Relaxation of Plasma Carriers in Graphene: An Approach by Frequency‐Dependent Optical Conductivity Measurement

Abstract The relaxation time and mobility of electrons on graphene are deduced quantitatively by complex permittivity analysis with microwave dielectric loss spectroscopy. Hexagonal graphene sheets with uniform domain sizes from 6 to 30 µm, which target the mean free path of electrons on graphene based on the significantly longer turnover time of microwave probing at gigahertz (GHz) frequencies as compared to the relaxation time of electrons, are employed to determine the factors for carrier relaxation at the intradomain and grain boundaries. The changes in the complex permittivity by carrier injections clearly show an evidence of plasma carriers in graphene. This result validates the plasma carrier model with the restoring force in the GHz regime instead of the free carrier model, which is applied to analyze the optical conductivity over the terahertz (THz) regime. Based on Dirac massless fermions in graphene, the relaxation time of electrons is estimated to be 250–450 fs from the changes in complex permittivity, which are experimentally determined by frequency‐dependent microwave conductivity measurements. The obtained relaxation time shows a positive correlation with the domain size of graphene, implying that the scattering dominantly occurs at the domain boundary.