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1. Tuning moiré excitons and correlated electronic states through layer degree of freedom

   https://doi.org/10.1038/s41467-022-32493-9  

   Chen, Dongxue ; Lian, Zhen ; Huang, Xiong ; Su, Ying ; Rashetnia, Mina ; Yan, Li ; Blei, Mark ; Taniguchi, Takashi ; Watanabe, Kenji ; Tongay, Sefaattin ; et al ( August 2022 , Nature Communications)

   Moiré coupling in transition metal dichalcogenides (TMDCs) superlattices introduces flat minibands that enable strong electronic correlation and fascinating correlated states, and it also modifies the strong Coulomb-interaction-driven excitons and gives rise to moiré excitons. Here, we introduce the layer degree of freedom to the WSe₂/WS₂moiré superlattice by changing WSe₂from monolayer to bilayer and trilayer. We observe systematic changes of optical spectra of the moiré excitons, which directly confirm the highly interfacial nature of moiré coupling at the WSe₂/WS₂interface. In addition, the energy resonances of moiré excitons are strongly modified, with their separation significantly increased in multilayer WSe₂/monolayer WS₂moiré superlattice. The additional WSe₂layers also modulate the strong electronic correlation strength, evidenced by the reduced Mott transition temperature with added WSe₂layer(s). The layer dependence of both moiré excitons and correlated electronic states can be well described by our theoretical model. Our study presents a new method to tune the strong electronic correlation and moiré exciton bands in the TMDCs moiré superlattices, ushering in an exciting platform to engineer quantum phenomena stemming from strong correlation and Coulomb interaction.
2. Tunable strain soliton networks confine electrons in van der Waals materials

   https://doi.org/10.1038/s41567-020-0953-2  

   Edelberg, Drew ; Kumar, Hemant ; Shenoy, Vivek ; Ochoa, Héctor ; Pasupathy, Abhay N. ( July 2020 , Nature Physics)

   Twisting or sliding two-dimensional crystals with respect to each other gives rise to moiré patterns determined by the difference in their periodicities. Such lattice mismatches can exist for several reasons: differences between the intrinsic lattice constants of the two layers, as is the case for graphene on BN; rotations between the two lattices, as is the case for twisted bilayer graphene; and strains between two identical layers in a bilayer. Moiré patterns are responsible for a number of new electronic phenomena observed in recent years in van der Waals heterostructures, including the observation of superlattice Dirac points for graphene on BN, collective electronic phases in twisted bilayers and twisted double bilayers, and trapping of excitons in the moiré potential. An open question is whether we can use moiré potentials to achieve strong trapping potentials for electrons. Here, we report a technique to achieve deep, deterministic trapping potentials via strain-based moiré engineering in van der Waals materials. We use strain engineering to create on-demand soliton networks in transition metal dichalcogenides. Intersecting solitons form a honeycomb-like network resulting from the three-fold symmetry of the adhesion potential between layers. The vertices of this network occur in bound pairs with different interlayer stacking arrangements.more »One vertex of the pair is found to be an efficient trap for electrons, displaying two states that are deeply confined within the semiconductor gap and have a spatial extent of 2 nm. Soliton networks thus provide a path to engineer deeply confined states with a strain-dependent tunable spatial separation, without the necessity of introducing chemical defects into the host materials.« less
3. Localized interlayer excitons in MoSe2–WSe2 heterostructures without a moiré potential

   https://doi.org/10.1038/s41467-022-33082-6  

   Mahdikhanysarvejahany, Fateme ; Shanks, Daniel N. ; Klein, Matthew ; Wang, Qian ; Koehler, Michael R. ; Mandrus, David G. ; Taniguchi, Takashi ; Watanabe, Kenji ; Monti, Oliver L. A. ; LeRoy, Brian J. ; et al ( September 2022 , Nature Communications)

   Interlayer excitons (IXs) in MoSe₂–WSe₂heterobilayers have generated interest as highly tunable light emitters in transition metal dichalcogenide (TMD) heterostructures. Previous reports of spectrally narrow (<1 meV) photoluminescence (PL) emission lines at low temperature have been attributed to IXs localized by the moiré potential between the TMD layers. We show that spectrally narrow IX PL lines are present even when the moiré potential is suppressed by inserting a bilayer hexagonal boron nitride (hBN) spacer between the TMD layers. We compare the doping, electric field, magnetic field, and temperature dependence of IXs in a directly contacted MoSe₂–WSe₂region to those in a region separated by bilayer hBN. The doping, electric field, and temperature dependence of the narrow IX lines are similar for both regions, but their excitonic g-factors have opposite signs, indicating that the origin of narrow IX PL is not the moiré potential.
4. Tailoring excitonic states of van der Waals bilayers through stacking configuration, band alignment, and valley spin

   https://doi.org/10.1126/sciadv.aax7407  

   Hsu, Wei-Ting ; Lin, Bo-Han ; Lu, Li-Syuan ; Lee, Ming-Hao ; Chu, Ming-Wen ; Li, Lain-Jong ; Yao, Wang ; Chang, Wen-Hao ; Shih, Chih-Kang ( December 2019 , Science Advances)

   Excitons in monolayer semiconductors have a large optical transition dipole for strong coupling with light. Interlayer excitons in heterobilayers feature a large electric dipole that enables strong coupling with an electric field and exciton-exciton interaction at the cost of a small optical dipole. We demonstrate the ability to create a new class of excitons in hetero- and homobilayers that combines advantages of monolayer and interlayer excitons, i.e., featuring both large optical and electric dipoles. These excitons consist of an electron confined in an individual layer, and a hole extended in both layers, where the carrier-species–dependent layer hybridization can be controlled through rotational, translational, band offset, and valley-spin degrees of freedom. We observe different species of layer-hybridized valley excitons, which can be used for realizing strongly interacting polaritonic gases and optical quantum controls of bidirectional interlayer carrier transfer.
5. Observation of chiral surface excitons in a topological insulator Bi ₂ Se ₃

   https://doi.org/10.1073/pnas.1813514116  

   Kung, H. -H. ; Goyal, A. P. ; Maslov, D. L. ; Wang, X. ; Lee, A. ; Kemper, A. F. ; Cheong, S. -W. ; Blumberg, G. ( February 2019 , Proceedings of the National Academy of Sciences)

   The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin–orbit interaction in solids composed of heavy elements. Here, we study the composite particles—chiral excitons—formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI,${\mathrm{B}\mathrm{i}}_2{\mathrm{S}\mathrm{e}}_3$. Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin–orbit coupling. A theoretical model basedmore »on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin–orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.

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