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Register a new userSpin-layer locking of interlayer excitons trapped in moire potentials
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Added by
Mauro Brotons-Gisbert
Publication date
2019
fields
Physics
and research's language is
English
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Van der Waals heterostructures offer attractive opportunities to design quantum materials. For instance, transition metal dichalcogenides (TMDs) possess three quantum degrees of freedom: spin, valley index, and layer index. Further, twisted TMD heterobilayers can form moire patterns that modulate the electronic band structure according to atomic registry, leading to spatial confinement of interlayer exciton (IXs). Here we report the observation of spin-layer locking of IXs trapped in moire potentials formed in a heterostructure of bilayer 2H-MoSe$_2$ and monolayer WSe$_2$. The phenomenon of locked electron spin and layer index leads to two quantum-confined IX species with distinct spin-layer-valley configurations. Furthermore, we observe that the atomic registries of the moire trapping sites in the three layers are intrinsically locked together due to the 2H-type stacking characteristic of bilayer TMDs. These results identify the layer index as a useful degree of freedom to engineer tunable few-level quantum systems in two-dimensional heterostructures.
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Moire superlattices provide a powerful way to engineer properties of electrons and excitons in two-dimensional van der Waals heterostructures. The moire effect can be especially strong for interlayer excitons, where electrons and holes reside in different layers and can be addressed separately. In particular, it was recently proposed that the moire superlattice potential not only localizes interlayer exciton states at different superlattice positions, but also hosts an emerging moire quasi-angular momentum (QAM) that periodically switches the optical selection rules for interlayer excitons at different moire sites. Here we report the observation of multiple interlayer exciton states coexisting in a WSe2/WS2 moire superlattice and unambiguously determine their spin, valley, and moire QAM through novel resonant optical pump-probe spectroscopy and photoluminescence excitation spectroscopy. We demonstrate that interlayer excitons localized at different moire sites can exhibit opposite optical selection rules due to the spatially-varying moire QAM. Our observation reveals new opportunities to engineer interlayer exciton states and valley physics with moire superlattices for optoelectronic and valleytronic applications.
Photon antibunching, a hallmark of quantum light, has been observed in the correlations of light from isolated atomic and atomic-like solid-state systems. Two-dimensional semiconductor heterostructures offer a unique method to create a quantum light source: a small lattice mismatch or relative twist in a heterobilayer can create moire trapping potentials for excitons which are predicted to create arrays of quantum emitters. While signatures of moire trapped excitons have been observed, their quantum nature has yet to be confirmed. Here we report photon antibunching from single moire trapped interlayer excitons in a heterobilayer. Via polarization resolved magneto-optical spectroscopy, we demonstrate the discrete anharmonic spectra arise from bound band-edge electron-hole pairs trapped in moire potentials. Finally, using an out-of-plane electric field, we exploit the large permanent dipole of interlayer excitons to achieve large DC Stark tuning, up to 40 meV, of the quantum emitters. Our results confirm the quantum nature of moire confined excitons and open opportunities to investigate their inhomogeneity and interactions between the emitters or tune single emitters into resonance with cavity modes or other emitters.
The creation of moire patterns in crystalline solids is a powerful approach to manipulate their electronic properties, which are fundamentally influenced by periodic potential landscapes. In 2D materials, a moire pattern with a superlattice potential can form by vertically stacking two layered materials with a twist and/or finite lattice constant difference. This unique approach has led to emergent electronic phenomena, including the fractal quantum Hall effect, tunable Mott insulators, and unconventional superconductivity. Furthermore, theory predicts intriguing effects on optical excitations by a moire potential in 2D valley semiconductors, but these signatures have yet to be experimentally detected. Here, we report experimental evidence of interlayer valley excitons trapped in a moire potential in MoSe$_2$/WSe$_2$ heterobilayers. At low temperatures, we observe photoluminescence near the free interlayer exciton energy but with over 100 times narrower linewidths. The emitter g-factors are homogeneous across the same sample and only take two values, -15.9 and 6.7, in samples with twisting angles near 60{deg} and 0deg, respectively. The g-factors match those of the free interlayer exciton, which is determined by one of two possible valley pairing configurations. At a twist angle near 20deg, the emitters become two orders of magnitude dimmer, but remarkably, they possess the same g-factor as the heterobilayer near 60deg. This is consistent with the Umklapp recombination of interlayer excitons near the commensurate 21.8{deg} twist angle. The emitters exhibit strong circular polarization, which implies the preservation of three-fold rotation symmetry by the trapping potential. Together with the power and excitation energy dependence, all evidence points to their origin as interlayer excitons trapped in a smooth moire potential with inherited valley-contrasting physics.
Photoluminescence (PL) from excitons serves as a powerful tool to characterize the optoelectronic property and band structure of semiconductors, especially for atomically thin 2D transition metal chalcogenide (TMD) materials. However, PL quenches quickly when the thickness of TMD material increases from monolayer to few-layers, due to the change from direct to indirect band transition. Here we show that PL can be recovered by engineering multilayer heterostructures, with the band transition reserved to be direct type. We report emission from layer engineered interlayer excitons from these multilayer heterostructures. Moreover, as desired for valleytronic devices, the lifetime, valley polarization, and the valley lifetime of the generated interlayer excitons can all be significantly improved as compared with that in the monolayer-monolayer heterostructure. Our results pave the way for controlling the properties of interlayer excitons by layer engineering.
Moire patterns with a superlattice potential can be formed by vertically stacking two layered materials with a relative twist or lattice constant mismatch. The moire superlattice can generate flat bands that result in new correlated insulating, superconducting, and topological states. Strong electron correlations, tunable by the fractional filling, have been observed in both graphene and transition metal dichalcogenide (TMD) based systems. In addition, in TMD based systems, the moire potential landscape can trap interlayer excitons (IX) at specific atomic registries. Here we report that spatially isolated trapped IX in a molybdenum diselenide/tungsten diselenide heterobilayer device provide a sensitive optical probe of carrier filling in their immediate environment. By mapping the spatial positions of individual trapped IX, we are able to spectrally track the emitters as the moire lattice is filled with excess carriers. Upon initial doping of the heterobilayer, neutral trapped IX form charged IX (IX trions) uniformly with a binding energy of ~7 meV. Upon further doping, the empty superlattice sites sequentially fill, creating a Coulomb staircase: stepwise changes in the IX trion emission energy due to Coulomb interactions with carriers at nearest neighbour moire sites. This non-invasive, highly local technique can complement transport and non-local optical sensing techniques to characterise Coulomb interaction energies, visualise charge correlated states, or probe local disorder in a moire superlattice.
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