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Preparation, optical and non-linear optical power limiting properties of Cu, CuNi nanowires.
No full textOpen AccessMetallic nanowires show excellent Plasmon absorption which is tunable based on its aspect ratio and alloying nature. We prepared Cu and CuNi metallic nanowires and studied its optical and nonlinear optical behavior. Optical properties of nanowires are theoretically explained using Gans theory. Nonlinear optical behavior is studied using a single beam open aperture z-scan method with the use of 5 ns Nd: YAG laser. Optical limiting is found to arise from two-photon absorption
Non-affine fields in solid-solid transformations: the structure and stability of a product droplet.
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We describe the microstructure, morphology, and dynamics of growth of a droplet of martensite nucleating in a parent austenite during a solid–solid transformation, using a Landau theory written in terms of both conventional affine elastic deformations and non-affine deformations. Non-affineness, phgr, serves as a source of strain incompatibility and screens long-ranged elastic interactions. It is produced wherever the local stress exceeds a threshold and anneals diffusively thereafter. Using a variational calculation, we find three types of stable solution (labeled I, II, and III) for the structure of the product droplet, depending on the stress threshold and the scaled mobilities of phgr parallel and perpendicular to the parent–product interface. The profile of the non-affine field phgr is different in these three solutions: I is characterized by a vanishingly small phgr, II admits large values of phgr localized in regions of high stress within the parent–product interface, and III is a structure in which phgr completely wets the parent–product interface. The width l and size W of the twins follow the relation in solution I; this relation does not hold for II or III. We obtain a dynamical phase diagram featuring these solutions, and argue that they represent specific solid-state microstructure
Nanoparticles in the supramolecular order of discotic liquid crystals
No full textRestricted Access.This article primarily summarises our work, presented at the 24th International Liquid Crystals Conference in Mainz, on the dispersion of various functionalised nanoparticles such as gold nanoparticles, cadmium selenide quantum dots, single-wall carbon nanotubes and gold nanorods in the supramolecular order of columnar mesophases of discotic liquid crystals (DLCs). Dispersion of such nanomaterials in small concentration has negligible effect on the mesomorphic properties of the DLCs but improves the physical properties like conductivity of the system significantly. Nanoparticles embedded discotic nanoribbons can be prepared via simple solution processing. Such nanocomposites may emerge as advanced semiconducting materials for many device applications
Torque reversals and pulse profile of the pulsar 4U 1626-67
No full textOpen AccessWe review the pulse profile evolution of the unique accretion powered X-ray pulsar 4U 1626−67 over the last 40 yr since its discovery. This pulsar showed two distinct eras of steady spin-up separated by a steady spin-down episode for about 18 yr. In this work, using data from different observatories active during each phase of spin-up and spin-down, we establish a clear correlation between the accretion torque acting on this pulsar and its pulse profile. The energy-resolved pulse profiles are identical in both the spin-up eras and quite different in the spin-down era, especially in the low-energy band. This correlation, along with the already known feature of strong quasi-periodic oscillations (QPO) that was present only in the spin-down era, clearly establishes two different accretion modes on to the neutron star which produce different pulse profiles and only one of which produces the QPOs
Inter-relationship between the two emission cones of B1237+25
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)The origin of two distinct pairs of conal emission components in pulsars, associated with the "outer" and the "inner" emission cones, as well as the marked difference in their observed spectral properties, is poorly understood. The sub-pulse modulation in the corresponding conal components, if mapped back to the underlying system of sub-beams rotating around the magnetic axis in the polar cap, as envisioned by Ruderman & Sutherland, provides a potential way to investigate the emission morphologies in the two conal regions, and more importantly, any inter-relationship between them. The bright pulsar B1237+25 with its special viewing geometry where the sightline traverses almost through the magnetic axis, along with a rich variety in pulse-to-pulse fluctuations, provides an excellent but challenging opportunity to map the underlying emission patterns across the full transverse slice of its polar