Spontaneous Lorentz violation: the case of infrared QED

It is by now clear that the infrared sector of quantum electrodynamics (QED) has an intriguingly complex structure. Based on earlier pioneering work on this subject, two of us recently proposed a simple modification of QED by constructing a generalization of the U(1) charge group of QED to the ``Sky'' group incorporating the well-known spontaneous Lorentz violation due to infrared photons, but still compatible in particular with locality (Balachandran and Vaidya, Eur Phys J Plus 128:118, 2013). It was shown that the ``Sky'' group is generated by the algebra of angle-dependent charges and a study of its superselection sectors has revealed a manifest description of spontaneous breaking of the Lorentz symmetry. We further elaborate this approach here and investigate in some detail the properties of charged particles dressed by the infrared photons. We find that Lorentz violation due to soft photons may be manifestly codified in an angle-dependent fermion mass, modifying therefore the fermion dispersion relations. The fact that the masses of the charged particles are not Lorentz invariant affects their spin content, and time dilation formulas for decays should also get corrections.

Nonlinear Optical Properties and Temperature-Dependent UV-Vis Absorption and Photoluminescence Emission in 2D Hexagonal Boron Nitride Nanosheets

Recently, a lot of interest has been centred on the optical properties of hexagonal boron nitride (h-BN), which has a similar lattice structure to graphene. Interestingly, h-BN has a wide bandgap and is biocompatible, so it has potential applications in multiphoton bioimaging, if it can exhibit large nonlinear optical (NLO) properties. However, extensive investigation into the NLO properties of h-BN have not been done so far. Here, NLO properties of 2D h-BN nanosheets (BNNS) are reported for the first time, using 1064-nm NIR laser radiation with a pulse duration of 10 ns using the Z-scan technique. The reverse saturable absorption occurs in aqueous colloidal solutions of BNNS with a very large two-photon absorption cross section (sigma(2PA)) of approximate to 57 x 10(-46) cm(4) s(-1) photon(-1). Also, by using UV-Vis absorption spectroscopy, the temperature coefficient of the bandgap (dE(g)/dT) of BNNS is determined to be 5.9 meV K-1. Further defect-induced photoluminescence emission in the UV region is obtained in the 283-303 K temperature range, under excitations of different wavelengths. The present report of large sigma(2PA) combined with stability and biocompatibility could open up new possibilities for the application of BNNS as a potential optical material for multiphoton bioimaging and advanced photonic devices.

High aspect ratio, processable coordination polymer gel nanotubes based on an AIE-active LMWG with tunable emission

A new TPE based low molecular weight gelator (LMWG) which displays both AIE and MCIE phenomena in gel state has been synthesized. LMWG self-assembles to form 1D nanofibers which undergo morphology transformation to coordination polymer gel (CPG) nanotubes upon metal ion coordination. CPG shows enhanced mechanical stability along with tunable emission properties.

General framework for acoustic emission during plastic deformation

Despite the long history, so far there is no general theoretical framework for calculating the acoustic emission spectrum accompanying any plastic deformation. We set up a discrete wave equation with plastic strain rate as a source term and include the Rayleigh-dissipation function to represent dissipation accompanying acoustic emission. We devise a method of bridging the widely separated time scales of plastic deformation and elastic degrees of freedom. While this equation is applicable to any type of plastic deformation, it should be supplemented by evolution equations for the dislocation microstructure for calculating the plastic strain rate. The efficacy of the framework is illustrated by considering three distinct cases of plastic deformation. The first one is the acoustic emission during a typical continuous yield exhibiting a smooth stress-strain curve. We first construct an appropriate set of evolution equations for two types of dislocation densities and then show that the shape of the model stress-strain curve and accompanying acoustic emission spectrum match very well with experimental results. The second and the third are the more complex cases of the Portevin-Le Chatelier bands and the Luders band. These two cases are dealt with in the context of the Ananthakrishna model since the model predicts the three types of the Portevin-Le Chatelier bands and also Luders-like bands. Our results show that for the type-C bands where the serration amplitude is large, the acoustic emission spectrum consists of well-separated bursts of acoustic emission. At higher strain rates of hopping type-B bands, the burst-type acoustic emission spectrum tends to overlap, forming a nearly continuous background with some sharp acoustic emission bursts. The latter can be identified with the nucleation of new bands. The acoustic emission spectrum associated with the continuously propagating type-A band is continuous. These predictions are consistent with experimental results. More importantly, our study shows that the low-amplitude continuous acoustic emission spectrum seen in both the type-B and type-A band regimes is directly correlated to small-amplitude serrations induced by propagating bands. The acoustic emission spectrum of the Luders-like band matches with recent experiments as well. In all of these cases, acoustic emission signals are burstlike, reflecting the intermittent character of dislocation-mediated plastic flow.

