arXiv subjects

C. P. Malta

Publications and source records attributed to C. P. Malta.

10 recordsLinked to original sources

arXiv2007

Is it possible to grow amorphous normal nanosprings ?

Nanosprings have been object of intense investigations in recent years. They can be classified as normal or binormal depending on the geometry of their cross-section. Normal amorphous nanosprings have not been observed experimentally up to now, and only recently the synthesis of a crystalline ZnO normal nanohelix has been reported. We discuss the shape of the catalyst in terms of the cross-sectional shape of the nanospring, and show that, within the vapor-liquid-solid model, the growth of amorphous normal nanospring is not energetically favoured.

cond-mat.mtrl-sci↗

arXiv2006

Mechanical properties of amorphous nanosprings

Helical amorphous nanosprings have attracted particular interest due to their special mechanical properties. In this work we present a simple model, within the framework of the Kirchhoff rod model, for investigating the structural properties of nanosprings having asymmetric cross section. We have derived expressions that can be used to obtain the Young's modulus and Poisson's ratio of the nanospring material composite. We also address the importance of the presence of a catalyst in the growth process of amorphous nanosprings in terms of the stability of helical rods.

cond-mat.mtrl-sci↗

arXiv2006

Circular and helical equilibrium solutions of inhomogeneous rods

Real filaments are not perfectly homogeneous. Most of them have various materials composition and shapes making their stiffnesses not constant along the arclength. We investigate the existence of circular and helical equilibrium solutions of an intrinsically straight rod with varying bending and twisting stiffnesses, within the framework of the Kirchhoff model. The planar ring equilibrium solution only exists for a rod with a given form of variation of the bending stiffness. We show that the well known circular helix is not an equilibrium solution of the static Kirchhoff equations for a rod with non constant bending stiffness. Our results may provide an explanation for the variation of the curvature seen in small closed DNAs immersed in a solution containing Zn^{2+}, and in the DNA wrapped around a nucleosome.

physics.class-ph↗

arXiv2006

Elastic Properties of Nanowires

We present a model to study Young's modulus and Poisson's ratio of the composite material of amorphous nanowires. It is an extension of the model derived by two of us [Da Fonseca and Galvao, Phys. Rev. Lett. 92, 175502 (2004)] to study the elastic properties of amorphous nanosprings. The model is based on twisting and tensioning a straight nanowire and we propose an experimental setup to obtain the elastic parameters of the nanowire. We used the Kirchhoff rod model to obtain the expressions for the elastic constants of the nanowire.

cond-mat.mtrl-sci↗

arXiv2005

Lancret helices

Helical configurations of inhomogeneous symmetric rods with non-constant bending and twisting stiffness are studied within the framework of the Kirchhoff rod model. From the static Kirchhoff equations, we obtain a set of differential equations for the curvature and torsion of the centerline of the rod and the Lancret's theorem is used to find helical solutions. We obtain a free standing helical solution for an inhomogeneous rod whose curvature and torsion depend on the form of variation of the bending coefficient along the rod. These results are obtained for inhomogeneous rods without intrinsic curvature, and for a particular case of intrinsic curvature.

physics.bio-ph↗

arXiv2004

The Schrodinger picture and the zero-point radiation

Dalibard, Dupont-Roc and Cohen-Tannoudji (J. Physique 43 (1982) 1617; 45 (1984) 637) used the Heisenberg picture to show that the atomic transitions, and the stability of the ground state, can only be explained by introducing radiation reaction and vacuum fluctuation forces. Here we consider the simple case of nonrelativistic charged harmonic oscillator, in one dimension, to investigate how to take into account the radiation reaction and vacuum fluctuation forces within the Schrodinger picture. We consider classical vacuum fields and large mass oscillator.

quant-ph↗

arXiv2004

Resonant helical deformations in nonhomogeneous Kirchhoff filaments

We study the three-dimensional static configurations of nonhomogeneous Kirchhoff filaments with periodically varying Young's modulus. This type of variation may occur in long tandemly repeated sequences of DNA. We analyse the effects of the Young's modulus frequence and amplitude of oscillation in the stroboscopic maps, and in the regular (non chaotic) spatial configurations of the filaments. Our analysis shows that the tridimensional conformations of long filaments may depend critically on the Young's modulus frequence in case of resonance with other natural frequencies of the filament. As expected, far from resonance the shape of the solutions remain very close to that of the homogeneous case. In the case of biomolecules, it is well known that various other elements, besides sequence-dependent effects, combine to determine their conformation, like self-contact, salt concentration, thermal fluctuations, anisotropy and interaction with proteins. Our results show that sequence-dependent effects alone may have a significant influence on the shape of these molecules, including DNA. This could, therefore, be a possible mechanical function of the ``junk'' sequences.

physics.bio-ph↗

arXiv2003

Helical filaments with varying cross section radius

The tridimensional configuration and the twist density of helical rods with varying cross section radius are studied within the framework of the Kirchhoff rod model. It is shown that the twist density increases when the cross section radius decreases. Some tridimensional configurations of helix-like rods are displayed showing the effects of the nonhomogeneity considered here. Since the helix-like solutions of the nonhomogeneous rods do not present constant curvature and torsion a set of differential equations for these quantities is presented. We discuss the results and possible consequences.

physics.bio-ph↗

arXiv2002

Effect of symmetry breaking on level curvature distributions

We derive an exact general formalism that expresses the eigenvector and the eigenvalue dynamics as a set of coupled equations of motion in terms of the matrix elements dynamics. Combined with an appropriate model Hamiltonian, these equations are used to investigate the effect of the presence of a discrete symmetry in the level curvature distribution. An explanation of the unexpected behavior of the data regarding frequencies of acoustic vibrations of quartz block is provided.

quant-ph↗

arXiv1995

Maxwell Electromagnetic Theory, Planck's Radiation Law and Bose-Einstein Statistics

We give an example in which it is possible to understand quantum statistics using classical concepts. This is done by studying the interaction of charged matter oscillators with the thermal and zeropoint electromagnetic fields characteristic of quantum electrodynamics and classical stochastic electrodynamics. Planck's formula for the spectral distribution and the elements of energy $ \hbarω$ are interpreted without resorting to discontinuities. We also show the aspects in which our model calculation complement other derivations of blackbody radiation spectrum without quantum assumptions.

quant-ph↗