Origin of translocation barriers for polyelectrolyte chains

For single-file translocations of a charged macromolecule through a narrow pore, the crucial step of arrival of an end at the pore suffers from free energy barriers, arising from changes in intrachain electrostatic interaction, distribution of ionic clouds and solvent molecules, and conformational entropy of the chain. All contributing factors to the barrier in the initial stage of translocation are evaluated by using the self-consistent field theory for the polyelectrolyte and the coupled Poisson-Boltzmann description for ions, without radial symmetry. The barrier is found to be essentially entropic, due to conformational changes. For moderate and high salt concentrations, the barriers for the polyelectrolyte chain are quantitatively equivalent to that of uncharged self-avoiding walks. Electrostatic effects are shown to increase the free energy barriers, but only slightly. The degree of ionization, electrostatic interaction strength, decreasing salt concentration and the solvent quality all result in increases in the barrier.Comment: J.Chem. Phys. 131, 21 (2009) - to be appeare

Elliptical flow and isospin effects in heavy-ion collisions at intermediate energies

The elliptical flow of fragments is studied for different systems at incident energies between 50 and 1000 MeV/nucleon using the isospin-dependent quantum molecular dynamics (IQMD) model. Our findings reveal that elliptical flow shows a transition from positive (in-plane) to negative (out-of-plane) values in the midrapidity region at a certain incident energy known as the transition energy. This transition energy is found to depend on the model ingredients, size of the fragments, and composite mass of the reacting system as well as on the impact parameter of the reaction. A reasonable agreement is observed for the excitation function of elliptical flow between the data and our calculations. Interestingly, the transition energy is found to exhibit a power-law mass dependence.Comment: 16 pages, 8 figure

Generation of scale invariant magnetic fields in bouncing universes

We consider the generation of primordial magnetic fields in a class of bouncing models when the electromagnetic action is coupled non-minimally to a scalar field that, say, drives the background evolution. For scale factors that have the power law form at very early times and non-minimal couplings which are simple powers of the scale factor, one can easily show that scale invariant spectra for the magnetic field can arise before the bounce for certain values of the indices involved. It will be interesting to examine if these power spectra retain their shape after the bounce. However, analytical solutions for the Fourier modes of the electromagnetic vector potential across the bounce are difficult to obtain. In this work, with the help of a new time variable that we introduce, which we refer to as the ${\rm e}$-${\cal N}$-fold, we investigate these scenarios numerically. Imposing the initial conditions on the modes in the contracting phase, we numerically evolve the modes across the bounce and evaluate the spectra of the electric and magnetic fields at a suitable time after the bounce. As one could have intuitively expected, though the complete spectra depend on the details of the bounce, we find that, under the original conditions, scale invariant spectra of the magnetic fields do arise for wavenumbers much smaller than the scale associated with the bounce. We also show that magnetic fields which correspond to observed strengths today can be generated for specific values of the parameters. But, we find that, at the bounce, the backreaction due to the electromagnetic modes that have been generated can be significantly large calling into question the viability of the model. We briefly discuss the implications of our results.Comment: v1: 19 pages, 5 figures; v2: 20 pages, 5 figures, minor revisions, to appear in JCA

Leading Gravitational Corrections and a Unified Universe

Leading order gravitational corrections to the Einstein-Hilbert action can lead to a consistent picture of the universe by unifying the epochs of inflation and dark energy in a single framework. While the leading local correction induces an inflationary phase in the early universe, the leading non-local term leads to an accelerated expansion of the universe at the present epoch. We argue that both the leading UV and IR terms can be obtained within the framework of a covariant effective field theory of gravity. The perturbative gravitational corrections therefore provide a fundamental basis for understanding a possible connection between the two epochs.Comment: 5 pages, 2 figures. This essay received "Honorable Mention" in the 2016 Gravity Research Foundation Awards for Essays on Gravitation. arXiv admin note: substantial text overlap with arXiv:1603.0002

On the non-Gaussian correlation of the primordial curvature perturbation with vector fields

We compute the three-point cross-correlation function of the primordial curvature perturbation generated during inflation with two powers of a vector field in a model where conformal invariance is broken by a direct coupling of the vector field with the inflaton. If the vector field is identified with the electromagnetic field, this correlation would be a non-Gaussian signature of primordial magnetic fields generated during inflation. We find that the signal is maximized for the flattened configuration where the wave number of the curvature perturbation is twice that of the vector field and in this limit, the magnetic non-linear parameter becomes as large as |b_{NL}| ~ 10^3. In the squeezed limit where the wave number of the curvature perturbation vanishes, our results agree with the magnetic consistency relation derived in arXiv:1207.4187.Comment: 18 pages. V3: some typos fixed, matches version published in JCA