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Lithium isotopic composition of benthic foraminifera: A new proxy for paleo-pH reconstruction
No full textThe lithium isotopic composition of foraminifera is an established tracer of long-term changes in the global silicate weathering cycle, following the assumption that foraminifera faithfully record the lithium isotopic composition (delta Li-7) of seawater. In this study, we demonstrate by utilising benthic foraminifera (Amphistegina lessonii) that were cultured under decoupled pH-CO32-] conditions, that foraminifera delta Li-7 is strongly dependent on pH. This is reinforced with delta Li-7 data from globally distributed core-top samples of Cibicidoides mundulus and Cibicidoides wuellerstorfi, which show the same negative correlation with pH. The dependency of delta Li-7 on pH is perhaps a surprising result given that lithium speciation in seawater is independent of both pH and carbonate ion speciation. The dependence of lithium incorporation on growth rate was assessed by measuring the calcium isotopic composition; no growth rate dependent incorporation was observed. Instead, we propose that the strength of the Li-6 and Li-7 hydration spheres (and hence their respective desolvation energy) is pH-dependent, resulting in a significant isotopic fractionation during the incorporation of lithium into foraminifer calcite. The core-top derived delta Li-7-pH calibration is used to demonstrate the applicability of this delta B-11-independent pH proxy in reconstructing deglacial variations in pH in the South Pacific. The use of foraminifera delta Li-7 to compliment delta B-11-based pH reconstructions has the potential to provide insight into time-dependent variations in porewater/seawater delta B-11, temperature and salinity, which were previously unresolvable. (C) 2018 Elsevier Ltd. All rights reserved
Enhanced coherence and decoupled surface states in topological insulators through structural disorder
No full textTo harness the true potential of topological insulators as quantum materials for information processing, it is imperative to maximise topological surface state conduction while simultaneously improving their quantum coherence. However, these goals have turned out to be contradictory. Surface dominated transport in topological insulators has been achieved primarily through compensation doping of bulk carriers which introduces tremendous electronic disorder and drastically deteriorates electronic coherence. In this work, we use structural disorder instead of electronic disorder to manipulate the electrical properties of thin films of topological insulator Bi2Se3. We achieve decoupled surface state transport in our samples and observe significantly suppressed carrier dephasing rates in the coupled surface state regime. As the film thickness is decreased, the dephasing rate evolves from a linear to a super-linear temperature dependence. While the former is consistent with Nyquist electron-electron interactions, the latter leads to significantly enhanced coherence at low temperatures and is indicative of energy exchange due to frictional drag between the two surface states. Our work opens up the way to harness topological surface states, without being afflicted by the deleterious effects of compensation doping. Published by AIP Publishing
Proteomic changes during adult stage in pre-optic, hypothalamus, hippocampus and pituitary regions of female rat brain following neonatal exposure to estradiol-17 beta
No full textAlthough neonatal exposure to estrogen or estrogenic compounds results in irreversible changes in the brain function and reproductive abnormalities during adulthood but the underlying mechanisms are still largely unknown. The present study has attempted to compare the protein profiles of sexually dimorphic brain regions of adult female rats which were exposed to estradiol- 17 beta during neonatal period. The total proteins extracted from pre-optic area (POA), hypothalamus, hippocampus and pituitary of control and neonatally E2 treated female rats was subjected to 2D-SDS-PAGE and differentially expressed proteins were identified by MALDI TOF/TOF-MS. Our results revealed that a total of 21 protein spots which were identified as differentially expressed in all the four regions analyzed; the differential expression was further validated by RT-PCR and western blotting. The differentially expressed proteins such as 14-3-3 zeta/delta (POA), LMNA (hippocampus), Axin2 (hypothalamus), Syntaxin-7 (hippocampus), prolactin and somatotropin (pituitary) which have very important functions in the process of neuronal differentiation, migration, axon outgrowth, formation of dendritic spine density and synaptic plasticity and memory have not been previously reported in association with neonatal estrogen exposure. The affected brain functions are very important for the establishment of sex specific brain morphology and behavior. Our results suggest that the differentially expressed proteins may play an important role in irreversible changes in the brain function as well as reproductive abnormalities observed in the female rats during adulthood
New frequency-domain (2)-stability criteria for nonlinear MIMO discrete-time systems with constant and varying time delays
