Weak localization (WL) and weak antilocalization (WAL) of electrons in a disordered conductor refer to the decrease or increase of the conductivity caused by the quantum interference of the electronic motion. In graphene, the nontrivial Berry phase and the chiral nature of the charge carriers imply that both phenomena can be observed, as it has been firmly established in direct current (DC) magnetotransport experiments. However, studies of alternating current (AC) magnetoconductivity at frequencies comparable to, and much higher than, the dephasing rate (${\tau }_{\varphi }^{-1}$) remain unexplored. Using a combination of transport and spectroscopic measurements at low ($\omega /2\pi <20\mathrm{GHz}$) and high ($\omega /2\pi =90–350\mathrm{GHz}$) frequencies compared to the dephasing rate, we demonstrate a frequency dependence in the optical magnetoconductivity of graphene in the limit $\omega {\tau }_{\varphi }\gg 1$. We prove that spectroscopic measurements at low frequencies are well reproduced by the WL theory in the static limit ($\omega =0$), with no appreciable changes in the magnetoconductivity at different frequencies as long as the condition $\omega {\tau }_{\varphi }<1$ holds. At higher frequencies, the spectra exhibit transitions between WL and WAL induced solely by the external electromagnetic field, as they occur at constant temperature and carrier density. By analyzing the results with a WL model extended to finite frequencies, we observe that the usual decoherence mechanisms of inelastic scattering, in addition to the elastic processes of intervalley and long-range scattering, do not explain the observed behavior. Our results point to the need for a revised model for the decoherence dynamics in the limit $\omega {\tau }_{\varphi }\gg 1$.

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• Magneto-optics
• Magnetotransport
• Optical & microwave phenomena
• Weak antilocalization
• Weak localization