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

This work deal 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.Comment: 9 pages, 7 figures; new versio

Rotatory power reversal induced by magnetic-current in bi-isotropic media

Bi-isotropic media constitute a proper scenario for scrutinizing optical effects stemming from magnetoelectric parameters. Chiral magnetic current is a macroscopic effect arising from the chiral magnetic effect that enriches the phenomenology of a classical dielectric medium. This work examines optical aspects of bi-isotropic media in the presence of magnetic conductivity. The full isotropic scenario manifests circular birefringence described by a dispersive rotatory power that changes sign at a given frequency. For a bi-isotropic medium with antisymmetric magnetic conductivity, an intricate dispersive rotatory power is attained, supplied with sign reversal as well. This scenario also indicates a handedness reversion of the medium, an unusual property in dielectrics, which may work as a signature of bi-isotropic media supporting chiral magnetic current.Comment: 9 pages, 3 figure

Probing ferroelectricity in highly conducting materials through their elastic response: persistence of ferroelectricity in metallic BaTiO3-d

The question whether ferroelectricity (FE) may coexist with a metallic or highly conducting state, or rather it must be suppressed by the screening from the free charges, is the focus of a rapidly increasing number of theoretical studies and is finally receiving positive experimental responses. The issue is closely related to the thermoelectric and multiferroic (also magnetic) applications of FE materials, where the electrical conductivity is required or spurious. In these circumstances, the traditional methods for probing ferroelectricity are hampered or made totally ineffective by the free charges, which screen the polar response to an external electric field. This fact may explain why more than 40 years passed between the first proposals of FE metals and the present experimental and theoretical activity. The measurement of the elastic moduli, Young's modulus in the present case, versus temperature is an effective method for studying the influence of doping on a FE transition because the elastic properties are unaffected by electrical conductivity. In this manner, it is shown that the FE transitions of BaTiO3-d are not suppressed by electron doping through O vacancies; only the onset temperatures are depressed, but the magnitudes of the softenings, and hence of the piezoelectric activity, are initially even increased

Cold plasma waves in the chiral Maxwell-Carroll-Field-Jackiw electrodynamics

In this work, we study the propagation and absorption of plasma waves in the chiral Maxwell-Carroll-Field-Jackiw (MCJF) electrodynamics. The Maxwell equations are rewritten for a cold, uniform, and collisionless fluid plasma model, allowing us to determine the new refractive indices and propagating modes. The cases of propagation parallel and orthogonal to the magnetic field are examined considering a purely timelike CFJ background that plays the role of the magnetic conductivity chiral parameter. The collective electromagnetic modes are associated with four distinct refractive indices associated with right-circularly polarized and left-circularly polarized waves. For each index, the propagation and absorption zones are illustrated for some specific parameter values. In low-frequency regime, we have obtained modified helicons with right- and left-circularly polarizations. The optical behavior is investigated by means of the rotatory power (RP) and dichroism coefficient. The existence of a negative refraction zone enhances the rotatory power. It is also observed RP sign reversal, a feature of rotating plasmas.Comment: revtex style, two columns, 19 figures, 19 pages, version published in the PR

Curves and surfaces making a constant angle with a parallel transported direction in Riemannian spaces

In the last two decades, much effort has been dedicated to studying curves and surfaces according to their angle with a given direction. How- ever, most findings were obtained using a case-by-case approach, and it is often unclear what is a consequence of specificities of the ambient manifold and what could be generic. In this work, we propose a theo- retical framework to unify parts of these findings. We study curves and surfaces by prescribing the angle they make with a parallel transported vector field. We show that the characterization of Euclidean helices in terms of their curvature and torsion is also valid in any Riemannian manifold. Among other properties, we prove that surfaces making a con- stant angle with a parallel transported direction are extrinsically flat ruled surfaces. We also investigate the relation between their geodesics and the so-called slant helices. We prove that surfaces of constant angle are the rectifying surface of a slant helix, i.e., the ruled surface with rulings given by the Darboux field of the directrix. We characterize recti- fying surfaces of constant angle or, equivalently, when their geodesics are slant helices. As a corollary, we show that if every geodesic of a surface of constant angle is a slant helix, the ambient manifold is flat. Finally, we characterize surfaces in the product of a Riemannian surface with the real line making a constant angle with the vertical real direction