Excitons

Correlated excitons in van der Waals magnets

Excitons in strongly correlated materials are not simple electron–hole pairs, but emergent quasiparticles whose properties are profoundly shaped by the surrounding spin and orbital degrees of freedom. In magnetic van der Waals materials, local excitations arising from Hund’s coupling or crystal-field transitions can propagate through the lattice by exchanging energy and momentum with magnons, giving rise to magnon-dressed quasiparticles with unconventional dispersion and dynamics. Because many of these excitations are spin-forbidden, they remain optically “dark” and inaccessible to conventional spectroscopies, yet they can be probed directly with resonant inelastic x-ray scattering (RIXS).

Using RIXS, we uncovered dispersive Hund’s excitons in the van der Waals antiferromagnet NiPS₃, where the exciton bandwidth provides a direct measure of the underlying superexchange interactions. We then mapped dispersive dark excitons in the van der Waals ferromagnet CrI₃ and in the low-symmetry magnet CrSBr, revealing strongly anisotropic exciton dynamics shaped by the interplay of magnetic exchange, spin–orbit coupling, and lattice anisotropy. Together, these results establish RIXS as a powerful probe of correlated excitonic physics and point to new opportunities for magneto-optical functionality in two-dimensional quantum materials.

References

2025

Observation of Anisotropic Dispersive Dark-Exciton Dynamics in CrSBr

J. Sears, B. Zager, W. He, C. A. Occhialini, Y. Shen, M. Lajer, J. W. Villanova, T. Berlijn, F. Yakhou-Harris, N. B. Brookes, D. G. Chica, X. Roy, E. Baldini, J. Pelliciari, V. Bisogni, S. Johnston, M. Mitrano, and M. P. M. Dean

Phys. Rev. Lett. 135, 146503 (2025)

DOI Bib PDF Supp

@article{sears2025observation,
  title = {Observation of Anisotropic Dispersive Dark-Exciton Dynamics in CrSBr},
  author = {Sears, J. and Zager, B. and He, W. and Occhialini, C. A. and Shen, Y. and Lajer, M. and Villanova, J. W. and Berlijn, T. and Yakhou-Harris, F. and Brookes, N. B. and Chica, D. G. and Roy, X. and Baldini, E. and Pelliciari, J. and Bisogni, V. and Johnston, S. and Mitrano, M. and Dean, M. P. M.},
  journal = {Phys. Rev. Lett.},
  volume = {135},
  issue = {14},
  pages = {146503},
  numpages = {9},
  year = {2025},
  month = oct,
  publisher = {American Physical Society},
  doi = {10.1103/fz3h-6jdx},
  url = {https://link.aps.org/doi/10.1103/fz3h-6jdx}
}

Dispersive Dark Excitons in van der Waals Ferromagnet CrI₃

W. He, J. Sears, F. Barantani, T. Kim, J. W. Villanova, T. Berlijn, M. Lajer, M. A. McGuire, J. Pelliciari, V. Bisogni, S. Johnston, E. Baldini, M. Mitrano, and M. P. M. Dean

Phys. Rev. X 15, 011042 (2025)

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Spin-flip dark excitons are optical-dipole-forbidden quasiparticles with remarkable potential in optoelectronics, especially when they are realized within cleavable van der Waals materials. Despite this potential, dark excitons have not yet been definitively identified in ferromagnetic van der Waals materials. Here, we report two dark excitons in a model ferromagnetic material CrI₃ using high-resolution resonant inelastic x-ray scattering and show that they feature narrower linewidths compared to the bright excitons previously reported in this material. These excitons are shown to have spin-flip character, to disperse as a function of momentum, and to change through the ferromagnetic transition temperature. Given the versatility of van der Waals materials, these excitons hold promise for new types of magneto-optical functionality.
@article{he2025dispersive, dimensions = {true}, title = {Dispersive Dark Excitons in van der Waals Ferromagnet CrI3}, author = {He, W. and Sears, J. and Barantani, F. and Kim, T. and Villanova, J. W. and Berlijn, T. and Lajer, M. and McGuire, M. A. and Pelliciari, J. and Bisogni, V. and Johnston, S. and Baldini, E. and Mitrano, M. and Dean, M. P. M.}, journal = {Phys. Rev. X}, volume = {15}, issue = {1}, pages = {011042}, numpages = {10}, year = {2025}, month = feb, publisher = {American Physical Society}, doi = {10.1103/PhysRevX.15.011042}, url = {https://link.aps.org/doi/10.1103/PhysRevX.15.011042} }

