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

Abstract

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 $n={n}_{c}\ensuremath{\approx}2.5\ifmmode\times\else\texttimes\fi{}{10}^{11}$ cm${}^{\ensuremath{-}2}$ for both positive and negative carriers, separating two distinct regions with $d\ensuremath{\rho}/dT<0$ and $d\ensuremath{\rho}/dT>0$, 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 ($T\ensuremath{\ll}10$ 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.