No full textNew frequency-domain 2-stability criteria are derived for nonlinear discrete-time MIMO systems, having a linear time-invariant block with the transfer function (z), in negative feedback with an aperiodic matrix gain A(k), k = 0, 1, 2,..., and a linear combination of a vector of certain classes of (generalised) first-and-third-quadrant non-monotone nonlinearities.(center dot), having arguments with constant and time-varying delays, but without restrictions on their slopes. The framework does not employ Lyapunov-Krasovskii functionals involving linear matrix inequalities (LMIs) or their equivalent. The new stability criteria seem to be the most general for nonlinear and time variying time-delay systems, and have the following structure: (1) positive definiteness of the real part (as evaluated on | z| = 1) of the product of (z) and an algebraic sum of general causal and anticausal matrix multiplier functions of z. (2) An upper bound on the L1-norm of the inverse Fourier transform of the multiplier function, the L1-norm being weighted by certain novel, quantitative characteristic parameters (CPs) of the nonlinearities.(center dot) without the asumptions of monotonicity, slope restrictions and the like. And (3) constraints on certain global averages of the generalised eigenvalues of (A(k + 1), A(k)), k = 1, 2,..., that are expressed in terms of (i) CPs of the nonlinearities, (ii) their coefficients, and, in general, (iii) time-delays in their arguments, a trade-off among all the three being possible. These global averages imply a restriction on the rate of variation of A(k) in a new sense. The literature results turn out to be special cases of the results of the present paper. Examples illustrate the stability theorems
Isoform-specific hyperactivation of calpain-2 occurs presymptomatically at the synapse in Alzheimer's disease mice and correlates with memory deficits in human subjects
No full textCalpain hyperactivation is implicated in late-stages of neurodegenerative diseases including Alzheimer's disease (AD). However, calpains are also critical for synaptic function and plasticity, and hence memory formation and learning. Since synaptic deficits appear early in AD pathogenesis prior to appearance of overt disease symptoms, we examined if localized dysregulation of calpain-1 and/or 2 contributes to early synaptic dysfunction in AD. Increased activity of synaptosomal calpain-2, but not calpain-1 was observed in presymptomatic 1 month old APP(swe)/PS1.E9 mice (a mouse model of AD) which have no evident pathological or behavioural hallmarks of AD and persisted up to 10 months of age. However, total cellular levels of calpain-2 remained unaffected. Moreover, synaptosomal calpain-2 was hyperactivated in frontal neocortical tissue samples of post-mortem brains of AD-dementia subjects and correlated significantly with decline in tests for cognitive and memory functions, and increase in levels of beta-amyloid deposits in brain. We conclude that isoform-specific hyperactivation of calpain-2, but not calpain-1 occurs at the synapse early in the pathogenesis of AD potentially contributing to the deregulation of synaptic signaling in AD. Our findings would be important in paving the way for potential therapeutic strategies for amelioration of cognitive deficits observed in ageing-related dementia disorders like AD
Magnetic field dependent steady-state shear response of Fe3O4 micro-octahedron based magnetorheological fluids
No full textWe report the synthesis of single crystalline octahedron-shaped magnetite microcrystals, the preparation of magnetorheological fluids (MRFs) and their magnetorheological properties under steady-state shear conditions. The magnetite microcrystals were synthesized via the template-free hydrothermal route. MRFs with three different particle concentrations (10, 20 and 40 weight%) were prepared and were subjected to steady shear conditions at various externally applied magnetic fields of strength up to 1.2 T. The shear rates were chosen up to high enough values to observe the yield behaviour of the MRFs. The dynamic yield strengths of MRFs, estimated using the Bingham plastic model fit to the steady-state shear response curves, showed that they scale-up with the applied magnetic field strength and amount of magnetic particles in the fluid. The origin of the mechanical strength in the MRFs due to the inter-particle interaction is explained using a simple dipolar model. The observed high yield strengths of the MRFs were explained on the basis of the particle shape (octahedrons) and magnetic nature (saturation magnetization). By comparing the values of the yield strength with the on-state to off-state viscosity ratio for the MRFs (for each particle concentration), an optimum content of particles in the carrier fluid to obtain high efficiency is suggested. Because the particles are single crystalline, the off-state viscosity of the MRFs even at the highest studied (40 wt%) particle concentration was very low, which is ideal for their application as quickly responding MRFs
Kinetics of intrasubband electron energy relaxation in quantum wells in a quantizing magnetic field
No full textHerein we report the intrasubband relaxation of electron energy in the Landau level system of quantum well as a function of magnetic field strength. The relative role of scattering processes contributing to relaxation kinetics, the electron-electron scattering which redistributes the electrons between Landau levels and the optical phonon emission by the electrons reaching the Landau levels lying near and above the optical phonon energy is revealed. It was shown the most important factor determining the kinetics of energy relaxation is the electron-electron scattering processes, delivering electrons to Landau levels close to optical phonon energy. The flux of these electrons depends on the number of the Landau levels lying below the energy of the optical phonon and increases substantially with a decrease in this number. On the contrary, the electron-phonon scattering rate dependence of the energy relaxation time was found to be much weaker
Deformation Behavior of AM30 Magnesium Alloy
