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1. Strong interaction between interlayer excitons and correlated electrons in WSe2/WS2 moiré superlattice

   https://doi.org/10.1038/s41467-021-23732-6  

   Miao, Shengnan ; Wang, Tianmeng ; Huang, Xiong ; Chen, Dongxue ; Lian, Zhen ; Wang, Chong ; Blei, Mark ; Taniguchi, Takashi ; Watanabe, Kenji ; Tongay, Sefaattin ; et al ( December 2021 , Nature Communications)

   null (Ed.)

   Abstract Heterobilayers of transition metal dichalcogenides (TMDCs) can form a moiré superlattice with flat minibands, which enables strong electron interaction and leads to various fascinating correlated states. These heterobilayers also host interlayer excitons in a type-II band alignment, in which optically excited electrons and holes reside on different layers but remain bound by the Coulomb interaction. Here we explore the unique setting of interlayer excitons interacting with strongly correlated electrons, and we show that the photoluminescence (PL) of interlayer excitons sensitively signals the onset of various correlated insulating states as the band filling is varied. When the system is in one of such states, the PL of interlayer excitons is relatively amplified at increased optical excitation power due to reduced mobility, and the valley polarization of interlayer excitons is enhanced. The moiré superlattice of the TMDC heterobilayer presents an exciting platform to engineer interlayer excitons through the periodic correlated electron states.
2. Proximity-enhanced valley Zeeman splitting at the WS ₂ /graphene interface

   https://doi.org/10.1088/2053-1583/acd5df  

   Faria Junior, Paulo E. ; Naimer, Thomas ; McCreary, Kathleen M. ; Jonker, Berend T. ; Finley, Jonathan J. ; Crooker, Scott A. ; Fabian, Jaroslav ; Stier, Andreas V. ( May 2023 , 2D Materials)

   The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS₂, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS₂/graphene stacks employing first-principles calculations and find that the lower conduction band of WS₂at the$K/K^{\mathrm{\prime }}$valleys (the${\mathrm{C}\mathrm{B}}^{-}$band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at theQpoint of WS₂mediates the coupling between${\mathrm{C}\mathrm{B}}^{-}$and graphene due to resonant energy conditions and strong coupling to the Dirac cone. Thismore »leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

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3. Moiré potential renormalization and ultra-flat bands induced by quasiparticle-plasmon coupling

   https://doi.org/10.1038/s41524-023-00963-3  

   Zhu, Linghan ; Wang, Haonan ; Yang, Li ( January 2023 , npj Computational Materials)

   Moiré potential profile can form flat electronic bands and manifest correlated states of electrons, where carrier doping is essential for observing those correlations. In this work, we uncover a hidden but remarkable many-electron effect: doped carriers form a two-dimensional plasmon and strongly couple with quasiparticles to renormalize moiré potential and realize ultra-flat bands. Using many-body perturbation theory, we demonstrate this effect in twisted MoS₂/WS₂heterobilayer. The moiré potential is significantly enhanced upon carrier doping, and the bandwidth is reduced by order of magnitude, leading to drastic quenching of electronic kinetic energy and stronger correlation. We further predict that the competition between correlated mechanisms can be effectively controlled via doping, giving hope to a quantum transition between Mott and charge-transfer insulating states. Our work reveals that the potential renormalization effect of doping is much more significant in determining and controlling many-electron electronic correlations than sole filling-factor tuning in semiconducting moiré crystals.
4. Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2

   https://doi.org/10.1038/s41467-021-25499-2  

   Lin, Kai-Qiang ; Ong, Chin Shen ; Bange, Sebastian ; Faria Junior, Paulo E. ; Peng, Bo ; Ziegler, Jonas D. ; Zipfel, Jonas ; Bäuml, Christian ; Paradiso, Nicola ; Watanabe, Kenji ; et al ( December 2021 , Nature Communications)

   Abstract Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe 2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K -valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW -BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons.
5. Radiative pattern of intralayer and interlayer excitons in two-dimensional WS2/WSe2 heterostructure

   https://doi.org/10.1038/s41598-022-10851-3  

   Aly, Mohammed Adel ; Shah, Manan ; Schneider, Lorenz Maximilian ; Kang, Kyungnam ; Koch, Martin ; Yang, Eui-Hyeok ; Rahimi-Iman, Arash ( April 2022 , Scientific Reports)

   Two-dimensional (2D) heterostructures (HS) formed by transition-metal dichalcogenide (TMDC) monolayers offer a unique platform for the study of intralayer and interlayer excitons as well as moiré-pattern-induced features. Particularly, the dipolar charge-transfer exciton comprising an electron and a hole, which are confined to separate layers of 2D semiconductors and Coulomb-bound across the heterojunction interface, has drawn considerable attention in the research community. On the one hand, it bears significance for optoelectronic devices, e.g. in terms of charge carrier extraction from photovoltaic devices. On the other hand, its spatially indirect nature and correspondingly high longevity among excitons as well as its out-of-plane dipole orientation render it attractive for excitonic Bose–Einstein condensation studies, which address collective coherence effects, and for photonic integration schemes with TMDCs. Here, we demonstrate the interlayer excitons’ out-of-plane dipole orientation through angle-resolved spectroscopy of the HS photoluminescence at cryogenic temperatures, employing a tungsten-based TMDC HS. Within the measurable light cone, the directly-obtained radiation profile of this species clearly resembles that of an in-plane emitter which deviates from that of the intralayer bright excitons as well as the other excitonic HS features recently attributed to artificial superlattices formed by moiré patterns.