Shot noise and conductivity at high bias in bilayer graphene: Signatures of electron-optical phonon coupling

We have studied electronic conductivity and shot noise of bilayer graphene (BLG) sheets at high bias voltages and low bath temperature T-0 = 4.2 K. As a function of bias, we find initially an increase of the differential conductivity, which we attribute to self-heating. At higher bias, the conductivity saturates and even decreases due to backscattering from optical phonons. The electron-phonon interactions are also responsible for the decay of the Fano factor at bias voltages V > 0.1 V. The high bias electronic temperature has been calculated from shot-noise measurements, and it goes up to similar to 1200 K at V = 0.75 V. Using the theoretical temperature dependence of BLG conductivity, we extract an effective electron-optical phonon scattering time tau(e-op). In a 230-nm-long BLG sample of mobility mu = 3600 cm(2) V-1 s(-1), we find that tau(e-op) decreases with increasing voltage and is close to the charged impurity scattering time tau(imp) = 60 fs at V = 0.6 V.