The quantum scattering time and its implications on scattering sources in graphene

We determine the quantum scattering time tau_q in six graphene samples with mobility of 4,400 < mu < 17,000 cm^2/Vs over a wide range of carrier density (1.2 < n < 6x10^{12}/cm^2). tau_q derived from Shubnikov-de Haas oscillations ranges ~25-74 fs, corresponding to a single-particle level broadening of 4.5-13 meV. The ratio of the transport to quantum scattering time tau_t/tau_q 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 SiO_2 surface play a dominant role in limiting carrier mobility in current samples.