Temperature- and density-dependent transport regimes in a h-BN/bilayer graphene/h-BN heterostructure

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=nc≈2.5×1011 cm−2 for both positive and negative carriers, separating two distinct regions with dρ/dT<0 and dρ/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>nc, while the T dependence close to the neutrality point is the one expected from the parabolic energy-momentum relation. At low temperature (T≪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.