Nonlinear optical investigations in nine-atom silver quantum clusters and graphitic carbon Nitride Nanosheets

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)Absorption saturation due to surface plasmon resonance affects the optical limiting efficiency of metal nanoparticles (NPs) by raising the limiting threshold to higher laser fluences. It has been shown that in gold, compared to the larger NPs, smaller quantum clusters (QCs) exhibit better optical limiting with lower limiting thresholds due to the absence of absorption saturation. Here we report optical limiting properties of two novel materials, namely, nine-atom silver (Ag9) QCs and graphitic carbon nitride (GCN) nanosheets. The relatively large nonlinear absorption of Ag9 QCs compared to Ag NPs is revealed from open-aperture Z-scan measurements carried out using 532 nm, 5 ns laser pulses. Optical nonlinearity in the QCs arises mostly from free carrier absorption and a relatively weak saturable absorption. The superior limiting efficiency of Ag9 QCs is complemented by excellent chemical stability, which makes silver quantum clusters ideal candidates for optical limiting applications. The two-dimensional sheet-like structure of GCN is ideal for grafting metals and semiconductors, and we show that even though the nonlinearity of pristine GCN is low it can be improved substantially by grafting lightly with Ag9 QC

Stacking of ultra-thin reduced graphene oxide nanoparticles in supramolecular structures for optoelectronic applications

Restricted Access.Octadecylamine functionalized ultra-thin reduced graphene oxide nanoparticles were synthesized and dispersed in the supramolecular order of discotic liquid crystals for the first time. The insertion and properties of the graphene nanoparticles in the columnar mesophase were studied using field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy, UV-vis spectroscopy, photoluminescence spectroscopy, polarized optical microscopy, differential scanning calorimetry, X-ray diffraction and DC conductivity. Experimental studies indicate the stacking of two-dimensional graphene nanoparticles in the supramolecular order of the columnar mesophase. The dispersion of graphene nanoparticles improves the order in the columnar phase and thus enhances the conductivity of the syste

Evolution of multiple supernova remnants

Open AccessHeating of the interstellar medium (ISM) by multiple supernova (SN) explosions is at the heart of producing galaxy-scale outflows in starburst galaxies. Standard models of outflows assume a high efficiency of SNe in heating the gas to X-ray emitting temperatures and filling the central region of starburst with hot gas, in order to launch vigorous outflows. We use hydrodynamical simulations to study the efficiency of multiple SNe in heating the ISM and filling the volume with gas of high temperatures. We argue that it is important for SN remnants to have a large filling factor and a large heating efficiency. For this, they have to be clustered in space and time, and keep exploding until the hot gas percolates through the whole region, in order to compensate for the radiative loss. In the case of a limited number of SNe, we find that although the filling factor can be large, the heating efficiency declines after reaching a large value. In the case of a continuous series of SNe, the hot gas (T ≥ 3 × 106 K) can percolate through the whole region after the total volume filling factor reaches a threshold of ∼0.3. The efficiency of heating the gas to X-ray temperatures can be ≥0.1 after this percolation epoch, which occurs after a period of ≈10 Myr for a typical starburst SN rate density of νSN ≈ 10−9 pc−3 yr−1 and gas density of n ≈ 10 cm−3 in starburst nuclei regions. This matches the recent observations of a time delay of similar order between the onset of star formation and galactic outflows. The efficiency to heat gas up to X-ray temperatures (≥106.5 K) roughly scales as ν0.2SNn−0.6. For a typical SN rate density and gas density in starburst nuclei, the heating efficiency is ∼0.15, also consistent with previous interpretations from X-ray observations. We discuss the implications of our results with regard to observational diagnostics of ionic ratios and emission measures in starburst nuclei regions

Numerical studies of dynamo action in a turbulent shear flow. I.

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We perform numerical experiments to study the shear dynamo problem where we look for the growth of a large-scale magnetic field due to non-helical stirring at small scales in a background linear shear flow in previously unexplored parameter regimes. We demonstrate the large-scale dynamo action in the limit where the fluid Reynolds number ($\operatorname{Re}$) is below unity while the magnetic Reynolds number (${\rm Rm}$) is above unity; the exponential growth rate scales linearly with shear, which is consistent with earlier numerical works. The limit of low $\operatorname{Re}$ is particularly interesting, as seeing the dynamo action in this limit would provide enough motivation for further theoretical investigations, which may focus attention on this analytically more tractable limit of $\operatorname{Re}\lt 1$ compared to the more formidable limit of $\operatorname{Re}\gt 1$. We also perform simulations in the regimes where (i) both ($\operatorname{Re}$, ${\rm Rm}$) < 1, and (ii) $\operatorname{Re}\gt 1$ and ${\rm Rm}\lt 1$, and compute all of the components of the turbulent transport coefficients (${{\alpha }_{ij}}$ and ${{\eta }_{ij}}$) using the test-field method. A reasonably good agreement is observed between our results and the results of earlier analytical works in similar parameter regimes

Cosmic ray antiprotons from nearby cosmic accelerators

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)The antiproton flux measured by PAMELA experiment might have originated from Galactic sources of cosmic rays. These antiprotons are expected to be produced in the interactions of cosmic ray protons and nuclei with cold protons. Gamma rays are also produced in similar interactions inside some of the cosmic accelerators. We consider a few nearby supernova remnants observed by Fermi LAT. Many of them are associated with molecular clouds. Gamma rays have been detected from these sources which most likely originate in decay of neutral pions produced in hadronic interactions. The observed gamma ray fluxes from these SNRs are used to find out their contributions to the observed diffuse cosmic ray antiproton flux near the earth

Gravitational Radiation

Restricted Acces

Crystal structure of 4-bromo­phenyl-2-oxo-2H-chromene-3-carboxyl­ate

Restricted Access.In the title compound, C16H9BrO4, the coumarin ring system is approximately planar, with an r.m.s deviation of the ten fitted non-H atoms of 0.031 Å, and forms a dihedral angle of 25.85 (10)° with the bromo­benzene ring. The carbonyl atoms are syn. In the crystal, mol­ecules are connected along [001] via C—H⋯O inter­actions, forming C(6) chains. Neighbouring C(6) chains are connected via several π–π inter­actions [range of centroid–centroid distances = 3.7254 (15)–3.7716 (16) Å], leading to sheets propagating in the bc plan

Luminescent CdTe quantum dots incarcerated in a columnar matrix of discotic liquid crystals for optoelectronic applications

Restricted Access.Here in we demonstrate, for the first time, the effects of highly luminescent alkylamine-capped semiconductor cadmium telluride quantum dot (CdTe QD) dispersion on the optical, electrical, and thermal properties and supramolecular order of a discotic liquid crystal (DLC). The insertion and properties of CdTe quantum dots in the columnar mesophase were studied by UV-vis spectroscopy, photoluminescence spectroscopy, polarized optical microscopy, differential scanning calorimetry, X-ray diffraction and DC conductivity. Results indicate uniform dispersion of CdTe QDs in a columnar matrix without disrupting the mesophase but enhancing the conductivity of the system significantly

Gravitational Radiation

Restricted Acces