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Huijse P.,University of Chile | Estevez P.A.,University of Chile | Zegers P.,University of Los Andes, Chile | Principe J.C.,University of Florida | And 2 more authors.
IEEE Signal Processing Letters | Year: 2011

In this letter, we propose a method for period estimation in light curves from periodic variable stars using correntropy. Light curves are astronomical time series of stellar brightness over time, and are characterized as being noisy and unevenly sampled. We propose to use slotted time lags in order to estimate correntropy directly from irregularly sampled time series. A new information theoretic metric is proposed for discriminating among the peaks of the correntropy spectral density. The slotted correntropy method outperformed slotted correlation, string length, VarTools (Lomb-Scargle periodogram and Analysis of Variance), and SigSpec applications on a set of light curves drawn from the MACHO survey. © 2011 IEEE. Source

News Article
Site: http://phys.org/physics-news/

Daniel Elton and Marivi Fernandez-Serra used computer simulation models of water developed at Stony Brook’s Institute for Advanced Computational Science to discover its molecular properties are similar to ice. For more than 100 years, scientists have debated what the underlying molecular structure of water is, and the common view has been that H2O molecules are either "water-like" or "ice-like." Now through computer simulation conducted at the Institute for Advanced Computational Science (IACS) at Stony Brook University, researchers can illustrate that the structure and dynamics of hydrogen bonding in liquid water is more similar to ice than previously thought. The finding, published in Nature Communications , changes the common understanding of the molecular nature of water and has relevance to many fields, such as climate science and molecular biophysics, and technologies such as desalinization and water-based energy production. In condensed matter physics, phonons are considered to be a solid-state phenomenon and can be visualized as collective vibrations that propagate through a material. More precisely, a phonon is the fundamental quantum mechanical unit of lattice vibration. Optical phonons are a type of phonon that interact with electromagnetic radiation. These can be visualized as peaks in the infrared absorption spectrum in ice. In the paper, "The hydrogen-bond network of water supports propagating optical phonon-like modes," lead author Daniel C. Elton, a PhD candidate, and Marivi Fernandez-Serra, PhD, Associate Professor, in the Department of Physics and Astronomy and IACS, show that propagating vibrations or phonons can exist in water, just as in ice. "No microscopes can allow us to directly see the behavior of water molecules and their pattern of hydrogen bonding. Therefore by simulating liquid water using the fundamental laws of physics, the structure and motion of molecules in water can be analyzed in great detail beyond what microscopes can reveal of liquid water," said Elton. "Our method involved both experimental data and extensive molecular dynamics simulations, and we found that the optical phonon coupling leads to similar absorption peaks also found in ice." The authors used a new high-powered computer cluster at Stony Brook's IACS to create the water dynamics simulations. By centering on water's unique hydrogen bond network, they routinely demonstrated that optical phonon-like modes can propagate the hydrogen bond network, just as in ice. Unlike in ice, however, hydrogen bonds in water are constantly being broken and reformed, so the phonons only last for about one trillionth of a second yet can travel over long distances up to two nanometers. "Our findings challenge older ideas about water dynamics, which characterized peaks in the absorption spectrum as being due to the vibrational motions of at most a few molecules, as in ice," said Professor Fernandez-Serra. "We found water peaks in spectra correspond to two different types of phonons, called longitudinal and transverse. The shifting of the position of the longitudinal and transverse peaks with temperature can be related to important structural changes in the hydrogen bond network, which provides a new window into how water's structure changes with temperature." Additionally, by comparing several different simulation techniques, the authors also found that the current non-polarizable water models used in biophysics fail to capture the higher frequency optical phonons. This work builds on their  previous work , which showed that polarizable models are more accurate than the more often used non-polarizable models. More information: Daniel C. Elton et al. The hydrogen-bond network of water supports propagating optical phonon-like modes, Nature Communications (2016). DOI: 10.1038/ncomms10193 D. C. Elton et al. Polar nanoregions in water: A study of the dielectric properties of TIP4P/2005, TIP4P/2005f and TTM3F, The Journal of Chemical Physics (2014). DOI: 10.1063/1.4869110

