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Organic dye impregnated poly(vinyl alcohol) nanocomposite as an efficient optical limiter: structure, morphology and photophysical properties
No full textRestricted Access.A new polymer based nanocomposite system comprising the organic chromophore Light Green (LG) has been fabricated using poly(vinyl alcohol) (PVA) as the host template, and its structure, morphology, and linear and nonlinear optical properties have been investigated. Microstructural analysis reveals a semicrystalline nature of the nanocomposites, with a uniform dispersion of nanoclusters of dye molecules encapsulated between the molecular chains of the host polymer PVA. XRD, FTIR and electrical conductivity studies indicate considerable interactions between the dye molecules and the polymer chains. LG–PVA nanocomposite films exhibit PL emission in the red region of the visible spectrum, when excited in the vicinity of their absorption maximum (645 nm). The composite films also display nonlinear absorption and optical limiting behavior under nanosecond (5 ns) Nd:YAG laser light excitation at 532 nm. The LG–PVA nanocomposite films are found to possess a high laser damage threshold, improved thermal and photostability, and excellent durability, which signify the scope of utilizing them as smart materials for applications in optoelectronic nanodevices
Electric-field-induced switchable dark conglomerate phases in a bent-core liquid crystal exhibiting reverse columnar phases.
No full textOpen AccessElectric-field-induced transitions into switchable dark conglomerate (DC) phases from two types of reverse columnar mesophases have been observed in the bent-core (BC) compound 2,7-naphthylene bis[4-(3-methyl-4-n-tetradecyloxybenzoyloxy)] benzoate. Optical and x-ray studies show that the higher temperature columnar phase corresponds to the orthogonal B1rev phase, whereas the lower temperature columnar phase is a variant of the B1revtilt phase. As the layer fragments in this phase are modulated in order to relieve the steric hindrance caused by an anticlinic tilting in adjacent blocks, it has been named B1revtiltM. The shape of the chiral domains are different in the DC phases viz. DC-B1rev and DC-B1revtiltM obtained by applying the electric field in the B1rev and B1revtiltM phases, respectively. While the chiral domains in the DC-B1rev phase appear similar to those observed in other DC phases, the shape of the domains in the DC-B1revtiltM phase appear to have some similarity to the domains in the banana leaf texture in the B1revtiltM phase implying that the detailed structure in this DC phase may be different. Optical observations, electro-optics, and dielectric studies show that the DC-B1rev and DC-B1revtiltM phases are both switchable and possess a local SmCSPF type of structure. As the temperature is decreased the switching behavior changes from ferroelectric to antiferroelectric. The temperature at which this changeover starts occurring coincides with the temperature at which the layer modulation occurs to overcome anticlinic tilt and the B1rev to B1revtiltM phase transition takes place without the application of the electric field. The change in switching behavior is attributed to a transformation into flat layers with the SmCAPA type of structure as also evidenced by the nucleation of bright regions alongside the chiral domains
Environments of extended radio sources in the Australia telescope low-brightness survey.
