Emergence of Interfacial Polarons from Electron–Phonon Coupling in Graphene/h-BN van der Waals Heterostructures

Abstract

van der Waals heterostructures, vertical stacks of layered materials, offer new opportunities for novel quantum phenomena which are absent in their constituent components. Here we report the emergence of polaron quasiparticles at the interface of graphene/hexagonal boron nitride (h-BN) heterostructures. Using nanospot angle-resolved photoemission spectroscopy, we observe zone-corner replicas of h-BN valence band maxima, with energy spacing coincident with the highest phonon energy of the heterostructure, an indication of Fröhlich polaron formation due to forward-scattering electron-phonon coupling. Parabolic fitting of the h-BN bands yields an effective mass enhancement of ∼2.3, suggesting an intermediate coupling strength. Our theoretical simulations based on Migdal-Eliashberg theory corroborate the experimental results, allowing the extraction of microscopic physical parameters. Moreover, renormalization of graphene π-band is observed due to the hybridization with the h-BN band. Our work generalizes the polaron study from transition metal oxides to van der Waals heterostructures with higher material flexibility, highlighting interlayer coupling as an extra degree of freedom to explore emergent phenomena.

Visualizing the Effect of an Electrostatic Gate with Angle-Resolved Photoemission Spectroscopy

“…This was solved with the advent of hexagonal boron nitride (hBN) as a supporting substrate. 8,9 The second challenge was to enhance the spatial resolution of ARPES to permit probing of micron-scale devices made with 2D materials.…”

Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3

“…First, the reduced dielectric screening in atomically thin samples enhances both the excitonic effect 7 and the electron-phonon (e-ph) coupling 8 , which is expected to promote the polaronic effect of excitons. Second, unlike bulk materials in which the e-ph coupling is largely determined by intrinsic electronic and phonon band structures with limited tunability, 2D materials provide greater flexibility for engineering e-ph coupling through a number of approaches including carrier doping 9,10 and interfacial coupling [11][12][13][14][15] , as well as dimensionality modulation 8 , and therefore hold high promise for the future development of optoelectronic devices.…”

Polarons Get the Full Treatment

“…The functional E p [ψ, { τ κα p }] defined by this equation constitutes the DFT counterpart of the Laundau-Pekar functional in Eq. (9). Also in this case we can take into account the normalization constraint on the wave function by introducing the Lagrange multiplier ε.…”

“…This feedback mechanism alters the energetics and dynamics of the charge carrier and may induce self-trapping. With the improvement in the energy and momentum resolution of angle-resolved photoelectron spectroscopy (ARPES), it has become possible to probe these quasiparticles in many systems of interest, from transition metal oxides [4][5][6][7] to two-dimensional materials [8][9][10]. These experiments and related theoretical investigations contributed to reinvigorating the interest in polaron physics [6,[11][12][13].…”