Department of Physics

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Aqueous dispersion of 4-8 nm size stable ZnO quantum dots (QDs) exhibiting luminescence in the visible region have been synthesized by a simple solution growth technique at room temperature. Silica has been used as capping agent to control the particle size as well as to achieve uniform dispersion of QDs in aqueous medium. X-ray diffractometer (XRD) analysis reveals formation phase pure ZnO particles having wurzite (hexagonal) structure. Atomic force microscope (AFM) images show that the particles are spherical in shape, having average crystalline sizes ∼4, 5.5 and 8 nm for samples prepared at pH values of 10, 12 and 14, respectively. From the optical absorption studies, the band gap energy of QDs is found to be blue shifted as compared to bulk ZnO (3.36 eV) due to the quantum confinement effect and is consistent with the band gap calculated by using effective-mass approximation model. The photoluminescence (PL) observed in these QDs has been attributed to the presence of defect centers. © 2008 Elsevier B.V. All rights reserved.

World University root node

Journal of Luminescence

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

The filler alloy of nominal composition Cu-40Mn-10Ni (all in wt%) was prepared in the form of ribbon of 40 μm thick by melt spinning technique. The ribbon exhibits narrow melting zone and comprise single phase of Cu-Mn-Ni solid solution. The melt spun ribbon successfully brazed 304 stainless steel butt joints. The formation of solid solution in the joining area without any intermetallics is observed. The bonding strength of filler alloy is achieved around 456 MPa. © 2009 Elsevier B.V. All rights reserved.

Materials Science and Engineering A

Development of a copper-based filler alloy for brazing stainless steels

Large area arrays of highly oriented sulfur (S2-) doped ZnO nanorods are grown on silica glass substrate by wet chemical decomposition of zinc-thiourea precursor in an aqueous medium. A series of five samples were synthesized by varying the concentrtaions of sulfur ions in solution. The synthesized materials were studied by using XRD, SEM, AFM, UV-ViS spectroscopy and photoluminescence techniques. Microscopy studies reveal the formation of conical nanorods having length ∼3.0μm with tip size ∼ 50nm. XRD studies of these nanorods show formation of hexagonal phase of pure ZnO with c-axis preferred orientation. Reduction in band gap energy from 3.2 to 2.9 eV was observed from optical absorption studies when different mole concentrations of thourea was varied in the precursor solution. The photoluminiscence efficiency of the materials was found to decrease with increase in the concentartion of sulfur ions in ZnO nanorods. © (2009) Trans Tech Publications.

Advanced Materials Research

Growth and studies of S2- doped ZnO nanorods by solution growth technique

We report on the electric-field tuning of a magnetic phase transition temperature (TL) in multiferroic Ni3V2O 8 thin films. The simultaneous magnetic and ferroelectric transition in Ni3V2O8 exhibits a clear dielectric anomaly; we monitored TL under applied electric and magnetic fields using dielectric measurements. The transition temperature increases by 0.2 K±0.05 K when the sample is biased approximately 25 MV/m compared to zero bias. This electric-field control of the magnetic transition can be qualitatively understood using a mean-field model incorporating a tri-linear coupling between the magnetic order parameters and spontaneous polarization. The shape of the electric field-temperature phase boundary is consistent with the proper order parameter for the multiferroic phase in Ni3V 2O8 being a linear combination of the magnetic and ferroelectric correlation functions. © EPLA, 2009.

Fulltext

Electric-field control of a magnetic phase transition in Ni 3V2O8

We develop an information theoretic interpretation of the number-phase complementarity in atomic systems, where phase is treated as a continuous positive operator valued measure (POVM). The relevant uncertainty principle is obtained as an upper bound on a sum of knowledge of these two observables for the case of two-level systems. A tighter bound characterizing the uncertainty relation is obtained numerically in terms of a weighted knowledge sum involving these variables. We point out that complementarity in these systems departs from mutual unbiasededness in two significant ways: first, the maximum knowledge of a POVM variable is less than log (dimension) bits; second, surprisingly, for higher dimensional systems, the unbiasedness may not be mutual but unidirectional in that phase remains unbiased with respect to number states, but not vice versa. Finally, we study the effect of non-dissipative and dissipative noise on these complementary variables for a single-qubit system. © 2009 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

European Physical Journal D

Complementarity in atomic (finite-level quantum) systems: An information-theoretic approach

We have studied phase transitions in multiferroic Ni3V2O8, through dielectric measurements and by investigating the effects of Zn doping. The divergence of the dielectric constant near TL = 6.4 K can be approximately fit by power law behaviour, but exhibits different critical exponents above and below TL. The two higher temperature magnetic transitions in (Ni1-xZnx)3V2O8, are strongly suppressed as the Zn doping fraction x is increased. The rates of the suppression of both TH and TL with x are consistent with two-dimensional spin systems, with the suppression of TH consistent with Ising spins and the suppression of TL consistent with Heisenberg spins.