2024

Magnetically propagating Hund’s exciton in van der Waals antiferromagnet NiPS₃

W. He, Y. Shen, K. Wohlfeld, J. Sears, J. Li, J. Pelliciari, M. Walicki, S. Johnston, E. Baldini, V. Bisogni, M. Mitrano, and M. P. M. Dean

Nature Communications 15, 3496 (2024)

[BNL Press Release]

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Magnetic van der Waals (vdW) materials have opened new frontiers for realizing novel many-body phenomena. Recently NiPS3 has received intense interest since it hosts an excitonic quasiparticle whose properties appear to be intimately linked to the magnetic state of the lattice. Despite extensive studies, the electronic character, mobility, and magnetic interactions of the exciton remain unresolved. Here we address these issues by measuring NiPS3 with ultra-high energy resolution resonant inelastic x-ray scattering (RIXS). We find that Hund’s exchange interactions are primarily responsible for the energy of formation of the exciton. Measuring the dispersion of the Hund’s exciton reveals that it propagates in a way that is analogous to a double-magnon. We trace this unique behavior to fundamental similarities between the NiPS3 exciton hopping and spin exchange processes, underlining the unique magnetic characteristics of this novel quasiparticle.
@article{he2024magnetically, dimensions = {true}, author = {He, W. and Shen, Y. and Wohlfeld, K. and Sears, J. and Li, J. and Pelliciari, J. and Walicki, M. and Johnston, S. and Baldini, E. and Bisogni, V. and Mitrano, M. and Dean, M. P. M.}, title = {Magnetically propagating Hund's exciton in van der Waals antiferromagnet NiPS3}, journal = {Nature Communications}, year = {2024}, month = apr, day = {25}, volume = {15}, number = {1}, pages = {3496}, issn = {2041-1723}, doi = {10.1038/s41467-024-47852-x}, url = {https://doi.org/10.1038/s41467-024-47852-x}, note = {[<a href="https://www.bnl.gov/newsroom/news.php?a=221937" target="_blank">BNL Press Release</a>]} }
Exploring Quantum Materials with Resonant Inelastic X-Ray Scattering

M. Mitrano, S. Johnston, Young-June Kim, and M. P. M. Dean

Phys. Rev. X 14, 040501 (2024)

[BNL Press Release]

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Understanding quantum materials—solids in which interactions among constituent electrons yield a great variety of novel emergent quantum phenomena—is a forefront challenge in modern condensed matter physics. This goal has driven the invention and refinement of several experimental methods, which can spectroscopically determine the elementary excitations and correlation functions that determine material properties. Here we focus on the future experimental and theoretical trends of resonant inelastic x-ray scattering (RIXS), which is a remarkably versatile and rapidly growing technique for probing different charge, lattice, spin, and orbital excitations in quantum materials. We provide a forward-looking introduction to RIXS and outline how this technique is poised to deepen our insight into the nature of quantum materials and of their emergent electronic phenomena.
@article{mitrano2024exploring, dimensions = {true}, nosupp = {}, title = {Exploring Quantum Materials with Resonant Inelastic X-Ray Scattering}, author = {Mitrano, M. and Johnston, S. and Kim, Young-June and Dean, M. P. M.}, journal = {Phys. Rev. X}, volume = {14}, issue = {4}, pages = {040501}, numpages = {32}, year = {2024}, month = dec, publisher = {American Physical Society}, doi = {10.1103/PhysRevX.14.040501}, url = {https://link.aps.org/doi/10.1103/PhysRevX.14.040501}, note = {[<a href="https://www.bnl.gov/newsroom/news.php?a=222277" target="_blank">BNL Press Release</a>]} }