Temperature dependance of the tunneling density of states in sub-micron planar metal/oxide/graphene junctions

We present the tunneling measurements of sub-micron metal/insulator/ graphene planar tunnel junctions up to room temperature. We observe a gate independent gap, as previously observed only by low temperature STM [Y. Zhang, Nat. Phys. 4, 627 (2008)]. No gap appears at temperatures above 150 K, which is four times smaller than the theoretically expected T_c, from the accepted mean field model [T. O. Wehling, Phys. Rev. Lett. 101, 216803 (2008)]. We show that taking into account an additional vibrational effect of out-of-plane phonon soft modes the gap may disappear from the measurements at temperatures much lower than the calculated T_c.