Analogous stress and electric field driven structural transformation and decrease in polarization coherence on poling around the morphotropic phase boundary in BiScO3-PbTiO3

The nature of the stress and electric field driven structural and microstructural transformations in the morphotropic phase boundary (MPB) compositions of the high Curie point piezoelectric system BiScO3-PbTiO3 has been examined by ex situ based techniques. Using a powder poling technique, which is based on the concept of exploiting the irreversible structural change that occurs after the application of a strong electric field and stress independently, it was possible to ascertain that both moderate stress and electric field induce identical structural transformation-a fraction of the monoclinic phase transforms irreversibly to the tetragonal phase. Moreover, analysis of the dielectric response before and after poling revealed a counterintuitive phenomenon of poling induced decrease in the spatial coherence of polarization for compositions around the MPB and not so for compositions far away from the MPB range. Exploiting the greater sensitivity of this technique, we demonstrate that the criticality associated with the interferroelectric transition spans a wider composition range than what is conventionally reported in the literature based on bulk x-ray/neutron powder diffraction techniques.

Polarization switching and high piezoelectric response in Sn-modified BaTiO3

BaTiO3 is shown to exhibit anomalous piezoelectric response, comparable to that of lead-zirconate titanate, by dilute Sn modification (1-4 mol%). Using a newly discovered powder poling technique it is shown that the mechanism associated with this anomalous strain response involves electric-field-induced switching of polarization vector from 001] towards 101] pseudocubic direction. This switchability is significantly enhanced by tuning the tetragonal-orthorhombic first-order criticality near to room temperature.

Wave propagation of phonon and phason displacement modes in quasicrystals: Determination of wave parameters

The paper presents the study of wave propagation in quasicrystals. Our interest is in the computation of the wavenumber (k(n)) and group speed (c(g)) of the phonon and phason displacement modes of one, two, and three dimensional quasicrystals. These wave parameter expressions are derived and computed using the elasto-hydrodynamic equations for quasicrystals. For the computation of the wavenumber and group speeds, we use Fourier transform approximation of the phonon and the phason displacement modes. The characteristic equations obtained are a polynomial equation of the wavenumber (k(n)), with frequency as a parameter. The corresponding group speeds (c(g)) for different frequencies are then computed from the wavenumber k(n). The variation of wavenumber and group speeds with frequency is plotted for the 1-D quasicrystal, 2-D decagonal Al-Ni-Co quasicrystals, and 3-D icosahedral Al-Pd-Mn and Zn-Mg-Sc quasicrystals. From the wavenumber and group speeds plots, we obtain the cut-off frequencies for different spatial wavenumber eta(m). The results show that for 1-D, 2-D, and 3-D quasicrystals, the phonon displacement modes are non-dispersive for low values of eta(m) and becomes dispersive for increasing values of eta(m). The cut-off frequencies are not observed for very low values of eta(m), whereas the cut-off frequency starts to appear with increasing eta(m). The group speeds of the phason displacement modes are orders of magnitude lower than that of the phonon displacement modes, showing that the phason modes do not propagate, and they are essentially the diffusive modes. The group speeds of the phason modes are also not influenced by eta(m). The group speeds for the 2-D quasicrystal at 35 kHz is also simulated numerically using Galerkin spectral finite element methods in frequency domain and is compared with the results obtained using wave propagation analysis. The effect of the phonon and phason elastic constants on the group speeds is studied using 3-D icosahedral Al-Pd-Mn and Zn-Mg-Sc quasicrystals. It is also shown that the phason elastic constants and the coupling coefficient do not affect the group speeds of the phonon displacement modes. (C) 2015 AIP Publishing LLC.

Characterizing the Impact of Feedback Delays on Wideband Rate Adaptation

In contemporary orthogonal frequency division multiplexing (OFDM) systems, such as Long Term Evolution (LTE), LTE-Advanced, and WiMAX, a codeword is transmitted over a group of subcarriers. Since different subcarriers see different channel gains in frequency-selective channels, the modulation and coding scheme (MCS) of the codeword must be selected based on the vector of signal-to-noise-ratios (SNRs) of these subcarriers. Exponential effective SNR mapping (EESM) maps the vector of SNRs into an equivalent flat-fading SNR, and is widely used to simplify this problem. We develop a new analytical framework to characterize the throughput of EESM-based rate adaptation in such wideband channels in the presence of feedback delays. We derive a novel accurate approximation for the throughput as a function of feedback delay. We also propose a novel bivariate gamma distribution to model the time evolution of EESM between the times of estimation and data transmission, which facilitates the analysis. These are then generalized to a multi-cell, multi-user scenario with various frequency-domain schedulers. Unlike prior work, most of which is simulation-based, our framework encompasses both correlated and independent subcarriers and various multiple antenna diversity modes; it is accurate over a wide range of delays.