emission region. We present here our analysis on a number of pulse sequences from this star to map and study any relationship between the underlying patterns responsible for emission in the two pairs of presumed conal components and a core component of this pulsar. The results from our correlation analysis of the two conal emission patterns strongly support the view that the two cones of this pulsar (the outer and the inner cone) originate from a common system of sub-beams. We also see evidence for a twist in the emission columns, most likely associated with a corresponding twist in the magnetic field structure. We discuss these results, and their implications, including a possibility that the core component of this pulsar shares its origin partly with the conal counterparts
Enhanced optical limiting and carrier dynamics in metal oxide-hydrogen exfoliated graphene hybrids
No full textRestricted Access.Hydrogen exfoliated graphene (HEG) is an interesting class of few-layer graphene, which is synthesized via hydrogen induced simultaneous exfoliation-reduction of graphite oxide. HEG exhibits strong optical limiting (OL) due to defect states arising from a large number of structural defects as well as oxygen functionalities present on its surface. Recently, we have shown that OL in HEG can be improved by simple acid functionalization, as it results in an increased number of defects. In the present study, we demonstrate that the OL performance of functionalized HEG (f-HEG) can be further improved, in both the short-pulse (nanosecond) and ultrafast (femtosecond) laser excitation regimes, using hybrids of f-HEG with transition metal oxide nanoparticles (NPs) such as CuO. The enhancement in the OL efficiency of the hybrid arises from strong nonlinear absorption in CuO NPs, which is determined mostly by interband and intraband transitions. The presence of defect states in the samples is confirmed using ultrafast pump–probe measurements, which reveal a delayed carrier relaxation due to carrier trapping by these states. Furthermore, we show that the occurrence of induced thermal scattering is minimal in these water dispersed systems, such that OL occurs predominantly due to nonlinear absorption
Large-scale dynamo action due to α alpha fluctuations in a linear shear flow
No full textOpen AccessWe present a model of large-scale dynamo action in a shear flow that has stochastic, zero-mean fluctuations of the α parameter. This is based on a minimal extension of the Kraichnan–Moffatt model, to include a background linear shear and Galilean-invariant α-statistics. Using the first-order smoothing approximation we derive a linear integro-differential equation for the large-scale magnetic field, which is non-perturbative in the shearing rate S , and the α-correlation time τα . The white-noise case, τα = 0 , is solved exactly, and it is concluded that the necessary condition for dynamo action is identical to the Kraichnan–Moffatt model without shear; this is because white-noise does not allow for memory effects, whereas shear needs time to act. To explore memory effects we reduce the integro-differential equation to a partial differential equation, valid for slowly varying fields when τα is small but non-zero. Seeking exponential modal solutions, we solve the modal dispersion relation and obtain an explicit expression for the growth rate as a function of the six independent parameters of the problem. A non-zero τα gives rise to new physical scales, and dynamo action is completely different from the white-noise case; e.g. even weak α fluctuations can give rise to a dynamo. We argue that, at any wavenumber, both Moffatt drift and Shear always contribute to increasing the growth rate. Two examples are presented: (a) a Moffatt drift dynamo in the absence of shear and (b) a Shear dynamo in the absence of Moffatt drift
Anti-fouling response of gold-carbon nanotubes composite for enhanced ethanol electrooxidation
No full textRestricted Access.We report the synthesis of gold carbon nanotubes composite through a one-pot surfactant free approach and its utility for ethanol electrooxidation reaction (EOR). The method involves the application of laser ablation for nanoparticle synthesis and simultaneous assembly of these on carbon nanotubes. The catalyst has been characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDAX) and UV–vis spectroscopic techniques. A systematic study of gold carbon nanotubes modified carbon paste electrode for EOR has been pursued. The kinetic study revealed the excellent stability of the modified electrode even after 200 cycles of EOR and with an Arrhenius energy as low as ∼28 kJ mol−1. Tafel slopes that are the measure of electrode activity have been monitored as a function of temperature of the electrolyte. The results indicate that despite an increase in the reaction rate with temperature, the electrode surface has not been significantly passivated by carbonaceous species produced at high temperatures