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Composition dependent crossover from ferroelectric to relaxor-ferroelectric in NBT-ST-KNN ceramics
Singha A.,Praharaj S.,Rout S.K.,Rout D. 한국물리학회 2022 Current Applied Physics Vol.36 No.-
Composition driven crossover from ferroelectric (FE) to relaxor-ferroelectric (RFE) is systematically investigated in (0.8-x)(Na0.5Bi0.5)TiO3-0.2SrTiO3-x(K0.5Na0.5)NbO3 (NBT-ST-xKNN) ceramics in the light of ferroelectric and dielectric (ε) response. The appearance of constricted polarization-electric field loop and double current densityelectric field peaks in the proximity of x = 0.02 suggests a co-existence of FE and RFE phases. The system attains a maximumε ~4200 and tan δ < 0.1 at x = 0.02. Broad ε(T) peaks reveal a greater degree of disorder due to KNN substitution and their deconvolution demonstrates three different relaxation/polarization processes related to low and high-temperature nanodomains. The composition with x = 0.1 exhibits maximum broadness in ε(T) profiles resulting in a temperature-insensitive permittivity variation of ±15% (ε125 ◦C,100 kHz~2007) over a span of ~280 ◦ C. A remarkably low (TCε)100kHz of − 75 ppm/◦ C is also achieved for x = 0.1 from 100 to 300 ◦C proposing it to be a potential capacitor material.
STUDY OF ULTRALUMINOUS X-RAY SOURCES IN SOME NEARBY GALAXIES
Akram Chandrajit Singha,A Senorita Devi 한국천문학회 2019 Journal of The Korean Astronomical Society Vol.52 No.1
We present the results of the spectral and temporal analysis of eight X-ray point sources in five nearby (distance $< 20$ Mpc) galaxies observed with {\it Chandra}. For spectral analysis, an absorbed powerlaw and an absorbed diskblackbody were used as empirical models. Six sources were found to be equally fitted by both the models while two sources were better fitted by the powerlaw model. Based on model parameters, we estimate the X-ray luminosity of these sources in the energy range $0.3 - 10.0$ keV, to be of the order of $\sim 10^{39} ergs\ s^{-1}$ except for one source (X-8) with $L_{X} > 10^{40} ergs\ s^{-1}$. Five of these maybe classified as Ultraluminous X-ray sources (ULXs) with powerlaw photon index within the range, $\Gamma \sim 1.63-2.63$ while the inner disk temperature, kT $\sim 0.68-1.93$ keV, when fitted with the disk blackbody model. The black hole masses harboured by the X-ray point sources were estimated using the disk blackbody model to be in the stellar mass range, however, the black hole mass of one source (X-6) lies within the range $68.37M_{\odot} \le M_{BH} \le 176.32 M_{\odot}$, which at the upper limit comes under the Intermediate mass black hole range. But if the emission is considered to be beamed by a factor $\sim 5$, the black hole mass reduces to $\sim 75M_{\odot}$. The timing analysis of these sources does not show the presence of any short term variations in the kiloseconds timescales.
Singh, Priya,Sharma, Veerendra Kumar,Singha, Subhankar,Garcí,a Sakai, Victoria,Mukhopadhyay, Ramaprosad,Das, Ranjan,Pal, Samir Kumar American Chemical Society 2019 Langmuir Vol.35 No.13
<P>The maintenance of cell membrane fluidity is of critical importance for various cellular functions. At lower temperatures when membrane fluidity decreases, plants and cyanobacteria react by introducing unsaturation in the lipids, so that the membranes return to a more fluidic state. To probe how introduction of unsaturation leads to reduced membrane fluidity, a model cationic lipid dioctadecyldimethylammonium bromide (DODAB) has been chosen, and the effects of an unsaturated lipid monoolein (MO) on the structural dynamics and phase behavior of DODAB have been monitored by quasielastic neutron scattering and time-resolved fluorescence measurements. In the coagel phase, fluidity of the lipid bilayer increases significantly in the presence of MO relative to pure DODAB vesicles and becomes manifest in significantly enhanced dynamics of the constituent lipids along with faster hydration and orientational relaxation dynamics of a fluorophore. On the contrary, MO restricts both lateral and internal motions of the lipid molecules in the fluid phase (>330 K), which is consistent with relatively slow hydration and orientational relaxation dynamics of the fluorophore embedded in the mixed lipid bilayer. The present study illustrates how incorporation of an unsaturated lipid at lower temperatures (below the phase transition) assists the model lipid (DODAB) in regulating fluidity via enhancement of dynamics of the constituent lipids.</P> [FIG OMISSION]</BR>