Plasmon losses due to electron-phonon scattering: the case of graphene encapsulated in hexagonal Boron Nitride

TL;DR

This paper develops a theoretical model for plasmon losses in graphene encapsulated in hexagonal Boron Nitride, highlighting acoustic phonon scattering as the main damping mechanism at room temperature.

Contribution

It introduces a detailed theory of hybrid plasmon-phonon modes in hBN/graphene/hBN stacks, emphasizing the dominant role of acoustic phonon scattering in plasmon damping.

Findings

Acoustic phonon scattering limits plasmon lifetime at room temperature.

Hybrid plasmon-phonon modes have inverse damping ratios of 50-60.

Plasmon lifetime can be anti-correlated with mobility.

Abstract

Graphene sheets encapsulated between hexagonal Boron Nitride (hBN) slabs display superb electronic properties due to very limited scattering from extrinsic disorder sources such as Coulomb impurities and corrugations. Such samples are therefore expected to be ideal platforms for highly-tunable low-loss plasmonics in a wide spectral range. In this Article we present a theory of collective electron density oscillations in a graphene sheet encapsulated between two hBN semi-infinite slabs (hBN/G/hBN). Graphene plasmons hybridize with hBN optical phonons forming hybrid plasmon-phonon (HPP) modes. We focus on scattering of these modes against graphene's acoustic phonons and hBN optical phonons, two sources of scattering that are expected to play a key role in hBN/G/hBN stacks. We find that at room temperature the scattering against graphene's acoustic phonons is the dominant limiting factor for hBN/G/hBN stacks, yielding theoretical inverse damping ratios of hybrid plasmon-phonon modes of the order of $50$-$60$, with a weak dependence on carrier density and a strong dependence on illumination frequency. We confirm that the plasmon lifetime is not directly correlated with the mobility: in fact, it can be anti-correlated.