No full textIn this study, deformation behavior of AM30 magnesium alloy is investigated in the hot working regime and empirical relations are developed. Compression tests were performed in the 0.001-10s(-1) strain rate range and 423-623K temperature range to obtain the mechanical properties. The compression curves were fitted using two approaches, viz. piecewise linear fitting and a Johnson-Cook type relation. Piecewise linear fit is found to be appropriate for describing the mechanical behavior in the dynamic recrystallization (DRX) regime as compared to the proposed Johnson-Cook type relation. The Johnson-Cook type model is not able to capture the essential feature of DRX, particularly at high strain rates
Physical Insight into the Mechanism of Electromagnetic Shielding in Polymer Nanocomposites Containing Multiwalled Carbon Nanotubes and Inverse-Spinel Ferrites
No full textA surge in the usage of electronic devices has led to a new kind of problem; electromagnetic (EM) interference. In a quest toward providing effective shielding, which offers design flexibility, lightweight, and ease to integrate and embed, the right combination of materials needs to be synthesized and dispersed in a polymer matrix to design composites that can shield EM radiation. However, selection of nanoparticles from a vast library is quite challenging and, hence, this study attempts to provide a physical insight into the mechanism of shielding in polymer nanocomposites containing a conducting phase (here multi-walled carbon nanotubes, MWCNTs) and a magnetic phase here inverse-spinel ferrites, MFe2O4 (M = Fe, Co, Ni)]. We adopted a biphasic co-continuous blend (consisting of polycarbonate and polyvinylidene fluoride) as the matrix to incorporate the conducting and the magnetic phases. MWENTs, which offer interconnected conductive fence, and ferrites, which provide magnetic dipoles that couple with incoming EM radiation, can absorb the incoming EM radiation. The detailed mechanistic insight regarding absorption of EM radiation reveals that high saturation magnetization, high consolidated loss, better impedance matching, higher attenuation constant, high hysteresis loss, and comparable eddy current loss help Fe3O4, compared to the other ferrites employed here, to effectively shield the EM wave in the X and Ku band frequency through absorption. In addition, better impedance matching, low skin depth, and enhanced dielectric and/or interfacial polarization losses because of pi-electrons in MWCNTs suggest a synergistic effect from both the phases. As a result, -31 dB shielding effectiveness is observed in the case of Fe2O4 and MWCNTs, which is 19% higher when compared with CoFe2O4 + MWCNT-containing blends and 24% higher when compared with NiFe2O4 + MWCNT-containing blends. Interestingly, when the nanoparticles are forced to localize in different components of the blends, the overall shielding efficiency enhances further because of their higher consolidated loss parameters. Hence, the mechanistic insight provided in this paper will help guide researchers working in this field from both academic and industry perspectives
Insights into the dynamics of conical breakdown modes in coaxial swirling flow field
No full textThe main idea of this paper is to understand the fundamental vortex breakdown mechanisms in the coaxial swirling flow field. In particular, the interaction dynamics of the flow field is meticulously addressed with the help of high fidelity laser diagnostic tools. Time-resolved particle image velocimetry (PIV) (similar to 1500 frames s(-1)) is employed in y-r and multiple r-theta planes to precisely delineate the flow dynamics. Experiments are carried out for three sets of co-annular flow Reynolds number Re-a = 4896, 10 545, 17 546. Furthermore, for each Re-a condition, the swirl number `SG' is varied independently from 0 <= S-G <= 3. The global evolution of flow field across various swirl numbers is presented using the time-averaged PIV data. Three distinct forms of vortex breakdown namely, pre-vortex breakdown (PVB), central toroidal recirculation zone (CTRZ; axisymmetric toroidal bubble type breakdown) and sudden conical breakdown are witnessed. Among these, the conical form of vortex breakdown is less explored in the literature. In this paper, much attention is therefore focused on exploring the governing mechanism of conical breakdown. It is should be interesting to note that, unlike other vortex breakdown modes, conical breakdown persists only for a very short band of S-G. For any small increase/decrease in S-G beyond a certain threshold, the flow spontaneously reverts back to the CTRZ state. Energy ranked and frequency-resolved/ranked robust structure identification methods - proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) respectively - are implemented over instantaneous time-resolved PIV data sets to extract the dynamics of the coherent structures associated with each vortex breakdown mode. The dominant structures obtained from POD analysis suggest the dominance of the Kelvin-Helmholtz (KH) instability (axial + azimuthal; accounts for similar to 80 % of total turbulent kinetic energy, TKE) for both PVB and CTRZ while the remaining energy is contributed by shedding modes. On the other hand, shedding modes contribute the majority of the TKE in conical breakdown. The frequency signatures quantified from POD temporal modes and DMD analysis reveal the occurrence of multiple dominant frequencies in the range of similar to 10-400 Hz with conical breakdown. This phenomenon may be a manifestation of high energy contribution by shedding eddies in the shear layer. Contrarily, with PVB and CTRZ, the dominant frequencies are observed in the range of similar to 20-40 Hz only. We have provided a detailed exposition of the mechanism through which conical breakdown occurs. In addition, the current work explores the hysteresis (path dependence) phenomena of conical breakdown as functions of the Reynolds and Rossby numbers. It has been observed that the conical mode is not reversible and highly dependent on the initial conditions