515 research outputs found

    Entanglement entropy and D1-D5 geometries

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    http://dx.doi.org/10.1103/PhysRevD.90.066004Giusto, Stefano, and Rodolfo Russo. "Entanglement Entropy and D1-D5 geometries." Physical Review D 90.6 (2014): 066004

    Carbon nitride-coated transparent glass vials as photoinitiators for radical polymerization

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    Benign polymerization routes offer new perspectives in current polymer technology. Especially for automated or continuous flow synthesis of polymers, new devices and principles have to be considered by the means of minimizing addition or separation sequences as well as the type of a polymer initiation. Near-UV and visible light-induced polymerization utilizing metal-free semiconductor polymeric carbon nitride (pCN) as heterogeneous photocatalyst was a first step into this direction. Moving from heterogeneous powder catalysis (which still requests catalyst separation) to surface photocatalysis via coating glass tubes or vials with pCN thin films is presented. Performance and effectivity of those photoactive reactors are proven by free radical photopolymerization of variety of monomers. Reusability of vials is demonstrated via reversible addition-fragmentation chain-transfer polymerization-assisted block copolymer synthesis. This strategy eliminates the necessity of adding or removing initiators, works at room temperature, and offers a platform for cheap and effective polymer synthesis at the age of automated synthesis

    RFI mitigation with phase-only adaptive beamforming

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    Connected radio interferometers are sometimes used in the tied-array mode: signals from antenna elements are coherently added and the sum signal applied to a VLBI backend or pulsar processing machine. Usually there is no computer-controlled amplitude weighting in the existing radio interferometer facilities. Radio frequency interference (RFI) mitigation with phase-only adaptive beamforming is proposed for this mode of observation. Small phase perturbations are introduced in each of the antenna's signal. The values of these perturbations are optimized in such a way that the signal from a radio source of interest is preserved and RFI signals suppressed. An evolutionary programming algorithm is used for this task. Computer simulations, made for both one-dimensional and two-dimensional array set-ups, show considerable suppression of RFI and acceptable changes to the main array beam in the radio source direction.Comment: 7 pages, 11 figure

    Optical Anisotropy of Carbon Nitride Thin Films and Photografted Polystyrene Brushes

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    Polymer brushes on surfaces enable advanced material design. In the present contribution, transparent and flat photoactive polymeric carbon nitride (pCN) thin films are employed as a photoactive substrate and primer layer to grow polystyrene (PS) brushes. These films are then characterized by ellipsometry. For the first time herein is reported on the optical anisotropy of pCN thin films revealing a high positive birefringence up to 0.71 with an in-plane nD of 2.54 making this material of high interest for photonic devices. Furthermore and rather surprising, the photografted polystyrene brushes exhibit an unusual high negative birefringence, too. This negative birefringence can be attributed to a practically complete stretching of the polymer chains throughout growth in the radical chain process. As the stretched PS brushes grafted from the pCN surfaces also provide unusual surface properties, the overall system can be of great interest for photonics, but also as mechanical coating and membranes for gas separation

    Reversible morphology-resolved chemotactic actuation and motion of Janus emulsion droplets

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    We report, for the first time, a chemotactic motion of emulsion droplets that can be controllably and reversibly altered. Our approach is based on using biphasic Janus emulsion droplets, where each phase responds differently to chemically induced interfacial tension gradients. By permanently breaking the symmetry of the droplets’ geometry and composition, externally evoked gradients in surfactant concentration or effectiveness induce anisotropic Marangoni-type fluid flows adjacent to each of the two different exposed interfaces. Regulation of the competitive fluid convections then enables a controllable alteration of the speed and the direction of the droplets’ chemotactic motion. Our findings provide insight into how compositional anisotropy can affect the chemotactic behavior of purely liquid-based microswimmers. This has implications for the design of smart and adaptive soft microrobots that can autonomously regulate their response to changes in their chemical environment by chemotactically moving towards or away from a certain target, such as a bacterium.<br

