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Register a new userResolving spin, valley, and moire quasi-angular momentum of interlayer excitons in WSe2/WS2 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.
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Moire superlattices provide a powerful tool to engineer novel quantum phenomena in two-dimensional (2D) heterostructures, where the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states, and the Hofstadter butterfly can emerge in different types of graphene/boron nitride moire superlattices, while correlated insulating states and superconductivity have been reported in twisted bilayer graphene moire superlattices. In addition to their dramatic effects on the single particle states, moire superlattices were recently predicted to host novel excited states, such as moire exciton bands. Here we report the first observation of moire superlattice exciton states in nearly aligned WSe2/WS2 heterostructures. These moire exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinctly different from that of the A exciton in WSe2 monolayers and in large-twist-angle WSe2/WS2 heterostructures. The observed phenomena can be described by a theoretical model where the periodic moire potential is much stronger than the exciton kinetic energy and creates multiple flat exciton minibands. The moire exciton bands provide an attractive platform to explore and control novel excited state of matter, such as topological excitons and a correlated exciton Hubbard model, in transition metal dichalcogenides.
Moire superlattices in van der Waals heterostructures have emerged as a powerful tool for engineering novel quantum phenomena. Here we report the observation of a correlated interlayer exciton insulator in a double-layer heterostructure composed of a WSe2 monolayer and a WS2/WSe2 moire bilayer that are separated by an ultrathin hexagonal boron nitride (hBN). The moire WS2/WSe2 bilayer features a Mott insulator state at hole density p/p0 = 1, where p0 corresponds to one hole per moire lattice site. When electrons are added to the Mott insulator in the WS2/WSe2 moire bilayer and an equal number of holes are injected into the WSe2 monolayer, a new interlayer exciton insulator emerges with the holes in the WSe2 monolayer and the electrons in the doped Mott insulator bound together through interlayer Coulomb interactions. The excitonic insulator is stable up to a critical hole density of ~ 0.5p0 in the WSe2 monolayer, beyond which the system becomes metallic. Our study highlights the opportunities for realizing novel quantum phases in double-layer moire systems due to the interplay between the moire flat band and strong interlayer electron interactions.
MoSe2-WSe2 heterostructures host strongly bound interlayer excitons (IXs) which exhibit bright photoluminescence (PL) when the twist-angle is near 0{deg} or 60{deg}. Over the past several years, there have been numerous reports on the optical response of these heterostructures but no unifying model to understand the dynamics of IXs and their temperature dependence. Here, we perform a comprehensive study of the temperature, excitation power, and time-dependent PL of IXs. We observe a significant decrease in PL intensity above a transition temperature that we attribute to a transition from localized to delocalized IXs. Astoundingly, we find a simple inverse relationship between the IX PL energy and the transition temperature, which exhibits opposite power dependent behaviors for near 0{deg} and 60{deg} samples. We conclude that this temperature dependence is a result of IX-IX exchange interactions, whose effect is suppressed by the moire potential trapping IXs at low temperature.
Transition metal dichalcogenides (TMDCs) heterostructure with a type II alignment hosts unique interlayer excitons with the possibility of spin-triplet and spin-singlet states. However, the associated spectroscopy signatures remain elusive, strongly hindering the understanding of the Moire potential modulation of the interlayer exciton. In this work, we unambiguously identify the spin-singlet and spin-triplet interlayer excitons in the WSe2/MoSe2 hetero-bilayer with a 60-degree twist angle through the gate- and magnetic field-dependent photoluminescence spectroscopy. Both the singlet and triplet interlayer excitons show giant valley-Zeeman splitting between the K and K valleys, a result of the large Lande g-factor of the singlet interlayer exciton and triplet interlayer exciton, which are experimentally determined to be ~ 10.7 and ~ 15.2, respectively, in good agreement with theoretical expectation. The PL from the singlet and triplet interlayer excitons show opposite helicities, determined by the atomic registry. Helicity-resolved photoluminescence excitation (PLE) spectroscopy study shows that both singlet and triplet interlayer excitons are highly valley-polarized at the resonant excitation, with the valley polarization of the singlet interlayer exciton approaches unity at ~ 20 K. The highly valley-polarized singlet and triplet interlayer excitons with giant valley-Zeeman splitting inspire future applications in spintronics and valleytronics.
Two-dimensional (2D) materials, such as graphene1, boron nitride2, and transition metal dichalcogenides (TMDs)3-5, have sparked wide interest in both device physics and technological applications at the atomic monolayer limit. These 2D monolayers can be stacked together with precise control to form novel van der Waals heterostructures for new functionalities2,6-9. One highly coveted but yet to be realized heterostructure is that of differing monolayer TMDs with type II band alignment10-12. Their application potential hinges on the fabrication, understanding, and control of bonded monolayers, with bound electrons and holes localized in individual monolayers, i.e. interlayer excitons. Here, we report the first observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by both photoluminescence and photoluminescence excitation spectroscopy. The energy and luminescence intensity of interlayer excitons are highly tunable by an applied vertical gate voltage, implying electrical control of the heterojunction band-alignment. Using time resolved photoluminescence, we find that the interlayer exciton is long-lived with a lifetime of about 1.8 ns, an order of magnitude longer than intralayer excitons13-16. Our work demonstrates the ability to optically pump interlayer electric polarization and provokes the immediate exploration of interlayer excitons for condensation phenomena, as well as new applications in 2D light-emitting diodes, lasers, and photovoltaic devices.
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