memoria de 2009 - Instituto de Estructura de la Materia - Consejo ...

temperature to check its divergence against the strength of the pairing instability, finding out that the latter prevails
throughout all the range of relevant values of the bare Coulomb repulsion and chemical potential about the Van
Hove singularity. Combined with the experimental side of the collaboration, our results have supported the idea
that doped graphene can achieve an electronically-mediated superconductivity, provided that the doping is in the
vicinity of the Van Hove singularity and the lattice symmetry is preserved, i.e. the chemical dopants do not
introduce new states near the singularity to disrupt the bandstructure.
On the other hand, we have studied the effects of strong curvature in graphene continuing with our investigation of
the carbon nanotube-graphene junctions. In this regard, it is quite promising that such hybrid structures have been
already produced in the Fujitsu laboratories, while it is also conceivable that graphene wormholes may be
fabricated starting from the easily available graphene bilayers. Our theoretical analysis has focused on wormhole
geometries in which a short nanotube acts as a bridge between two graphene sheets, where the honeycomb carbon
lattice is curved from the presence of 12 heptagonal defects. By taking nanotube bridges of very small length
compared to the radius, we have developed an effective theory of Dirac fermions to account for the low-energy
electronic properties of the wormholes in the continuum limit. In this construction, we have included appropriately
the effect that the heptagonal carbon rings induce on the Dirac fields encoding the low-energy electronic
excitations of the carbon material. This action has been mimicked by attaching a line of fictitious gauge flux at
each topological defect, following the same procedure applied long time ago in the case of the fullerene lattices.
The graphene wormholes represent actually an instance which can be considered to some extent dual to the case of
the fullerenes, as the 12 pentagonal carbon rings in those closed lattices play a role opposite to that of the
heptagonal defects in the wormhole. We have found in particular that, when the effective gauge flux from the
topological defects becomes maximal, the zero-energy modes of the Dirac equation can be arranged into two
triplets, which can be thought as the counterpart of the two triplets of zero modes that arise in the continuum limit
of large spherical fullerenes. We have further investigated the graphene wormhole spectra by performing a
numerical diagonalization of tight-binding Hamiltonians for very large lattices realizing the wormhole geometry.
In this way, we have shown the correspondence between the number of localized electronic states observed in the
numerical approach and the effective gauge flux predicted in the continuum limit. We have then concluded that
graphene wormholes can be consistently described by an effective theory of two Dirac fermion fields in the curved
geometry of the wormhole, opening the possibility of using real samples of the carbon material as a playground to
experiment with the interaction between the background curvature and the Dirac fields. It is therefore plausible
that the study of these condensed matter systems may allow the investigation of relevant gravitational effects
related to the Dirac character of the electron quasiparticles, which otherwise would be only accessible at the much
higher energies typical of the astrophysical phenomena.
2B.2 NUCLEAR PHYSICS AND STATISTICAL PHYSICS DEPARTMENT
RESEARCH LINES:
‣ Nuclear Structure studied in reactions with stable and exotic nuclei.
‣ Nuclear Structure studies done by beta-decay. Decay modes and breakup mechanisms of excited states.
‣ Spectroscopic studies of light nuclei close to the nuclear drip-lines. Characterization of the nuclear
structure by elastic and break-up reactions as well as in beta decay. R&D in detectors and electronics.
‣ Magnetic moment measurements in exotic nuclei using radioactive ion beams.
‣ Fundamental Nuclear Physics.
‣ Nuclear structure.
‣ Nuclear Structure and Reactions of Stable and Exotic Nuclei.
‣ Three-body systems in Nuclear Physics.
‣ Nuclear reactions involving few-body systems.
‣ Strongly correlated and mesoscopic systems.
‣ Quantum integrability and exactly solvable models.
‣ Systems devoid of long-range order.
‣ Advanced instrumentation.
RESEARCH SUBLINES:
Halo nuclei.
R&D for FAIR (Facility for Antiprotons and Ion Research).
Study of the nuclear break up mechanism by multiparticle detection.
Study of the structure and dynamics of halo nuclei.
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