    D1D5 microstate geometries from string amplitudes

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    We reproduce the asymptotic expansion of the D1D5 microstate geometries by computing the emission amplitudes of closed string states from disks with mixed D1D5 boundary conditions. Thus we provide a direct link between the supergravity and D-brane descriptions of the D1D5 microstates at non-zero string coupling. Microscopically, the profile functions characterizing the microstate solutions are encoded in the choice of a condensate for the twisted open string states connecting D1 and D5 branes.Comment: 21 pages; added reference

    Red Carbon Thin Film: A Carbon-Oxygen Semiconductor with Tunable Properties by Amine Vapors and Its Carbonization toward Carbon Thin Films

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    The requirements for organic semiconductor materials and new methods for their synthesis at low temperature have risen over the last decades, especially due to concerns of sustainability. Herein, the synthesis of a carbon/oxygen molecular semiconductor thin film, which is promptly reactive toward amines, is presented. This allows for tuning the semiconductor properties and application as amine vapor sensors for a scope of analogous amines. The gas-to-solid phase reaction causes a significant change of the films’ optical properties, blue-shifting the absorption and the photoluminescence spectra from the red to the near UV spectral range. The irreversible chemical reaction between the thin film and the amine vapor is also exploited for the preparation of nitrogen-containing thin carbon films. The herein presented materials and methods will be of interest for gas sensing applications as well as for the development of tunable semiconductors and heteroatom-doped thin films

    Microstrata

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    Microstrata are the non-extremal analogues of superstrata: they are smooth, non-extremal (non-BPS) solitonic solutions to IIB supergravity whose deep-throat limits approximate black holes. Using perturbation theory and numerical methods, we construct families of solutions using a consistent truncation to three-dimensional supergravity. The most general families presented here involve two continuous parameters, or amplitudes, and four quantized parameters that set the angular momenta and energy levels. Our solutions are asymptotic to the vacuum of the D1-D5 system: AdS3×S3×T4_3 \times S^3 \times T^4. Using holography, we show that the they are dual to multi-particle states in the D1-D5 CFT involving a large number of mutually non-BPS supergravitons and we determine the anomalous dimensions of these states from the binding energies in supergravity. These binding energies are uniformly negative and depend non-linearly on the amplitudes of the states. In one family of solutions, smoothness restricts some of the fields to lie on a special locus of the parameter space. Using precision holography we show that this special locus can be identified with the multi-particle states constructed via the standard OPE of the single-particle constituents. Our numerical analysis shows that microstrata are robust at large amplitudes and the solutions can be obtained to very high precision.Comment: 84 pages, 14 figures, Mathematica file to be attached in v

    Habemus superstratum! A constructive proof of the existence of superstrata

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    We construct the first example of a superstratum: a class of smooth horizonless supergravity solutions that are parameterized by arbitrary continuous functions of (at least) two variables and have the same charges as the supersymmetric D1-D5-P black hole. We work in Type IIB string theory on T^4 or K3 and our solutions involve a subset of fields that can be described by a six-dimensional supergravity with two tensor multiplets. The solutions can thus be constructed using a linear structure, and we give an explicit recipe to start from a superposition of modes specified by an arbitrary function of two variables and impose regularity to obtain the full horizonless solutions in closed form. We also give the precise CFT description of these solutions and show that they are not dual to descendants of chiral primaries. They are thus much more general than all the known solutions whose CFT dual is precisely understood. Hence our construction represents a substantial step toward the ultimate goal of constructing the fully generic superstratum that can account for a finite fraction of the entropy of the three-charge black hole in the regime of parameters where the classical black hole solution exists.Comment: 55 pages, 5 figure

    Hamiltonian Formulation of Open WZW Strings

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    Using a Hamiltonian approach, we construct the classical and quantum theory of open WZW strings on a strip. (These are the strings which end on WZW branes.) The development involves non-abelian generalized Dirichlet images in an essential way. At the classical level, we find a new non-commutative geometry in which the equal-time coordinate brackets are non-zero at the world-sheet boundary, and the result is an intrinsically non-abelian effect which vanishes in the abelian limit. Using the classical theory as a guide to the quantum theory, we also find the operator algebra and the analogue of the Knizhnik-Zamolodchikov equations for the the conformal field theory of open WZW strings.Comment: 34 pages. Added an equation in Appendix C; some typos corrected. Footnote b changed. Version to appear on IJMP
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