Thursday, May 23, 2013

1305.4936 (D. S. L. Abergel et al.)

Inter-layer excitonic superfluidity in graphene    [PDF]

D. S. L. Abergel, M. Rodriguez-Vega, Enrico Rossi, S. Das Sarma
We describe the conditions under which the predicted (but not yet observed) zero-field inter-layer excitonic condensation in double layer graphene has a critical temperature high enough to allow detection. Both double monolayer and double bilayer graphene are considered in our theory. We present the calculation of the critical temperature for the inter-layer excitonic superfluid transition within the mean-field BCS theory for both optimistic and conservative approximations for the screening of the inter-layer Coulomb interaction, and discuss how disorder arising from charged impurities and corrugation in the lattice structure affect formation of the condensate via the charge inhomogeneity they induce. In the former case, we use a numerical Thomas-Fermi-Dirac theory to describe the local fluctuations in the electronic density in double layer graphene devices and estimate the effect these realistic fluctuations have on the formation of the condensate. We also estimate the effect of allowing dynamic contributions to the inter-layer screening. We then conduct similar calculations for double quadratic bilayer graphene, showing that the quadratic nature of the low-energy bands produces only a small qualitative change in the pairing strength. We compare the inter-layer interaction potential for these two systems and find that, since screening is stronger in quadratic bilayer graphene, it is probably not advantageous to search for the condensate in this system. We believe that disorder-free closely spaced double layers of monolayer graphene provide the most likely system for excitonic inter-layer superfluidity to manifest itself.
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