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Register a new userOne-Dimensional Moire Excitons in Transition-Metal Dichalcogenide Heterobilayers
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The formation of interfacial moire patterns from angular and/or lattice mismatch has become a powerful approach to engineer a range of quantum phenomena in van der Waals heterostructures. For long-lived and valley-polarized interlayer excitons in transition-metal dichalcogenide (TMDC) heterobilayers, signatures of quantum confinement by the moire landscape have been reported in recent experimental studies. Such moire confinement has offered the exciting possibility to tailor new excitonic systems, such as ordered arrays of zero-dimensional (0D) quantum emitters and their coupling into topological superlattices. A remarkable nature of the moire potential is its dramatic response to strain, where a small uniaxial strain can tune the array of quantum-dot-like 0D traps into parallel stripes of one-dimensional (1D) quantum wires. Here, we present direct evidence for the 1D moire potentials from real space imaging and the corresponding 1D moire excitons from photoluminescence (PL) emission in MoSe2/WSe2 heterobilayers. Whereas the 0D moire excitons display quantum emitter-like sharp PL peaks with circular polarization, the PL emission from 1D moire excitons has linear polarization and two orders of magnitude higher intensity. The results presented here establish strain engineering as a powerful new method to tailor moire potentials as well as their optical and electronic responses on demand.
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We reveal by first-principles calculations that the interlayer binding in a twisted MoS2/MoTe2 heterobilayer decreases with increasing twist angle, due to the increase of the interlayer overlapping degree, a geometric quantity describing well the interlayer steric effect. The binding energy is found to be a Gaussian-like function of twist angle. The resistance to rotation, an analogue to the interlayer sliding barrier, can also be defined accordingly. In sharp contrast to the case of MoS2 homobilayer, here the energy band gap reduces with increasing twist angle. We find a remarkable interlayer charge transfer from MoTe2 to MoS2 which enlarges the band gap, but this charge transfer weakens with greater twisting and interlayer overlapping degree. Our discovery provides a solid basis in twistronics and practical instruction in band structure engineering of van der Waals heterostructures.
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 $delta=0.6 pm 0.1$ 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 $tau_D$ in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure $tau_D sim 110 pm 10$ 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.
Atomically thin layers of transition metal dichalcogenides (TMDCs) exhibit exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. The van der Waals nature of interlayer coupling makes it easy to integrate TMDC layers into heterostructures with different dielectric or metallic substrates. This allows to tailor electronic and optical properties of these materials, as Coulomb interaction inside atomically thin layers is very susceptible to screening by the environment. Here we theoretically investigate dynamical screening effects in TMDCs due to bulk substrates doped with carriers over a large density range, thereby offering three-dimensional plasmons as tunable degree of freedom. We report a wide compensation of renormalization effects leading to a spectrally more stable exciton than predicted for static substrate screening, even if plasmons and excitons are in resonance. We also find a nontrivial dependence of the single-particle band gap on substrate doping density due to dynamical screening. Our investigation provides microscopic insight into the mechanisms that allow for manipulations of TMDC excitons by means of arbitrary plasmonic environments on the nanoscale.
Small-twist-angle transition metal dichalcogenide (TMD) heterobilayers develop isolated flat moire bands that are approximately described by triangular lattice generalized Hubbard models [PhysRevLett.121.026402]. In this article we explore the metallic and insulating states that appear under different control conditions at a density of one-electron per moire period, and the transitions between them. By combining fully self-consistent Hartree-Fock theory calculations with strong-coupling expansions around the atomic limit, we identify four different magnetic states and one nonmagnetic state near the model phase diagrams metal-insulator phase-transition line. Ferromagnetic insulating states, stabilized by non-local direct exchange interactions, are surprisingly prominent.
Strain in two-dimensional (2D) transition metal dichalcogenide (TMD) has led to localized states with exciting optical properties, in particular in view of designing one photon sources. The naturally formed of the MoS2 monolayer deposed on hBN substrate leads to a reduction of the bandgap in the strained region creating a nanobubble. The photogenerated particles are thus confined in the strain-induced potential. Using numerical diagonalization, we simulate the spectra of the confined exciton states, their oscillator strengths and radiative lifetimes. We show that a single state of the confined exciton is optically active, which suggests that the MoS2/hBN nanobubble is a good candidate for the realisation of single-photon sources. Furthermore, the exciton binding energy, oscillator strength and radiative lifetime are enhanced due to the confinement effect.
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