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We present a study of the environments of extended radio sources in the Australia Telescope Low Brightness Survey (ATLBS). The radio sources were selected from the Extended Source Sample (ATLBS-ESS), which is a well defined sample containing the most extended of radio sources in the ATLBS sky survey regions. The environments were analyzed using 4-m CTIO Blanco telescope observations carried out for ATLBS fields in the SDSS band. We have estimated the properties of the environments using smoothed density maps derived from galaxy catalogs constructed using these optical imaging data. The angular distribution of galaxy density relative to the axes of the radio sources has been quantified by defining anisotropy parameters that are estimated using a new method presented here. Examining the anisotropy parameters for a sub-sample of extended double radio sources that includes all sources with pronounced asymmetry in lobe extents, we find good evidence for environmental anisotropy being the dominant cause for lobe asymmetry in that higher galaxy density occurs almost always on the side of the shorter lobe, and this validates the usefulness of the method proposed and adopted here. The environmental anisotropy parameters have been used to examine and compare the environments of FRI and FRII radio sources in two redshift regimes ( and ). Wide-angle tail sources and Head-tail sources lie in the most overdense environments. The Head-tail source environments (for the HT sources in our sample) display dipolar anisotropy in that higher galaxy density appears to lie in the direction of the tails. Excluding the Head-tail and Wide-angle tail sources, subsamples of FRI and FRII sources from the ATLBS survey appear to lie in similar moderately overdense environments, with no evidence for redshift evolution in the regimes studied herein
Azo-functionalised achiral bent-core liquid crystalline materials: e ect of presence of -N=N- linkage at di erent locations in the molecular architecture
No full textRestricted Access.Here, we report four new homologous series of azo-functionalised achiral bent-core compounds. The paper deals with the design, synthesis and liquid crystalline properties of four new homologous series of photochromic materials. In this study, we have investigated the effect of the presence of –N=N– linkage at different locations of the molecular architecture, on the mesomorphic properties. The molecular structures of all the newly synthesised compounds are established using the organic spectroscopic methods. The liquid crystalline properties are investigated using polarising optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction and electro-optical studies. They exhibit B1 (colr) and B2 (SmCAPA) mesophases. We find that the presence of the –N=N– linkage at different locations in the molecular architecture does not seem to have much effect on the mesogenic behaviour of such compounds. However, we clearly see a profound effect of the location of the –N=N– linkage on the photo-induced electro-optical properties of these compounds
Constraint algebra in loop quantum gravity reloaded. I. Toy model of a U(1)3 gauge theory
No full textOpen AccessWe analyze the issue of anomaly-free representations of the constraint algebra in loop quantum gravity (LQG) in the context of a diffeomorphism-invariant U(1)3 theory in three spacetime dimensions. We construct a Hamiltonian constraint operator whose commutator matches with a quantization of the classical Poisson bracket involving structure functions. Our quantization scheme is based on a geometric interpretation of the Hamiltonian constraint as a generator of phase space-dependent diffeomorphisms. The resulting Hamiltonian constraint at finite triangulation has a conceptual similarity with the μ¯ scheme in loop quantum cosmology and highly intricate action on the spin-network states of the theory. We construct a subspace of non-normalizable states (distributions) on which the continuum Hamiltonian constraint is defined which leads to an anomaly-free representation of the Poisson bracket of two Hamiltonian constraints in loop quantized framework. Our work, along with the work done in [C. Tomlin and M. Varadarajan, Phys. Rev. D 87, 044039 (2013)], suggests a new approach to the construction of anomaly-free quantum dynamics in Euclidean LQG
Towards a definition of locality in a manifoldlike causal set
No full textOpen Access.It is a common misconception that spacetime discreteness necessarily implies a violation of local Lorentz invariance. In fact, in the causal set approach to quantum gravity, Lorentz invariance follows from the specific implementation of the discreteness hypothesis. However, this comes at the cost of locality. In particular, it is difficult to define a “local” region in a manifoldlike causal set, i.e., one that corresponds to an approximately flat spacetime region. Following up on suggestions from previous work, we bridge this lacuna by proposing a definition of locality based on the abundance of m-element order-intervals as a function of m in a causal set. We obtain analytic expressions for the expectation value of this function for an ensemble of causal set that faithfully embeds into an Alexandrov interval in d-dimensional Minkowski spacetime and use it to define local regions in a manifoldlike causal set. We use this to argue that evidence of local regions is a necessary condition for manifoldlikeness in a causal set. This in addition provides a new continuum dimension estimator. We perform extensive simulations which support our claims
Cryo-TEM studies of two smectic phases of an asymmetric bent-core material. Invited Article
No full textRestricted Access. Invited articleCryo-TEM studies on two smectic phases of an asymmetric bent-core liquid crystal material are presented and compared to prior X-ray results obtained in bulk samples. While the bulk samples have layer-modulated structures, those modulations are not observable in the 100-nm-thick TEM samples, indicating surface-induced suppression of the layer modulations. The observed layer spacing is in agreement with the X-ray results in the lower temperature smectic phase, but distinctly larger in the higher temperature phase. This indicates surface-induced suppression of the director tilt. Cryo-TEM textures resolve the profiles of individual smectic layers at the scales down to few nanometres and reveal the presence of edge and screw dislocations, twist grain boundaries, small-angle and large-angle tilt grain boundaries
Possible detection of a cyclotron resonance scattering feature in the X-ray pulsar 4U 1909+07.