Synthesis, characterization and field emission properties of tin oxide nanowires

Tin oxide (SnO2) nanowires are synthesized by Au catalyzed chemical vapor deposition of Sn and C mixture at 900 degrees C by employing a continuous flow of Ar: O-2 (10:1) for an hour. X-ray diffraction and Raman spectroscopy studies indicate that the as-grown SnO2 nanowires are crystalline in nature with tetragonal rutile phase. Electron microscopy studies reveal towards high aspect ratio of nanowires. The field emission studies show that SnO2 nanowires grown on Si substrate exhibit low turn-on field of 1.75 V/mu m (at 0.1 mu A/cm(2)) and long-term emission stability over a period of more than 50 h with a current density of 4 mu A/cm(2) at a constant electric field of 2.25 V/mu m. Hardly any considerable degradation in the emission current is noticed even after 50 h which may be attributed to the high crystallinity of SnO2 nanowires. (C) 2015 Elsevier B.V. All rights reserved.

Plasmon-mediated emergence of collective emission and enhanced quantum efficiency in quantum dot films

We present experimental and theoretical results on monolayer colloidal cadmium selenide quantum dot films embedded with tiny gold nanoparticles. By varying the density of the embedded gold nanoparticles, we were able to engineer a plasmon-mediated crossover from emission quenching to enhancement regime at interparticle distances for which only quenching of emission is expected. This crossover and a nonmonotonic variation of photoluminescence intensity and decay rate, in experiments, is explained in terms of a model for plasmon-mediated collective emission of quantum emitters which points to the emergence of a new regime in plasmon-exciton interactions. The presented methodology to achieve enhancement in optical quantum efficiency for optimal doping of gold nanoparticles in such ultrathin high-density quantum dot films can be beneficial for new-generation displays and photodetectors.

DIFFUSE X-RAY EMISSION FROM STAR-FORMING GALAXIES

We study the diffuse X-ray luminosity (L-X) of star-forming galaxies using two-dimensional axisymmetric hydrodynamical simulations and analytical considerations of supernovae-(SNe-) driven galactic outflows. We find that the mass loading of the outflows, a crucial parameter for determining the X-ray luminosity, is constrained by the availability of gas in the central star-forming region, and a competition between cooling and expansion. We show that the allowed range of the mass loading factor can explain the observed scaling of L-X with star formation rate (SFR) as L-X alpha SFR2 for SFR greater than or similar to 1 M-circle dot yr(-1), and a flatter relation at low SFRs. We also show that the emission from the hot circumgalactic medium (CGM) in the halo of massive galaxies can explain the large scatter in the L-X-SFR relation for low SFRs (less than or similar to few M-circle dot yr(-1)). Our results suggest that galaxies with small SFRs and large diffuse X-ray luminosities are excellent candidates for the detection of the elusive CGM.

Complex patterns arise through spontaneous symmetry breaking in dense homogeneous networks of neural oscillators

There has been much interest in understanding collective dynamics in networks of brain regions due to their role in behavior and cognitive function. Here we show that a simple, homogeneous system of densely connected oscillators, representing the aggregate activity of local brain regions, can exhibit a rich variety of dynamical patterns emerging via spontaneous breaking of permutation or translational symmetries. Upon removing just a few connections, we observe a striking departure from the mean-field limit in terms of the collective dynamics, which implies that the sparsity of these networks may have very important consequences. Our results suggest that the origins of some of the complicated activity patterns seen in the brain may be understood even with simple connection topologies.