Structural investigation of resorcinol based symmetrical banana mesogens by XRD, NMR and polarization measurements

Synthesis and structural characterization of two novel symmetrical banana mesogens built from resorcinol with seven phenyl rings linked by ester and imine with a terminal dodecyl/dodecyloxy chain has been carried out. Density functional theory (DFT) has been employed for obtaining the geometry optimized structures, the dipole moments and C-13 NMR chemical shifts. The HOPM and DSC studies revealed enantiotropic B-2 and B-7 phases for the dodecyl and dodecyloxy homologs respectively. The powder X-ray studies of both the mesogens indicate the presence of layer ordering. The polarization measurements reveal an anti-ferroelectric switching for the B-2 phase of the dodecyl homolog whose structure has been identified as SmCSPA. The B-7 phase of the dodecyloxy homolog was found to be non-switchable. High resolution C-13 NMR study of the dodecyl homolog in its mesophase has been carried out. C-13-H-1 dipolar couplings obtained from the 2-dimensional separated local field spectroscopy experiment were used to obtain the orientational order parameters of the different segments of the mesogen. Very large C-13-H-1 dipolar couplings observed for the carbons of the central phenyl ring (9.7-12.3 kHz) in comparison to the dipolar couplings of those of the side arm phenyl rings (less than 3 kHz) are a direct consequence of the ordering in the banana phase and the shape of the molecule. From the ratio of the local order parameter values, the bent-angle of the mesogen could be determined in a straight forward manner to be 120.5 degrees.

Preferential polarization and its reversal in polycrystalline BiFeO3/La0.5Sr0.5CoO3 heterostructures

Polycrystalline BiFeO3 thin films were grown on La0.5Sr0.5CoO3 buffered Pt (200)/TiO2/SiO2/Si substrates under different oxygen partial pressures (10, 25, 50 and 100 mTorr) by puked laser ablation. Piezo-response Force Microscopy and Piezo-Force Spectroscopy have shown that all the films are ferroelectric in nature with locally switchable domains. It has also revealed a preferential downward domain orientation in as-grown films grown under lower oxygen partial pressure (10 and 25 mTorr) with a reversal of preferential domain orientation as the oxygen partial pressure is increased to 100 mTorr during laser ablation. Such phenomena are atypical of multi-grained polycrystalline ferroelectric films and have been discussed On the basis of detect formation with changing growth conditions. For the 50 mTorr grown film, asymmetric domain stability and retention during write-read studies has been observed which is attributed to grain-size-related defect concentration, affecting pinning centres that inhibit domain wall motion. (C) 2015 Elsevier Ltd. All rights reserved.

Acoustic response of vortex breakdown modes in a coaxial isothermal unconfined swirling jet

The present experimental work is concerned with the study of amplitude dependent acoustic response of an isothermal coaxial swirling jet. The excitation amplitude is increased in five distinct steps at the burner's Helmholtz resonator mode (i.e., 100 Hz). Two flow states are compared, namely, sub-critical and super-critical vortex breakdown (VB) that occur before and after the critical conical sheet breakdown, respectively. The geometric swirl number is varied in the range 2.14-4.03. Under the influence of external pulsing, global response characteristics are studied based on the topological changes observed in time-averaged 2D flow field. These are obtained from high resolution 2D PIV (particle image velocimetry) in the longitudinal-mid plane. PIV results also illustrate the changes in the normalized vortex core coordinates (r(vcc)/(r(vcc))(0) (Hz), y(vcc)/(y(vcc))(0) (Hz)) of internal recirculation zone (IRZ). A strong forced response is observed at 100 Hz (excitation frequency) in the convectively unstable region which get amplified based on the magnitude of external forcing. The radial extent of this forced response region at a given excitation amplitude is represented by the acoustic response region (b). The topological placement of the responsive convectively unstable region is a function of both the intensity of imparted swirl (characterized by swirl number) and forcing amplitude. It is observed that for sub-critical VB mode, an increase in the excitation amplitude till a critical value shifts the vortex core centre (particularly, the vortex core moves downstream and radially outwards) leading to drastic fanning-out/widening of the IRZ. This is accompanied by similar to 30% reduction in the recirculation velocity of the IRZ. It is also observed that b < R (R: radial distance from central axis to outer shear layer-OSL). At super-critical amplitudes, the sub-critical IRZ topology transits back (the vortex core retracts upstream and radially inwards) and finally undergoes a transverse shrinkage ((r(vcc))/(r(vcc))(0 Hz) decreases by similar to 20%) when b >= R. In contrast, the vortex core of super-critical breakdown mode consistently spreads radially outwards and is displaced further downstream. Finally, the IRZ fans-out at the threshold excitation amplitude. However, the acoustic response region b is still less than R. This is explained based on the characteristic geometric swirl number (S-G) of the flow regimes. The super-critical flow mode with higher S-G (hence, higher radial pressure drop due to rotational effect which scales as Delta P similar to rho u theta(2) and acts inwards towards the center line) compared to sub-critical state imposes a greater resistance to the radial outward spread of b. As a result, the acoustic energy supplied to the super-critical flow mode increases the degree of acoustic response at the pulsing frequency and energizes its harmonics (evident from power spectra). As a disturbance amplifier, the stronger convective instability mode within the flow structure of super-critical VB causes the topology to widen/fan-out severely at threshold excitation amplitude. (C) 2015 AIP Publishing LLC.

