Strong magnetophonon oscillations in extra-large graphene

Abstract

Van der Waals materials and their heterostructures offer a versatile platform for studying a variety of quantum transport phenomena due to their unique crystalline properties and the exceptional ability in tuning their electronic spectrum. However, most experiments are limited to devices that have lateral dimensions of only a few micrometres. Here, we perform magnetotransport measurements on graphene/hexagonal boron-nitride Hall bars and show that wider devices reveal additional quantum effects. In devices wider than ten micrometres we observe distinct magnetoresistance oscillations that are caused by resonant scattering of Landau-quantised Dirac electrons by acoustic phonons in graphene. The study allows us to accurately determine graphene’s low energy phonon dispersion curves and shows that transverse acoustic modes cause most of phonon scattering. Our work highlights the crucial importance of device width when probing quantum effects and also demonstrates a precise, spectroscopic method for studying electron-phonon interactions in van der Waals heterostructures.

Magnetophonon spectroscopy of Dirac fermion scattering by transverse and longitudinal acoustic phonons in graphene

“…This relation describes accurately the data shown in Fig. 1(b) and described in Kumaravadivel et al 11 . The measured magneto-oscillations in Fig.…”

“…We associate this peak with MPR due to LA phonons with speed v LA and we obtain v LA /v F = 0.0198. With a Fermi velocity of v F = 1.06 ± 0.05 × 10 6 ms −1 extracted from the temperature-dependent Shubnikov de Haas measurements on the devices reported by Kumaravadivel et al 11 , we obtain v T A = 13.6 ± 0.7 kms −1 and v LA = 21 ± 1 kms −1 . These values are in good agreement with calculations of the speeds of linearly dispersed acoustic phonons in graphene [17][18][19] .…”

“…Curves in (a) are offset by 0.75 Ω for clarity. tance oscillations that were observed recently in wide, gated Hall bars of monolayer graphene encapsulated in hexagonal boron nitride (hBN) 11 . The spectroscopic nature of MPR complements the extensive literature on the effects of electron-phonon interactions on the carrier mobility of graphene [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] of phonon-assisted tunnelling in stacked graphene-hBNgraphene devices 28,29 .…”

“…The measurements in Fig. 1(b), were made with a negative gate voltage applied to the device so that the charge carriers are holes with Fermi energy, E F , positioned below the Dirac point of graphene's band structure 11 . Our analysis considers the case when E F is above the Dirac point; electron-hole symmetry close to the Dirac point in graphene ensures that it is applicable to both types of charge carrier.…”

Bulk-boundary quantum oscillations in inhomogeneous Weyl semimetals

“…We note that the two sets of oscillations should be distinguishable in the experiment, similar to the purely bulk and bulk-boundary sets of oscillations in [25]. We also notice that the movement of one set of the oscillations with applied strain would help to clearly distinguish their bulk-boundary nature from the bulk set of quantum oscillation or more exotic phenomena like magnetophonon oscillations [42,43].…”