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1. Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

   https://doi.org/10.1038/s41467-023-40288-9  

   Lian, Zhen; Chen, Dongxue; Ma, Lei; Meng, Yuze; Su, Ying; Yan, Li; Huang, Xiong; Wu, Qiran; Chen, Xinyue; Blei, Mark; et al (August 2023, Nature Communications)

   Abstract Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe₂-WS₂-WSe₂trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe₂valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe₂layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe₂under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions. 

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2. Strained two-dimensional tungsten diselenide for mechanically tunable exciton transport

   https://doi.org/10.1038/s41467-024-55135-8  

   Kim, Jin_Myung; Jeong, Kwang-Yong; Kwon, Soyeong; So, Jae-Pil; Wang, Michael_Cai; Snapp, Peter; Park, Hong-Gyu; Nam, SungWoo (December 2024, Nature Communications)

   Abstract Tightly bound electron-hole pairs (excitons) hosted in atomically-thin semiconductors have emerged as prospective elements in optoelectronic devices for ultrafast and secured information transfer. The controlled exciton transport in such excitonic devices requires manipulating potential energy gradient of charge-neutral excitons, while electrical gating or nanoscale straining have shown limited efficiency of exciton transport at room temperature. Here, we report strain gradient induced exciton transport in monolayer tungsten diselenide (WSe₂) across microns at room temperature via steady-state pump-probe measurement. Wrinkle architecture enabled optically-resolvable local strain (2.4%) and energy gradient (49 meV/μm) to WSe₂. We observed strain gradient induced flux of high-energy excitons and emission of funneled, low-energy excitons at the 2.5 μm-away pump point with nearly 45% of relative emission intensity compared to that of excited excitons. Our results strongly support the strain-driven manipulation of exciton funneling in two-dimensional semiconductors at room temperature, opening up future opportunities of 2D straintronic exciton devices. 

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3. Evidence for equilibrium exciton condensation in monolayer WTe2

   https://doi.org/10.1038/s41567-021-01427-5  

   Sun, Bosong; Zhao, Wenjin; Palomaki, Tauno; Fei, Zaiyao; Runburg, Elliott; Malinowski, Paul; Huang, Xiong; Cenker, John; Cui, Yong-Tao; Chu, Jiun-Haw; et al (January 2022, Nature Physics)

   Abstract We present evidence that the two-dimensional bulk of monolayer WTe 2 contains electrons and holes bound by Coulomb attraction—excitons—that spontaneously form in thermal equilibrium. On cooling from room temperature to 100 K, the conductivity develops a V-shaped dependence on electrostatic doping, while the chemical potential develops a step at the neutral point. These features are much sharper than is possible in an independent-electron picture, but they can be accounted for if electrons and holes interact strongly and are paired in equilibrium. Our calculations from first principles show that the exciton binding energy is larger than 100 meV and the radius as small as 4 nm, explaining their formation at high temperature and doping levels. Below 100 K, more strongly insulating behaviour is seen, suggesting that a charge-ordered state forms. The observed absence of charge density waves in this state is surprising within an excitonic insulator picture, but we show that it can be explained by the symmetries of the exciton wavefunction. Therefore, in addition to being a topological insulator, monolayer WTe 2 exhibits strong correlations over a wide temperature range. 

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4. Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

   https://doi.org/10.1038/s41535-021-00351-4  

   Volkov, P. A.; Ye, Mai; Lohani, H.; Feldman, I.; Kanigel, A.; Blumberg, G. (May 2021, npj Quantum Materials)

   Abstract Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs—excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta₂NiSe₅disentangling it from the lattice phonons. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to noncritical lattice modes is shown to enhance the transition temperature. On cooling, we observe the gradual emergence of coherent superpositions of band states at the correlated insulator gap edge, with strong departures from mean-field theory predictions. Our results demonstrate the realization of a strongly correlated excitonic state in an equilibrium bulk material. 

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5. Antiferromagnetic excitonic insulator state in Sr3Ir2O7

   https://doi.org/10.1038/s41467-022-28207-w  

   Mazzone, D. G.; Shen, Y.; Suwa, H.; Fabbris, G.; Yang, J.; Zhang, S. -S.; Miao, H.; Sears, J.; Jia, Ke; Shi, Y. G.; et al (February 2022, Nature Communications)

   Abstract Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr₃Ir₂O₇. By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr₃Ir₂O₇indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase. 

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