Collective excitations in 2D materials

Abstract

Research on 2D materials has been one of the fastest-growing fields in condensed matter and materials science research in the past 10 years. The low dimensionality and strong correlations of 2D systems give rise to electronic and structural properties, in the form of collective excitations, that do not have counterparts in ordinary 3D materials used in modern technology. These 2D materials present extraordinary opportunities for new technologies, such as in flexible electronics. In this Review, we focus on plasmons, excitons, phonons and magnons in 2D materials. We discuss the theoretical formalism of these collective excitations and elucidate how they differ from their 3D counterparts.

Key points

  • Reduced screening in 2D materials leads to strong interactions and a breakdown of the single-particle picture.

  • 2D plasmons are soft modes with a tunable dispersion and a high confinement of the electric field.

  • Strong interaction produces various exciton-like quasiparticles in addition to the traditional excitons, such as biexcitons, trions and even quintons.

  • Phonons in 2D possess unique flexural modes not found in 3D materials.

  • Recent experimental advances have demonstrated 2D magnetism and topological excitations.

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Fig. 1: Experimental detection of plasmonic modes in graphene.
Fig. 2: Optically excited quasiparticles with the photoluminescence emission energies (ΔX, ΔT, ΔXX, ΔXT, ΔPT, ΔIX) indicated for each quasiparticle.
Fig. 3: Real-space representation of quasiparticle decomposition with the binding energies indicated for each quasiparticle.
Fig. 4: Relative positions of quasiparticle PL emission lines with the binding energies indicated.
Fig. 5: Emergence of new vibrational modes in multi-layer 2D materials.
Fig. 6: Magnon band structure in CrI3, a honeycomb 2D material.
Fig. 7: Chiral magnetic spin textures.

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Acknowledgements

This work was supported by the National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Programme and CRP award ‘Novel 2D materials with tailored properties: beyond graphene’ (grant no. R-144-000-295-281). A.R. acknowledges support by Yale-NUS College (through grant no. R-607-265-380-121). M.T. is supported by the Director’s Senior Research Fellowship from the Centre for Advanced 2D Materials.

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Glossary

Bessel and Struve functions

These are solutions of Bessel’s differential equation naturally occurring in problems with cylindrical symmetry.

Wannier–Mott picture

This is a model that assumes the exciton radius (or, in general, the mean electron–hole distance in exciton-like quasiparticles) to be much larger than the lattice constant, allowing for the effective mass description.

Fröhlich interactions

Interactions that couple electrons with phonons, assuming the continuum approximation and long-range forces.

Moiré potential

A potential resulting from a superposition of two periodic potentials with slightly different lattice constants or misalignment, resulting in a pattern with a much larger periodicity.

Ising model

A statistical model in which spin states at lattice sites are represented by discrete variables (‘up’ or ‘down’).

Heisenberg model

A statistical model in which the atomic spins are represented by vectors (classical Heisenberg model) or by spin or orbital angular momentum operators or their respective matrix representations (quantum Heisenberg model).

Dzyaloshinskii–Moriya interaction

A type of antisymmetric exchange interaction between neighbouring spins.

Domain walls

The boundaries between domains with different orientation of magnetization or other order parameter.

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Rodin, A., Trushin, M., Carvalho, A. et al. Collective excitations in 2D materials. Nat Rev Phys 2, 524–537 (2020). https://doi.org/10.1038/s42254-020-0214-4

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