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  • Open Access

Collective Modes in Multilayer Graphene/α−RuCl3 Heterostructures

Samuel L. Moore1,*, Miguel Sánchez Sánchez2, M. C. Strasbourg3, Y. Shao4, J. Pack1, Y. Wang5, D. J. Rizzo1, B. S. Jessen6, Matthew Cothrine7 et al.

David G. Mandrus7,8, Takashi Taniguchi9, Kenji Watanabe10, K. S. Burch11, C. R. Dean1, J. Hone3, M. Fogler12, A. J. Millis1, A. Rubio13, P. J. Schuck3, T. Stauber2, and D. N. Basov1

  • *Contact author: slm2236@columbia.edu
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Phys. Rev. X 15, 041011 – Published 21 October, 2025

DOI: https://doi.org/10.1103/rp3q-svws

Abstract

Collective modes in multilayer graphene, such as plasmons and phonons, exhibit sensitivity to displacement fields and interlayer coupling, distinguishing them from their counterparts in single-layer graphene. Here, we engineer collective modes in charge-transfer heterostructures composed of multilayer graphene and α−RuCl3. In heterostructures with a single α−RuCl3 interface, the charge transfer generates displacement fields up to 7 V/nm at the interface between α−RuCl3 and the adjacent graphene layer—the highest value achieved through charge-transfer methods. As a result of the broken inversion symmetry, we discover enhanced nonlinear optical response and modified phonon selection rules. Conversely, we find that multilayer graphene sandwiched between two α−RuCl3 flakes causes displacement fields to cancel. There, we achieve carrier densities as high as 8×1013  cm−2 in multilayer graphene and restore the phonon selection rules to their unperturbed state. Meanwhile, we demonstrate that plasmonic properties derive from the depletion of multiple valence bands. As a result of the quasilinear band dispersion, these “Dirac multiband plasmons” are relatively unaffected by displacement fields. On the other hand, the inverted heterostructure sequence—two multilayer graphene sheets encapsulating α−RuCl3—activates significant alteration of the plasmons via interlayer plasmon-plasmon coupling. Hence, multilayer graphene and α−RuCl3 heterostructures offer a gate-free platform for engineering collective modes derived from inversion symmetry and interlayer coupling.

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Physics Subject Headings (PhySH)

Corrections

3 December, 2025

Correction: A funding statement was missing from the Acknowledgments and has been inserted.

Popular Summary

Collective modes—coordinated oscillations of many interacting particles—are key to understanding how complex materials behave. Examples include plasmons—oscillations of free electrons responding to electromagnetic fields—and phonons—atomic vibrations in a solid that propagate as waves. While single-layer graphene has been studied extensively in this context, multilayer graphene is less explored. Here, we investigate how plasmons and phonons in multilayer graphene respond under extreme conditions of ultrahigh doping and strong electric fields.

We find that phonon modes become highly sensitive to electric fields, a result of inversion symmetry breaking. Meanwhile, plasmons remain largely unaffected, akin to what is observed in single-layer graphene. However, when the distance between two graphene layers aligns with the plasmon oscillation wavelength, we see a strong coupling effect where the plasmon mode splits in two and the wavelength shrinks dramatically.

Typically, charge-carrier density is controlled via a gate electrode. We instead utilize a gate-free method rooted in interfacial charge transfer from RuCl3. This approach provides much higher carrier densities and lets us tune electric fields and charge distributions with precision. It also permits optical interrogation with a suite of experimental tools. We employ second-harmonic generation to track symmetry changes, Raman spectroscopy to monitor phonons, and near-field nanoscopy to visualize ultrashort plasmon wavelengths. We also use theoretical models to estimate carrier concentrations and electric field strengths in our samples.

Our results show that this gate-free, highly tunable platform opens new pathways for studying collective modes in multilayer graphene. Looking ahead, we plan to explore how other exotic states of matter—such as excitons and superconducting Cooper pairs—behave in this regime, especially at low temperatures.

Article Text

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Outline

Information

Phys. Rev. X 15, 041011– Published 21 October, 2025
  • Received 5 December 2024
  • Revised 13 May 2025
  • Accepted 23 July 2025
  • Corrected 3 December 2025
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