Confining and repulsive potentials from effective non-Abelian gauge fields in graphene bilayers

9 págs.; 6 figs.We investigate the effect of shear and strain in graphene bilayers, under conditions where the distortion of the lattice gives rise to a smooth one-dimensional modulation in the stacking sequence of the bilayer. We show that strain and shear produce characteristic Moiré patterns which can have the same visual appearance on a large scale, but representing graphene bilayers with quite different electronic properties. The different features in the low-energy electronic bands can be ascribed to the effect of a fictitious non-Abelian gauge field mimicking the smooth modulation of the stacking order. Strained and sheared bilayers show a complementary behavior, which can be understood from the fact that the non-Abelian gauge field acts as a repulsive interaction in the former, expelling the electron density away from the stacking domain walls, while behaving as a confining interaction leading to localization of the electronic states in the sheared bilayers. In this latter case, the presence of the effective gauge field explains the development of almost flat low-energy bands, resembling the form of the zeroth Landau level characteristic of a Dirac fermion field. The estimate of the gauge field strength in those systems gives a magnitude of the order of several tens of tesla, implying a robust phenomenology that should be susceptible of being observed in suitably distorted bilayer samples. ©2016 American Physical SocietyWe acknowledge financial support from MINECO (Spain) through Grant No. FIS2014-57432-P.Peer Reviewe

Excitonic and marginal Fermi liquid instabilities in 2D and 3D Dirac semimetals

San Antonio, Texas, March 2 - 6 2015We study the quantum electrodynamics of 2D and 3D Dirac semimetals by means of a self-consistent resolution of the Schwinger-Dyson equations, aiming to obtain the respective phase diagrams in terms of the relative strength of the Coulomb interaction and the number N of Dirac fermions. In this framework, 2D Dirac semimetals have just a strong-coupling instability characterized by exciton condensation (and dynamical generation of mass) that we find at a critical coupling well above the estimates made with RPA screening (large-N approximation), thus explaining the absence of that instability in free-standing graphene samples. On the other hand, we show that 3D Dirac semimetals have a richer phase diagram, with a strong-coupling instability leading to dynamical mass generation up to N = 4 and a line of critical points for larger values of N characterized by the vanishing of the electron quasiparticle weight in the low-energy limit. Such a marginal Fermi liquid boundary marks the transition to a kind of strange metal that can still be defined in terms of electron quasiparticles, but with parameters that have large imaginary parts implying an increasing deviation at strong coupling from the conventional Fermi liquid picture.Peer Reviewe

La revolución de la física del carbono: fullerenos, nanotubos, grafeno

24 págs. ; XI Semana de la Ciencia de Madrid, 2011El carbono es el elemento químico más estudiado, es el elemento central de la biología y la  medicina, y también fundamental en la producción de energía y conservación del medio.Peer reviewe

Charge instabilities near a Van Hove singularity

11 págs.; 13 figs.; PACS number(s): 71.10.Fd, 71.27.1aThe charge instabilities of electron systems in the square lattice are analyzed near the Van Hove singularity by means of a Wilsonian renormalization group approach. We show that the method preserves the spin rotational invariance at all scales, allowing a rigorous determination of spin and charge instabilities of the t-t′ Hubbard model. Regarding the latter, repulsive interactions fall into two different universality classes. One of them has nonsingular response functions in the charge sector, while the other is characterized by the splitting of the Van Hove singularity. At the level of marginal perturbations, the Hubbard model turns out to be at the boundary between the two universality classes, while extended models with nearest-neighbor repulsive interactions belong to the latter class. In the case of open systems allowed to exchange particles with a reservoir, we show the existence of a range of fillings forbidden above and below the Van Hove singularity. This has the property of attracting the Fermi level in the mentioned range, as the system reaches its lowest energy when the Fermi energy is at the singularity. ©2001 American Physical SocietyPeer Reviewe

Localization of electronic states by fullerene charges in carbon nanotubes

4 págs.; 3 figs. ; PACS numberssd: 73.22.2f, 71.10.PmWe study the effects of the electrostatic interaction produced by charged metallofullerenes encapsulated in carbon nanotubes, showing that they are able to modify locally the electronic density of states in the hybrid system. In the cases where the interaction is felt as an attractive potential by the electrons in the nanotube, localized electronic states are formed in the nanotubes around the position of the fullerenes. This produces an effective narrowing of the gap in semiconducting nanotubes over a distance of a few nanometers, in agreement with the spatial modulation of the gap observed in the experiments. © 2005 The American Physical Society.The financial support of the Ministerio de Educación y Ciencias (Spain) through Grants No. MAT2002-0495-C02-01 sfor F.G.d and No. BFM2003-05317 for J.G. is gratefully acknowledged.Peer Reviewe