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1. Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5

   https://doi.org/10.1038/s41467-023-43365-1  

   Chen, Cheng ; Tang, Weichen ; Chen, Xiang ; Kang, Zhibo ; Ding, Shuhan ; Scott, Kirsty ; Wang, Siqi ; Li, Zhenglu ; Ruff, Jacob P. C. ; Hashimoto, Makoto ; et al ( November 2023 , Nature Communications)

   During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators – an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to strong coupling is not clear. In this work, using angle-resolved photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray diffraction (XRD), we investigate the broken symmetry state across the semimetal-to-semiconductor transition in a leading bulk excitonic insulator candidate system Ta₂Ni(Se,S)₅. A broken symmetry phase is found to be continuously suppressed from the semimetal side to the semiconductor side, contradicting the anticipated maximal excitonic instability around the Lifshitz transition. Bolstered by first-principles and model calculations, we find strong interband electron-phonon coupling to play a crucial role in the enhanced symmetry breaking on the semimetal side of the phase diagram. Our results not only provide insight into the longstanding debate of the nature of intertwined orders in Ta₂NiSe₅, but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems.

    

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2. 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)

   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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3. Exciton dispersion and exciton–phonon interaction in solids by time-dependent density functional theory

   https://doi.org/10.1063/5.0137326  

   Liu, Junyi ; Lu, Gang ; Zhang, Xu ( January 2023 , The Journal of Chemical Physics)

   Understanding, predicting, and ultimately controlling exciton band structure and exciton dynamics are central to diverse chemical and materials problems. Here, we have developed a first-principles method to determine exciton dispersion and exciton–phonon interaction in semiconducting and insulating solids based on time-dependent density functional theory. The first-principles method is formulated in planewave bases and pseudopotentials and can be used to compute exciton band structures, exciton charge density, ionic forces, the non-adiabatic coupling matrix between excitonic states, and the exciton–phonon coupling matrix. Based on the spinor formulation, the method enables self-consistent noncollinear calculations to capture spin-orbital coupling. Hybrid exchange-correlation functionals are incorporated to deal with long-range electron–hole interactions in solids. A sub-Hilbert space approximation is introduced to reduce the computational cost without loss of accuracy. For validations, we have applied the method to compute the exciton band structure and exciton–phonon coupling strength in transition metal dichalcogenide monolayers; both agree very well with the previous GW-Bethe–Salpeter equation and experimental results. This development paves the way for accurate determinations of exciton dynamics in a wide range of solid-state materials.

    

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4. Unconventional excitonic states with phonon sidebands in layered silicon diphosphide

   https://doi.org/10.1038/s41563-022-01285-3  

   Zhou, Ling ; Huang, Junwei ; Windgaetter, Lukas ; Ong, Chin Shen ; Zhao, Xiaoxu ; Zhang, Caorong ; Tang, Ming ; Li, Zeya ; Qiu, Caiyu ; Latini, Simone ; et al ( June 2022 , Nature Materials)

   Abstract Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP 2 ), where the bound electron–hole pair is composed of electrons confined within one-dimensional phosphorus–phosphorus chains and holes extended in two-dimensional SiP 2 layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP 2 as a platform for the study of excitonic physics and many-particle effects. 

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5. Screening induced crossover between phonon- and plasmon-mediated pairing in layered superconductors

   https://doi.org/10.1088/2053-1583/acf944  

   in’t Veld, Y. ; Katsnelson, M. I. ; Millis, A. J. ; Rösner, M. ( September 2023 , 2D Materials)

   Two-dimensional (2D) metals can host gapless plasmonic excitations that strongly couple to electrons and thus may significantly affect superconductivity. To investigate the dynamical interplay of the electron–electron and electron–phonon interactions in the theory of 2D superconductivity, we apply a full momentum- and frequency-dependent one-loop theory treating electron–phonon, electron–plasmon, and phonon–plasmon coupling with the same accuracy. We tune the strength of the Coulomb interaction by varying the external screening${\epsilon }_{\mathrm{e}\mathrm{x}\mathrm{t}}$to the layered superconductor and find three distinct regions. At weak screening, superconductivity is mediated by plasmons. In the opposite limit conventional electron–phonon interactions dominate. In between, we find a suppressed superconducting state. Our results show that even in conventional electron–phonon coupled layered materials, superconductivity can be significantly enhanced by the electron–plasmon coupling in a manner that can be controlled by the external screening.

    

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