No full textRestricted AccessWe present timing and broad band spectral studies of the high-mass X-ray binary pulsar 4U 1909+07 using data from Suzaku observations during 2010 November 2–3. The pulse period of the pulsar is estimated to be 604.11 ± 0.14 s. Pulsations are seen in the X-ray light curve up to ~70 keV. The pulse profile is found to be strongly energy-dependent: a complex, multi-peaked structure at low energy becomes a simple single peak at higher energy. We found that the 1–70 keV pulse-averaged continuum can be fit by the sum of a blackbody and a partial covering Negative and Positive power law with Exponential cutoff model. A weak iron fluorescence emission line at 6.4 keV was detected in the spectrum. An absorption-like feature at ~44 keV was clearly seen in the residuals of the spectral fitting, independent of the continuum model adopted. To check the possible presence of a cyclotron resonance scattering feature (CRSF) in the spectrum, we normalized the pulsar spectrum with the spectrum of the Crab Nebula. The resulting Crab ratio also showed a clear dip centered at ~44 keV. We performed statistical tests on the residuals of the spectral fitting and also on the Crab spectral ratio to determine the significance of the absorption-like feature and identified it as a CRSF of the pulsar. We estimated the corresponding surface magnetic field of the pulsar to be 3.8 × 1012 G
Low-frequency radio emission in the massive galaxy cluster MACS J0717.5 + 3745
No full textOpen AccessAims. To investigate the nonthermal emission mechanism and their interaction during cluster mergers, we analyze multiple low-frequency radio data for the X-ray luminous massive galaxy cluster MACS J0717.5 + 3745, located at z = 0.5548. Large-scale structure-formation models in the Universe suggest that galaxy clusters grow via constant accretion of gas and the merger of galaxy groups and smaller clusters. Low-frequency radio observations trace these mergers in the form of relics and halos. The dual frequency observations were performed on MACS J0717.5 + 3745 to investigate the spectral index pattern of the nonthermal emission and its interaction within the intracluster medium (ICM), during merger process.
Methods. Continuum observations were carried out using GMRT at 0.235 and 0.61 GHz on MACS J0717.5 + 3745 and archival data from the VLA (0.074 and 1.42 GHz) and WSRT (0.325 GHz) was used to complement the results. Furthermore, to explore the thermal and nonthermal interactions within the ICM and the morphological distribution, Chandra X-ray and HST data were used.
Results. A highly complex nonthermal radio emission distribution is seen in the cluster at very low frequencies, with a global spectral index . We have detected a giant radio halo within the cluster system with a linear size of 1.58 Mpc and a “Chair-shaped” filament structure between the merging subclusters of linear size 853 kpc at 0.235 GHz. This is the most powerful halo ever observed with P1.4 = 9.88 × 1025 WHz-1 and an equipartition magnetic field estimate of ~6.49 μG. The bright filament structure is well located in the central merging region of subclusters with enhanced temperature, as shown by Chandra and HST data analysis, further indicating the formation of this structure due to shock waves encountered within the ICM during the merger events
Azo-functionalised liquid crystalline dimers composed of bent-core and rod-like moieties: synthesis and mesomorphic properties
No full textRestricted Access.Three new series of azo-functionalised dimers composed of banana (bent-core) and rod-like moieties connected via flexible alkylene spacer have been synthesised and characterised. Here, the molecular structure – liquid crystalline property relationship has been studied, mainly by varying the spacer chain length in addition to the effect of polar –CN end group. All the compounds synthesised are characterised by the organic spectroscopic methods. The thermal behaviour of these dimers is investigated by the polarising optical microscopy, differential scanning calorimetry, electro-optical and X-ray diffraction studies. Interestingly, we observed a variety of mesophases in these dimers, namely, N, SmA, SmC, SmX, SmY, Colx and Coly. We even observed a re-entrant phenomena of the mesophases in one of these dimers. The nematic phases of some of these dimers exhibit electric field induced textural patterns. The dimers are photosensitive and the T NI was found to decrease as the illuminated light intensity increases