Reduced Look Ahead Orthogonal Matching Pursuit

Compressed Sensing (CS) is an elegant technique to acquire signals and reconstruct them efficiently by solving a system of under-determined linear equations. The excitement in this field stems from the fact that we can sample at a rate way below the Nyquist rate and still reconstruct the signal provided some conditions are met. Some of the popular greedy reconstruction algorithms are the Orthogonal Matching Pursuit (OMP), the Subspace Pursuit (SP) and the Look Ahead Orthogonal Matching Pursuit (LAOMP). The LAOMP performs better than the OMP. However, when compared to the SP and the OMP, the computational complexity of LAOMP is higher. We introduce a modified version of the LAOMP termed as Reduced Look Ahead Orthogonal Matching Pursuit (Reduced LAOMP). Reduced LAOMP uses prior information from the results of the OMP and the SP in the quest to speedup the look ahead strategy in the LAOMP. Monte Carlo simulations of this algorithm deliver promising results.

Majorana modes and transport across junctions of superconductors and normal metals

We study Majorana modes and transport in one-dimensional systems with a p-wave superconductor (SC) and normal metal leads. For a system with an SC lying between two leads, it is known that there is a Majorana mode at the junction between the SC and each lead. If the p-wave pairing Delta changes sign or if a strong impurity is present at some point inside the SC, two additional Majorana modes appear near that point. We study the effect of all these modes on the sub-gap conductance between the leads and the SC. We derive an analytical expression as a function of Delta and the length L of the SC for the energy shifts of the Majorana modes at the junctions due to hybridization between them; the shifts oscillate and decay exponentially as L is increased. The energy shifts exactly match the location of the peaks in the conductance. Using bosonization and the renormalization group method, we study the effect of interactions between the electrons on Delta and the strengths of an impurity inside the SC or the barriers between the SC and the leads; this in turn affects the Majorana modes and the conductance. Finally, we propose a novel experimental realization of these systems, in particular of a system where Delta changes sign at one point inside the SC.

Different Modes of Transactivation of Bacteriophage Mu Late Promoters by Transcription Factor C

Transactivator protein C is required for the expression of bacteriophage Mu late genes from lys, I, P and mom promoters during lytic life cycle of the phage. The mechanism of transcription activation of mom gene by C protein is well understood. C activates transcription at Pmom by initial unwinding of the promoter DNA, thereby facilitating RNA polymerase (RNAP) recruitment. Subsequently, C interacts with the (sic) subunit of RNAP to enhance promoter clearance. The mechanism by which C activates other late genes of the phage is not known. We carried out promoter-polymerase interaction studies with all the late gene promoters to determine the individual step of C mediated activation. Unlike at P-mom, at the other three promoters, RNAP recruitment and closed complex formation are not C dependent. Instead, the action of C at P-lys, P-I, and P-P is during the isomerization from closed complex to open complex with no apparent effect at other steps of initiation pathway. The mechanism of transcription activation of mom and other late promoters by their common activator is different. This distinction in the mode of activation (promoter recruitment and escape versus isomerization) by the same activator at different promoters appears to be important for optimized expression of each of the late genes.

