Quantum scattering time and its implications on scattering sources in graphene

We determine the quantum scattering time τq in six graphene samples with mobility of 4400<μ<17000 cm2 /Vs over a wide range of carrier density (1.2<n<6× 1012 / cm2). τq derived from Shubnikov-de Haas oscillation ranges ∼25-74fs, corresponding to a single-particle level broadening of 4.5-13 meV. The ratio of the transport to quantum scattering time τt / τq spans 1.5-5.1 in these samples, which can be quantitatively understood combining scattering from short-ranged centers and charged impurities located within 2 nm of the graphene sheet. Our results suggest that charges residing on the SiO2 surface play a dominant role in limiting carrier mobility in current samples.