and latticeconstant mismatches, heterostructures exhibit long period moiré patterns that can yield dramatic changes. Moiré pattern formed in graphene-based heterostructures has been extensively studied, and many interesting phenomena have been observed, for example gap opening at graphene's Dirac point [1,2], generation of secondary Dirac points [3,4] and Hofstadter-butterfly spectra in a strong magnetic field [1,5,6].In this Letter, we study the influence of moiré patterns on collective excitations, focusing on the important case of excitons in the transition metal dichalcogenide (TMD) 2D semiconductors [7,8] like MoS 2 and WS 2 . Exciton features dominate the optical response of these materials because electron-hole pairs are strongly bound by the Coulomb interaction [9][10][11][12]. An exciton inherits a pseudospin-1/2 valley degree of freedom from its constituent electron and hole, and the exciton valley pseudospin can be optically addressed [13][14][15][16], providing access to the valley Hall effect [17] and the valley selective optical Stark effect [18,19].As in the case of graphene/hexagonal-boron-nitride and graphene/graphene bilayers, a moiré pattern can be established in TMD bilayers by using two different materials with a small lattice mismatch, by applying a small twist, or by combining both effects. TMD heterostructures have been realized [20][21][22] experimentally and can host interesting effects, for example the observation of valley polarized interlayer excitons with long lifetimes [23], the theoretical prediction of multiple degenerate interlayer excitons [24], and the possibility of achieving spatially indirect exciton condensation [25,26]. Our focus here is instead on the intralayer excitons that are more strongly coupled to light. As we explain below, the moiré pattern produces a periodic potential, mixing momentum states separated by moiré reciprocal lattice vectors and producing satellite optical absorption peaks that are revealing. The exciton energy-momentum dispersion can be measured by tracking the dependence of satellite peak energies on twist angle.The valley pseudospin...

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Exciton band structure of monolayerMoS2

Cited by 309 publications

References 43 publications

High-temperature superfluidity of the two-component Bose gas in a transition metal dichalcogenide bilayer

High-temperature superfluidity of the two-component Bose gas in a transition metal dichalcogenide bilayer

Optical manipulation of valley pseudospin

Topological Exciton Bands in Moiré Heterojunctions

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