A. J. da Silva

A Consistent noncommutative field theory: The Wess-Zumino model

Abstract We show that the noncommutative Wess–Zumino model is renormalizable to all orders of perturbation theory. The noncommutative scalar potential by itself is non-renormalizable but the Yukawa terms demanded by supersymmetry improve the situation turning the theory into a renormalizable one. As in the commutative case, there are neither quadratic nor linear divergences. Hence, the IR/UV mixing does not give rise to quadratic infrared poles.

Stability investigation and thermal behavior of a hypothetical silicon nanotube

Abstract Even though silicon nanotubes have never been observed, this paper attempts to establish the theoretical similarities and differences between Si and C structures. Through the use of two alternative theoretical approaches, the first principles calculations and empirical potential, the electronic and structural properties of this hypothetical material are examined. The first principles calculations are based on the density-functional theory and it is shown that depending on their chiralities and diameters, the silicon nanotubes may present metallic (armchair) or semiconductor (zigzag and mixed) behaviors, similar to carbon structures. It is shown that the gap decreases in inverse proportion to the diameter, thus approaching zero for planar graphite, as was expected. In the second alternative approach, the Monte Carlo simulations are used with the Tersoffs empirical potential to present a systematic study on the thermal behavior of these new structures. It is shown that similarities like band structures and density of states are observed between the C and Si nanotubes. Nevertheless, there are relevant discrepancies in the thermal stabilities and energy differences between the cohesive energies per atom for the two tubes, compared with the corresponding bulks, implying the very improbable structure of the silicon nanotubes.

Braneworld solutions for F(R) models with non-constant curvature

Abstract This work deals with braneworld scenarios with generalized gravity. We investigate models where the potential of the scalar field is polynomial or nonpolynomial. We obtain exact and approximated solutions for the scalar field, warp factor and energy density, in the complex scenario with no restriction on the scalar curvature. In particular, we describe the case where the brane may split, engendering internal structure, with the splitting caused by the same parameter that controls deviation from standard gravity.

THE NONCOMMUTATIVE SUPERSYMMETRIC NONLINEAR SIGMA MODEL

We show that the noncommutativity of space–time destroys the renormalizability of the 1/N expansion of the O(N) Gross–Neveu model. A similar statement holds for the noncommutative nonlinear sigma model. However, we show that, up to the subleading order in 1/N expansion, the noncommutative supersymmetric O(N) nonlinear sigma model becomes renormalizable in D=3. We also show that dynamical mass generation is restored and there is no catastrophic UV/IR mixing. Unlike the commutative case, we find that the Lagrange multiplier fields, which enforce the supersymmetric constraints, are also renormalized. For D=2 the divergence of the four-point function of the basic scalar field, which in D=3 is absent, cannot be eliminated by means of a counterterm having the structure of a Moyal product.

On the equivalence of the self-dual and Maxwell-Chern-Simons models coupled to fermions

Abstract We study the exact equivalence between the self-dual model minimally coupled with a Dirac field and the Maxwell-Chern-Simons model with non-minimal magnetic coupling to fermions. We show that the fermion sectors of the models are equivalent only if a Thirring like interaction is included. Using functional methods we verify that, up to renormalizations, the equivalence persists at the quantum level.

Supergauge theories in aether superspace

Within the superfield method we extend the formulation of the Lorentz-breaking aether superspace for supergauge theories, both in three- and in four-dimensional cases.

Energetics and stability of vacancies in carbon nanotubes

In this work we present ab initio calculations of the formation energies and stability of different types of multi-vacancies in carbon nanotubes. We demonstrate that, as in the case of graphene, the reconstruction of the defects has drastic effects on the energetics of the tubes. In particular, the formation of pentagons eliminates the dangling bonds thus lowering the formation energy. This competition leads to vacancies having an even number of carbon atoms removed to be more stable. Finally the appearance of magic numbers indicating more stable defects can be represented by a model for the formation energies that is based on the number of dangling bonds of the unreconstructed system, the pentagons and the relaxation of the final form of the defect formed after the relaxation.

Lorentz violation in the linearized gravity

Abstract We study some physical consequences of the introduction of a Lorentz-violating modification term in the linearized gravity, which leads to modified dispersion relations for gravitational waves in the vacuum. We discuss two possible mechanisms for the induction of such a term in the Lagrangian. First, it is generated at the quantum level by a Lorentz-breaking coupling of the gravity field to a spinor field. Second, it appears as consequence of a particular modification of the Poisson algebra of the canonical variables, in the spirit of the so-called “noncommutative fields approach”.

Short linear atomic chains in copper nanowires

We have performed realistic molecular dynamics simulations of copper nanowires (NWs) under stress along some crystallographic directions until their rupture to help understand the properties of these NWs at an atomistic level. We compare the structural arrangement during the elongation with the dynamical evolution of copper NWs observed in high resolution transmission electron microscopy (HRTEM) experiments, and they are in good agreement. Finally, we report the formation of short linear atomic chains (LACs) before breaking that occurs in all cases indicating the possibility to use copper as metallic nanocontacts.

Lorentz violation bounds on Bhabha scattering

We investigate the effect of Lorentz-violating terms on Bhabha scattering in two distinct cases correspondent to vectorial and axial nonminimal couplings in QED. In both cases, we find significant modifications with respect to the usual relativistic result. Our results reveal an anisotropy of the differential cross section which imply new constraints on the possible Lorentz violating terms.