Circular dichroism of magnetophonon resonance in doped graphene

The polarization-resolved Raman-scattering response due to $E_{2g}$ phonons in monolayer graphene has been investigated in magnetic fields up to 29 T. The hybridization of the $E_{2g}$ phonon is only observed with the fundamental inter-Landau-level excitation (involving the $n=0$ Landau level) and in just one of the two configurations of the circularly cross-polarized excitation and scattered light. This polarization anisotropy of the magnetophonon resonance is shown to be inherent to relatively strongly doped graphene samples with carrier concentrations typical for graphene deposited on ${\mathrm{Si}/\mathrm{SiO}}_2$ substrates.

Figure 1

Raman-scattering spectra measured at a temperature of $T=4.2\mathrm{K}$ and under a 488-nm laser line excitation, showing the characteristic G and 2D bands of the graphene investigated. The inset shows the optical image of the sample.

Figure 2

Typical Raman-scattering spectra of the $G$-band feature in the two crossed circular polarization configurations (the dashed line corresponds to ${\sigma }^{+}/{\sigma }^{-}$) at various values of the magnetic field. Spectra are shifted for clarity.

Figure 3

Three regimes of the hole gas concentration and its impact on the magnetophonon effect. Left panels represent fan charts of the Landau levels with marked excitations being in resonance with the $E_{2g}$ phonon [panels (a) and (b)]. The solid arrows represent excitations in ${\sigma }^{-}/{\sigma }^{+}$ polarization, and the dashed arrows represent excitations in ${\sigma }^{+}/{\sigma }^{-}$ polarization. The thick line shows the Fermi level. Arrows in panel (c) represent nonresonant excitations (inter- and intrabands). Right panels show the evolution of the phonon energy versus the magnetic field with the same representation of polarizations by solid/dashed lines.

Figure 4

Simulation of the Raman spectra versus the magnetic field in two crossed circular polarization configurations for different values of the magnetic field.