Philosophical Magazine

Scaling behaviour of magnetic transitions in Ni3V2O8

We investigate the transition of a radiatively inefficient phase of a viscous two temperature accreting flow to a cooling dominated phase and vice versa around black holes. Based on a global sub-Keplerian accretion disk model in steady state, including explicit cooling processes self-consistently, we show that general advective accretion flow passes through various phases during its infall towards a black hole. Bremsstrahlung, synchrotron and inverse Comptonization of soft photons are considered as possible cooling mechanisms. Hence the flow governs a much lower electron temperature ∼ 108 - 109.5 K compared to the hot protons of temperature ∼ 10 10 2 - 1011.8 K in the range of the accretion rate in Eddington units 0.01 ≲ Ṁ ≲ 100. Therefore, the solutions may potentially explain the hard X-rays and the γ-rays emitted from AGNs and X-ray binaries. We finally compare the solutions for two different regimes of viscosity and conclude that a weakly viscous flow is expected to be cooling dominated compared to its highly viscous counterpart which is radiatively inefficient. The flow is successfully able to reproduce the observed luminosities of the under-fed AGNs and quasars (e.g. Sgr A*), ultra-luminous X-ray sources (e.g. SS433), as well as the highly luminous AGNs and ultra-luminous quasars (e.g. PKS 0743-67) at different combinations of the mass accretion rate and ratio of specific heats. © 2009 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd.

Research in Astronomy and Astrophysics

Transition from radiatively inefficient to cooling dominated phase in two temperature accretion disks around black holes

A novel method is reported here for the synthesis of optically clear and stable colloidal solutions of silver nanoparticles. According to size they show different colours depending upon their plasmonic absorption frequencies. The materials have been synthesized at room temperature by chemical reduction of silver ions (silver nitrate) coordinated with dendrigraft polymer, polyethyleneimine (PEI) using formaldehyde in aqueous medium. UV-vis absorption and transmission electron microscopy (TEM) studies show single-band absorption with peak maximum at 354 nm for ∼3 nm sized particles, whereas a side band at ∼400 nm was observed when the particle size increased to ∼20 nm. Highly narrow particle size distribution was observed in case of samples having ∼3 nm size silver particles and also the process of reduction could be completed within minutes. More interestingly, the 3-nm sized particles showed strong blue (474 nm) fluorescence under UV excitation. Thin films of all synthesized samples were prepared on silica substrate by fine spray coating technique. © 2008 Elsevier Ltd. All rights reserved.

Materials Research Bulletin

Dendrigraft polymer-based synthesis of silver nanoparticles showing bright blue fluorescence

We have probed changes in the magnetic, electrical, dielectric, optical, and thermodynamic properties of iron vanadate (FeVO4) at two magnetic phase transitions. FeVO4 exhibits two antiferromagnetic transitions at TN1 = 22K and TN2 = 15K. Below 15K FeVO4 develops an electric polarization, concomitant with the second antiferromagnetic transition and indicating strong magnetoelectric coupling. The powder averaged zero field electric polarization for the polycrystalline FeVO4 sample is 6μCm-2 and can be switched by reversing the poling voltage. The peaks for certain Raman modes at larger wavenumbers shift to slightly higher energies in the temperature range between TN1 and TN2, but there is practically no change in the Raman spectra between the paramagnetic and ground states. These Raman features help to clarify the microscopic mechanisms for magnetoelectric coupling in FeVO4. © 2009 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Development of electrical polarization at an antiferromagnetic transition in FeVO4

Pierre Auger experiment has detected at least a couple of cosmic ray events above energy 60 EeV from the direction of the radio-galaxy Centaurus A. Assuming those events are from Centaurus A, we have calculated the numbers of neutral cosmic ray events from this source for small values of the degree of violation in Lorentz invariance. Our results show that a comparison of our calculated numbers of events with the observed number of events at EeV energy from the direction of the source can probe extremely low value of the degree of this violation. © 2009 IOP Publishing Ltd and SISSA.