Depression of the Normal-Superfluid Transition Temperature in Gated Bilayer Graphene

It is shown that the normal-superfluid transition in bilayer graphene predicted to occur at a high temperature is strongly affected not only by the dielectric constants of the substrate, interlayer, and gate insulators but also by the proximity of ideal metal gates. Even assuming optimistically a completely unscreened interlayer Coulomb interaction-thus bypassing the controversial problems regarding the proper way to screen the interlayer Coulomb interactions-it is shown that employing a gate-insulator thickness smaller than about 2-to-5 nm of equivalent SiO2-thickness pushes the transition temperature significantly below 300K to the 1 K-1mK range, depending on the dielectric constant of the gate insulator and on the dielectric mismatch of the insulators employed. These results imply that thicker and low-dielectric-constant gate insulators should be employed to observe the phase transition, but exploiting the superfluid state of gated graphene-bilayers in room-temperature device applications may be challenging.