We report on multiterminal electrical transport measurements performed on a bilayer graphene sheet enclosed by two hexagonal boron nitride flakes. We characterize the temperature dependence of electrical resistivity from 300 mK to 50 K, varying the carrier densities with a back gate. The resistivity curves clearly show a temperature-independent crossing point at density $n=n_c\approx 2.5\times {10}^{11}$ cm${}^{-2}$ for both positive and negative carriers, separating two distinct regions with $d\rho /dT<0$ and $d\rho /dT>0$, respectively. Our analysis rules out the possibility of a zero-T quantum phase transition, revealing instead the onset of robust ballistic transport for $n>n_c$, while the T dependence close to the neutrality point is the one expected from the parabolic energy-momentum relation. At low temperature ($T\ll 10$ K), the data are compatible with transport via variable range hopping mediated by localized impurity sites, with a characteristic exponent 1/3 that is renormalized to 1/2 by Coulomb interaction in the high-density regime.

• Received 12 November 2013
• Revised 21 January 2014

DOI:https://doi.org/10.1103/PhysRevB.89.121404