Simulations of Oscillation Modes of the Solar Convection Zone

We use the three-dimensional hydrodynamic code of Stein and Nordlund to realistically simulate the upper layers of the solar convection zone in order to study physical characteristics of solar oscillations. Our first result is that the properties of oscillation modes in the simulation closely match the observed properties. Recent observations from SOHO/MDI and GONG have confirmed the asymmetry of solar oscillation line profiles, initially discovered by Duvall et al. In this paper we compare the line profiles in the power spectra of the Doppler velocity and continuum intensity oscillations from the SOHO/MDI observations with the simulation. We also compare the phase differences between the velocity and intensity data. We have found that the simulated line profiles are asymmetric and have the same asymmetry reversal between velocity and intensity as observed. The phase difference between the velocity and intensity signals is negative at low frequencies and jumps in the vicinity of modes as is also observed. Thus, our numerical model reproduces the basic observed properties of solar oscillations, and allows us to study the physical properties which are not observed.Comment: Accepted for publication in ApJ Letter

Analytical models for Cross-correlation signal in Time-Distance Helioseismology

In time-distance helioseismology, the time signals (Doppler shifts) at two points on the solar surface, separated by a fixed angular distance are cross-correlated, and this leads to a wave packet signal. Accurately measuring the travel times of these wave packets is crucial for inferring the sub-surface properties in the Sun. The observed signal is quite noisy, and to improve the signal-to-noise ratio and make the cross-correlation more robust, the temporal oscillation signal is phase-speed filtered at the two points in order to select waves that travel a fixed horizontal distance. Hence a new formula to estimate the travel times is derived, in the presence of a phase speed filter, and it includes both the radial and horizontal component of the oscillation displacement signal. It generalizes the previously used Gabor wavelet that was derived without a phase speed filter and included only the radial component of the displacement. This is important since it will be consistent with the observed cross-correlation that is computed using a phase speed filter, and also it accounts for both the components of the displacement. The new formula depends on the location of the two points on the solar surface that are being cross correlated and accounts for the travel time shifts at different locations on the solar surface.Comment: 27 pages, 8 figures, ApJ in pres

Lignin Reinforced Rubber Composites

Lignin is the country's second most abundant renewable biomass resource next to cellulose. The pulp and paper industries produce very large quantities of lignin, most of those are burned to recover energy, pulping chemicals, enzymatic or acid hydrolysis to sugars followed by fermentation to alcohols. Another emerging technology where the lignin being used is in the adhesives and asphalts. For the former, lignin partly replaces phenol in phenol-formaldehyde formulations, for the latter lignin is used as an extender. Lignin in polyurethanes is for good mechanical properties and reactions of lignin such as grafting and crosslinking agent are also well known. Novolak-hexamine based phenolic resins are commonly used as reinforcing and processing aids in nitrile rubber (NBR) compounds. Not only the oil and petrol resistances increase significantly , resin loading is also found to offer better heat resistant properties than carbon blacks. For seals, valves and gasket applications addition of phenolic resins provides superior abrasion resistance, ageing and negligible hardening effects at elevated temperatures. Poor tackiness of NBR compounds can easily be eliminated by using phenolic resin in place of carbon black. The study presented here is to explore the possibility of employing lignin into some value -added rubber based composites . Lignin, gymnosperms, angiosperms and glass varieties, contain alcohol and phenolic groups and also double bonds in their structural moiety and therefore, could resinify into prepolymer in presence of hexamine.Varied proportions of lignin upto 50 parts with respect to total rubber plus hexamine,have been incorporated in medium acrylonitrile (37%) NBR and studied for their processing characteristics and physico-mechanical properties e.g. reinforcement, oil and fuel resistances, ageing and thermal stability. Efficacy of lignin has been found to be superior to either phenolic resin-hexamine or carbon black filled compounds. Finally, attempt has also been made to modify the surface chemistry incorporating CO,> C=O etc. groups by additions of dicumyl peroxide and this modified lignin was found to offer improved rubber-filler adhesion, tackiness and physical properties

Role of Fe substitution on the anomalous magnetocaloric and magnetoresistance behavior in Tb(Ni1-xFex)2 compounds

We report the magnetic, magnetocaloric and magnetoresistance results obtained in Tb(Ni1-xFex)2 compounds with x=0, 0.025 and 0.05. Fe substitution leads to an increase in the ordering temperature from 36 K for x=0 to 124 K for x=0.05. Contrary to a single sharp MCE peak seen in TbNi2, the MCE peaks of the Fe substituted compounds are quite broad. We attribute the anomalous MCE behavior to the randomization of the Tb moments brought about by the Fe substitution. Magnetic and magnetoresistance results seem to corroborate this proposition. The present study also shows that the anomalous magnetocaloric and magnetoresistance behavior seen in the present compounds is similar to that of Ho(Ni,Fe)2 compounds

Origin of spin gapless semiconductor behavior in CoFeCrGa: theory and experiment

Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), which offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by electronic-structure calculations, we report evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (prototype LiMgPdSn) structure but exhibiting chemical disorder (DO3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (M-S) obtained at 8K agrees with the Slater-Pauling rule and the Curie temperature (T-C) is found to exceed 400 K. Carrier concentration (up to 250 K) and electrical conductivity are observed to be nearly temperature independent, prerequisites for SGS. The anomalous Hall coefficient is estimated to be 185 S/cm at 5K. Considering the SGS properties and high T-C, this material appears to be promising for spintronic applications.Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), which offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by electronic-structure calculations, we report evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (prototype LiMgPdSn) structure but exhibiting chemical disorder (DO3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (M-S) obtained at 8K agrees with the Slater-Pauling rule and the Curie temperature (T-C) is found to exceed 400 K. Carrier concentration (up to 250 K) and electrical conductivity are observed to be nearly temperature independent, prerequisites for SGS. The anomalous Hall coefficient is estimated to be 185 S/cm at 5K. Considering the SGS properties and high T-C, this material appears to be promising for spintronic applications.92415Agências de fomento estrangeiras apoiaram essa pesquisa, mais informações acesse artig

Field dependent competing magnetic ordering in multiferroic Ni3V2O8

The geometrically frustrated magnet Ni3V2O8 undergoes a series of competing magnetic ordering at low temperatures. Most importantly, one of the incommensurate phases has been reported to develop a ferroelectric correlation caused by spin frustration. Here we report an extensive thermodynamic, dielectric and magnetic study on clean polycrystalline samples of this novel multiferroic compound. Our low temperature specific heat data at high fields up to 14 Tesla clearly identify the development of a new magnetic field induced phase transition below 2 K that shows signatures of simultaneous electric ordering. We also report temperature and field dependent dielectric constant that enables us to quantitatively estimate the strength of magneto-electric coupling in this improper ferroelectric material.Comment: 18 pages, 4 figures. Accepted for publication in Euro. Phys. Let