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Synthesis and characterization of naphthalene-based banana-shaped liquid crystals for photoswitching properties
No full textRestricted Access.A series of banana-shaped monomers containing naphthalene as central units, azobenzene in side arms with terminal alkenes were synthesized and characterized. Polarizing optical microscopy, DSC and X-ray diffraction measurements reveal that one compound processes a nematic phase while other four compounds exhibit B6 phase. The absorption spectrum of trans-azobenzene displays high-intensity π-π* transition at 365 and low-intensity n-π* transition at 450 nm. These molecules exhibit strong photoisomerisation behaviour in solutions in which trans to cis isomerisation takes 55 seconds whereas reverse process takes about 32 hours. Such a long thermal back relaxation is useful for creation of optical image storage devices
Switching of absorptive nonlinearity from reverse saturation to saturation in polymer-ZnO nanotop composite films
No full textOpen AccessWe report an interesting switchover of optical nonlinearity from Reverse Saturable Absorption (RSA) to Saturable Absorption (SA) in polymer-Zinc Oxide (ZnO) nanotop composite films, investigated using the Z-scan technique. The nanocomposites have been prepared by in situ polymerization of the monomer in which ZnO nanotops are dispersed with the help of a dispersing agent. The films exhibit RSA for lower concentrations of ZnO nanotops, which changes to SA on increasing the loading concentration, irrespective of the monomer and dispersing agent used. These versatile films are good candidates for applications such as ultrafast optical switching and optical limiting
Fermi Transport
No full textOpen AccessThis article introduces Fermi Transport, starting with elementary examples and slowly rising in level. A number exercises are suggested for the reader’s active participation
The Koslowski- Sahlmann representation: Gauge and diffeomorphism invariance.
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)The discrete spatial geometry underlying loop quantum gravity (LQG) is degenerate almost everywhere. This is at apparent odds with the non-degeneracy of asymptotically flat metrics near spatial infinity. Koslowski generalized the LQG representation so as to describe states labeled by smooth non-degenerate triad fields. His representation was further studied by Sahlmann with a view to imposing gauge and spatial diffeomorphism invariance through group averaging methods. Motivated by the desire to model asymptotically flat quantum geometry by states with triad labels which are non-degenerate at infinity but not necessarily so in the interior, we initiate a generalization of Sahlmann's considerations to triads of varying degeneracy. In doing so, we include delicate phase contributions to the averaging procedure which are crucial for the correct implementation of the gauge and diffeomorphism constraints, and whose existence can be traced to the background exponential functions recently constructed by one of us. Our treatment emphasizes the role of symmetries of quantum states in the averaging procedure. Semianalyticity, influential in the proofs of the beautiful uniqueness results for LQG, plays a key role in our considerations. As a by product, we re-derive the group averaging map for standard LQG, highlighting the role of state symmetries and explicitly exhibiting the essential uniqueness of its specification
H I in the Arp 202 system and its tidal dwarf candidate
No full textOpen Access.We present results from our Giant Metrewave Radio Telescope (GMRT) H i observations of the interacting pair Arp 202 (NGC 2719 and NGC 2719A). Earlier deep ultraviolet (Galaxy Evolution Explorer) observations of this system revealed a tidal-tail-like extension with a diffuse object towards its end, proposed as a tidal dwarf galaxy (TDG) candidate. We detect H i emission from the Arp 202 system, including H i counterparts for the tidal tail and the TDG candidate. Our GMRT H i morphological and kinematic results clearly link the H i tidal tail and the H i TDG counterparts to the interaction between NGC 2719 and NGC 2719A, thus strengthening the case for the TDG. The Arp 202 TDG candidate belongs to a small group of TDG candidates with extremely blue colours. In order to gain a better understanding of this group we carried out a comparative study of their properties from the available data. We find that H i (and probably stellar) masses of this extremely blue group are similar to the lowest H i mass TDGs in the literature. However the number of such blue TDG candidates examined so far is too small to conclude whether or not their properties justify them to be considered as a subgroup of TDGs
Formation and rupture of Ca2+ induced pectin biopolymer gels†
No full textRestricted Access.When calcium salts are added to an aqueous solution of polysaccharide pectin, ionic cross-links form between pectin chains, giving rise to a gel network in dilute solution. In this work, dynamic light scattering (DLS) is employed to study the microscopic dynamics of the fractal aggregates (flocs) that constitute the gels, while rheological measurements are carried out to study the process of gel rupture. As the calcium salt concentration is increased, DLS experiments reveal that the polydispersity of the flocs increase simultaneously with the characteristic relaxation times of the gel network. Above a critical salt concentration, the flocs become interlinked to form a reaction-limited fractal gel network. Rheological studies demonstrate that the limits of the linear rheological response and the critical stresses required to rupture these networks both decrease with the increase in salt concentration. These features indicate that the ion-mediated pectin gels studied here lie in a ‘strong link’ regime that is characterised by inter-floc links that are stronger than intra-floc links. A scaling analysis of the experimental data presented here demonstrates that the elasticities of the individual fractal flocs exhibit power-law dependences on the added salt concentration. We conclude that when both pectin and salt concentrations are increased, the number of fractal flocs of pectin increases simultaneously with the density of crosslinks, giving rise to very large values of the bulk elastic modulus
Dislocations and metastable chevrons in the electroconvective inplane normal roll state of a bent core nematic liquid crystal
No full textRestricted Access.We report experimental results on the formation, dynamics, and annihilation of edge dislocations of opposite topological charge in the electroconvective inplane vortex state of a bent core nematic liquid crystal. The approach of paired, oppositely charged defects toward each other is a two-step process. Near constant velocity at large separation and accelerated motion close to annihilation are found, as in the case of nematic rolls belonging to standard electroconvection. Periodic arrays of dislocations of alternating polarity form upon a sudden, strong elevation of the control parameter. Chevron structures that appear between undulatory defect chains are metastable, and their decay with time is accompanied by an exponential reduction in the dislocation density. The initial periodicity of defect chains also drops exponentially with increasing fiel
Improved upper limits on the stochastic gravitational-wave background from 2009-2010 LIGO and Virgo Data
No full textOpen AccessGravitational waves from a variety of sources are predicted to superpose to create a stochastic background. This background is expected to contain unique information from throughout the history of the Universe that is unavailable through standard electromagnetic observations, making its study of fundamental importance to understanding the evolution of the Universe. We carry out a search for the stochastic background with the latest data from the LIGO and Virgo detectors. Consistent with predictions from most stochastic gravitational-wave background models, the data display no evidence of a stochastic gravitational-wave signal. Assuming a gravitational-wave spectrum of ΩGW(f)=Ωα(f/fref)α, we place 95% confidence level upper limits on the energy density of the background in each of four frequency bands spanning 41.5–1726 Hz. In the frequency band of 41.5–169.25 Hz for a spectral index of α=0, we constrain the energy density of the stochastic background to be ΩGW(f)<5.6×10−6. For the 600–1000 Hz band, ΩGW(f)<0.14(f/900 Hz)3, a factor of 2.5 lower than the best previously reported upper limits. We find ΩGW(f)<1.8×10−4 using a spectral index of zero for 170–600 Hz and ΩGW(f)<1.0(f/1300 Hz)3 for 1000–1726 Hz, bands in which no previous direct limits have been placed. The limits in these four bands are the lowest direct measurements to date on the stochastic background. We discuss the implications of these results in light of the recent claim by the BICEP2 experiment of the possible evidence for inflationary gravitational waves