Interaction-Enhanced Coherence Between Two-Dimensional Dirac Layers

Abstract

We estimate the strength of interaction-enhanced coherence between two graphene or topological insulator surface-state layers by solving imaginary-axis gap equations in the random phase approximation. Using a self-consistent treatment of dynamic screening of Coulomb interactions in the gapped phase, we show that the excitonic gap can reach values on the order of the Fermi energy at strong interactions. The gap is discontinuous as a function of interlayer separation and effective fine structure constant, revealing a first order phase transition between effectively incoherent and interlayer coherent phases. To achieve the regime of strong coherence the interlayer separation must be smaller than the Fermi wavelength, and the extrinsic screening of the medium embedding the Dirac layers must be negligible. In the case of a graphene double-layer we comment on the supportive role of the remote $\pi$-bands neglected in the two-band Dirac model.

14 pages, 9 figures

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• Excitonic condensation in spatially separated one-dimensional systems
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• Photoemission spectra of massless Dirac fermions on the verge of exciton condensation
• Phases of the excitonic condensate in two-layer graphene
• Diamagnetism and suppression of screening as hallmarks of electron-hole pairing in a double layer graphene system
• Superfluidity of a rarefied gas of electron-hole pairs in a bilayer system
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• Stationary waves in a superfluid gas of electron-hole pairs in bilayers
• Strong enhancement of third harmonic generation in a double layer graphene system caused by electron-hole pairing
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• Density of states in the bilayer graphene with the excitonic pairing interaction