Mukhopadhyay K.,University of Burdwan | Sutradhar S.,University of Burdwan | Modak S.,University of Burdwan | Roy S.K.,IACS | Chakrabarti P.K.,University of Burdwan
Journal of Physical Chemistry C | Year: 2012

Nanoparticles of GaFeO 3 (GFO) and Ni 0.4Zn 0.4Cu 0.2 Fe 2O 4 (NZCF) and their nanocomposite [(GaFeO 3) 0.50 (Ni 0.4Zn 0.4Cu 0.2Fe 2O 4) 0.50, GFONZCF] were prepared by chemical route. Nanoparticles of GFO were synthesized by sol-gel route, and those of NZCF were prepared by chemical coprecipitation method. The nanoparticles of GFO were incorporated in the matrix of NZCF by coprecipitating the salts required for NZCF in the presence of GFO particles, followed by subsequent washing and heat treatment at 500 °C. X-ray diffractograms (XRDs) were recorded to confirm the formation of the desired crystallographic phases of the samples. The sizes of the nanoparticles were estimated from the broadening of the well-defined peaks using the Debye-Scherrer equation. The nanoparticle size and its distribution, crystallographic phase, nanocrystallinity, and so on were studied by a high-resolution transmission electron microscope (HRTEM), and the extracted results were in good agreement with those obtained from the XRD patterns. The static and dynamic magnetic measurements were carried out. The observations of field-cooled (FC), zero-field-cooled (ZFC) magnetizations, and hysteresis loops (M-H loop) in the temperature range of 300 to 2 K were carried out in the static measurements. The static magnetic data were analyzed to evaluate the particle size, nanocrystalline anisotropy, and so on, and the agreement of these evaluated data are quite satisfactory, so far as the extracted results obtained from XRD and HRTEM are concerned. The maximum magnetization of the GFO sample has been drastically enhanced by incorporating them in the matrix of NZCF. Also, the nature of variation of the magnetization in all cases of FC, ZFC, and M-H curves of the nanoparticles of GFO has been drastically modulated by the NZCF. The dynamic magnetic measurements include the measurements of ac magnetization versus excitation curves, hysteresis loops at different frequencies at room temperatures, and so on. The remarkable enhancement of magnetization of the multiferroic system of GFO by the encapsulation of NZCF would be quite interesting for various applications. © 2012 American Chemical Society. Source

Islam S.M.,Kalyani University | Roy A.S.,Kalyani University | Mondal P.,Kalyani University | Tuhina K.,Bs College | And 2 more authors.
Tetrahedron Letters | Year: 2012

A new polymer-anchored Cu(II) complex has been synthesized and characterized. The catalytic performance of the complex has been tested for the oxidation of sulfides and in oxidative bromination reaction with hydrogen peroxide as the oxidant. Sulfides have been selectively oxidized to the corresponding sulfoxides in excellent yields and in the presence of KBr as the bromine source, organic substrates have been selectively converted to mono bromo substituted compounds using polymer-anchored Cu(II) catalyst. This catalyst showed excellent catalytic activity, high selectivity, and recyclability. The polymer-anchored Cu(II) catalyst could be easily recovered by filtration and reused more than five times without appreciable loss of its initial activity. © 2011 Published by Elsevier Ltd. Source

Hossain W.,Malda College | Ghosh M.,IACS | Sinha C.,Jadavpur University | Debnath D.K.,Malda College | Sarkar U.K.,Malda College
Chemical Physics Letters | Year: 2013

Raman spectra and SERS of 1H-2(Phenylazo) imidazole (PaiH) adsorbed on silver nano particles are reported. Monomolecular layer is formed at a concentration of 5 × 10 6 M. A trans-to-cis isomerisation of PaiH is suggested by the cis-signature peak at 570 cm 1. In absorption spectra a single π-π* band at 358 nm is observed at higher concentrations whereas the π-π* and the π-π* bands appear at 370 nm and 456 nm, respectively, at a concentration of 5 × 10 6 M. This is in support of the hypothesis of trans-to-cis-isomerisation with lowering of concentration. DFT calculations are shown. © 2013 Elsevier B.V. All rights reserved. Source

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