Monday, April 8, 2013

1304.1711 (Joaquín E. Drut et al.)

Fermi velocity renormalization and the excitonic insulator in graphene    [PDF]

Joaquín E. Drut, Timo A. Lähde
We compute the Fermi velocity of the Dirac quasiparticles in clean graphene at the charge neutrality point for strong Coulomb coupling alpha_g. We perform a Lattice Monte Carlo calculation within the low-energy Dirac theory, which includes an instantaneous, long-range Coulomb interaction. This method is non-perturbative and takes full account of quantum fluctuations. We find a finite renormalized Fermi velocity v_FR > v_F, where v_F = c/300. We introduce the critical Fermi velocity renormalization v_c = v_FR(alpha_gc)/v_F, where alpha_gc is the critical coupling for the semimetal-insulator transition due to excitonic pair formation. We compare our results with empirical studies of interaction-induced spectral changes in graphene. We find v_c = 3.3, which should be contrasted with v_FR/v_F = 2 - 3 for ultra-clean suspended graphene and v_FR/v_F = 1.2 for graphene on a boron nitride substrate. Our results are consistent with the non-observation of insulating states in suspended graphene in the absence of an external magnetic field. We also discuss the dynamical critical exponent z.
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