Identification of dominant modes in random dynamical and aeroelastic systems

Identification of dominant modes is an important step in studying linearly vibrating systems, including flow-induced vibrations. In the presence of uncertainty, when some of the system parameters and the external excitation are modeled as random quantities, this step becomes more difficult. This work is aimed at giving a systematic treatment to this end. The ability to capture the time averaged kinetic energy is chosen as the primary criterion for selection of modes. Accordingly, a methodology is proposed based on the overlap of probability density functions (pdf) of the natural and excitation frequencies, proximity of the natural frequencies of the mean or baseline system, modal participation factor, and stochastic variation of mode shapes in terms of the modes of the baseline system - termed here as statistical modal overlapping. The probabilistic descriptors of the natural frequencies and mode shapes are found by solving a random eigenvalue problem. Three distinct vibration scenarios are considered: (i) undamped arid damped free vibrations of a bladed disk assembly, (ii) forced vibration of a building, and (iii) flutter of a bridge model. Through numerical studies, it is observed that the proposed methodology gives an accurate selection of modes. (C) 2015 Elsevier Ltd. All rights reserved.

Structural modification and enhanced magnetic properties with two phonon modes in Ca-Co codoped BiFeO3 nanoparticles

Bi1-xCaxFe1-xCoxO3 nanoparticles with x=0.0, 0.05, 0.10 and 0.15 were successfully synthesized by cost effective tartaric acid based sol gel route. The alkali earth metal Ca2+ ions and transition metal Co3+ ions codoping at A and B-sites of BiFeO3 results in structural distortion and phase transformation. Rietveld refinement of XRD patterns suggested the coexistence of rhombohedral and orthorhombic phases in codoped BiFeO3 samples. Both XRD and Raman scattering studies showed the compressive lattice distortion in the samples induced by codoping of Ca2+ and Co3+ ions. Two-phonon Raman spectra exhibited the improvement of magnetization in these samples. X-ray photoelectron spectroscopy (XPS) showed the dominancy of Fe3+ and Co3+ oxidation states along with the shifting of the binding energy of Bi 4f orbital which confirms the substitution Ca2+ at Bi-site. The magnetic study showed the enhancement in room temperature ferromagnetic behavior with co-substitution consistent with Rama analysis. The gradual change in line shape of electron spin resonance spectra indicated the local distortion induced by codoping. (C) 2015 Published by Elsevier Ltd and Techna Group S.r.l.

Thermalization of Green functions and quasinormal modes

We develop a new method to study the thermalization of time dependent retarded Green function in conformal field theories holographically dual to thin shell AdS Vaidya space times. The method relies on using the information of all time derivatives of the Green function at the shell and then evolving it for later times. The time derivatives of the Green function at the shell is given in terms of a recursion formula. Using this method we obtain analytic results for short time thermalization of the Green function. We show that the late time behaviour of the Green function is determined by the first quasinormal mode. We then implement the method numerically. As applications of this method we study the thermalization of the retarded time dependent Green function corresponding to a minimally coupled scalar in the AdS 3 and AdS 5 thin Vaidya shells. We see that as expected the late time behaviour is determined by the first quasinormal mode. We apply the method to study the late time behaviour of the shear vector mode in AdS 5 Vaidya shell. At small momentum the corresponding time dependent Green function is expected to relax to equilibrium by the shear hydrodynamic mode. Using this we obtain the universal ratio of the shear viscosity to entropy density from a time dependent process.

Looking for BSM physics using top-quark polarization and decay-lepton kinematic asymmetries

We explore beyond-standard-model (BSM) physics signatures in the l + jets channel of the t (t) over bar pair production process at the Tevatron and the LHC. We study the effects of BSM physics scenarios on the top-quark polarization and on the kinematics of the decay leptons. To this end, we construct asymmetries using the lepton energy and angular distributions. Further, we find their correlations with the top polarization, net charge asymmetry and top forward-backward asymmetry. We show that when used together, these observables can help discriminate effectively between SM and different BSM scenarios, which can lead to varying degrees of top polarization at the Tevatron as well as the LHC. We use two types of colored mediator models to demonstrate the effectiveness of proposed observables, an s-channel axigluon and a u-channel diquark.

Ferromagnetism and the effect of free charge carriers on electric polarization in the double perovskite Y2NiMnO6

The double perovskite Y2NiMnO6 displays ferromagnetic transition at T-c approximate to 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (Ms) and also validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P2(1)/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of the frequency-dependent dielectric constant and equivalent circuit analysis of impedance data take into account the bulk contribution to the total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T-c). Pyrocurrent measurements register a current flow with onset near T-c and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to reorientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material.