Coulomb interaction effects in graphene bilayers: electron–hole pairing and plasmaron formation

Abstract

We report a theoretical study of the many-body effects of electron-electron interaction on the ground-state and spectral properties of double-layer graphene. Using a projector-based renormalization method we show that if a finite-voltage difference is applied between the graphene layers, electron-hole pairs can be formed and-at very low temperatures-an excitonic instability might emerge in a double-layer graphene structure. The single-particle spectral function near the Fermi surface exhibits a prominent quasiparticle peak, different from neutral (undoped) graphene bilayers. Away from the Fermi surface, we find that the charge carriers strongly interact with plasmons, thereby giving rise to a broad plasmaron peak in the angle-resolved photoemission spectrum. 2 1. MotivationGraphene-based structures are most likely the building blocks of future nanoelectronic devices. The reasons for this are manifold, but one may highlight that many of the exceptional properties of this new class of low-dimensional materials, which arise from the special form of the energy spectrum near the so-called Dirac nodal points and the related non-trivial topological structure of the wave function, can be easily modified by the application of external electric and magnetic fields, as well as by confining the sample geometry, by chemical doping or by edge functionalization and substrate manipulation; for recent reviews see [1][2][3].Single-layer graphene (SLG) [4,5], a truly two-dimensional (2D) crystal with remarkable mechanical properties, can be considered to be a gapless semiconductor with zero density of states at the Fermi level and a linear energy dispersion close to the (inequivalent) corners of the Brillouin zone (K, K Dirac points). Thus the low-energy electrons are massless, chiral Dirac fermions. As a consequence, any backscattering is suppressed and the charge carriers are almost insensitive to disorder and electron-electron interactions. Bilayer graphene (BLG) [6,7], consisting of two AB-stacked, chemically bound graphene monolayers, is also a zero-gap semiconductor, if unbiased, but with a parabolic band dispersion. That is, in this material the low-energy electrons acquire a finite quasiparticle mass. Interestingly, there exists another physical realization of a graphene-based double-layer structure that can be fabricated: in this system-usually referred to as a graphene bilayer or double-layer graphene (DLG)-the two graphene sheets are separated by a dielectric so that the tunneling between the layers can be neglected [8][9][10][11].In BLG and DLG, the charge carrier concentration can be controlled by the simple application of a gate voltage [7,12]. This electric field effect is fundamental for potential technological applications. For BLG even the band structure might be manipulated by an electric field, so that a gap between the valence and conduction bands varies between zero and midinfrared energies [13].BLG, besides being at present the only known semiconductor with a tunable band gap, is intriguing also from a many-p...

Fate of the excitonic insulator in the presence of phonons

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