arXiv subjects

Ashok Kumar

Publications and source records attributed to Ashok Kumar.

At least 91 records · Page 5Linked to original sources

arXiv2015

Electron-Beam Driven Relaxation Oscillations in Ferroelectric Nanodisks

Using a combination of computational simulations, atomic-scale resolution imaging and phenomenological modelling, we examine the underlying mechanism for nanodomain restructuring in lead zirconate titanate (PZT) nanodisks driven by electron beams. The observed subhertz nanodomain dynamics are identified with relaxation oscillations where the charging/discharging cycle time is determined by saturation of charge traps and nanodomain wall creep. These results are unusual in that they indicate very slow athermal dynamics in nanoscale systems.

cond-mat.mtrl-sci↗

arXiv2015

Ferroelectric Capped Magnetization in Multiferroic PZT/LSMO Tunnel Junctions

Self-poled ultra-thin ferroelectric PbZr0.52Ti0.48O3 (PZT) (5 and 7 nm) films have been grown by pulsed laser deposition technique on ferromagnetic La0.67Sr0.33MnO3 (LSMO) (30 nm) to check the effect of polar capping on magnetization for ferroelectric tunnel junction (FTJ) devices. PZT/LSMO heterostructures with thick polar PZT (7 nm) capping show nearly 100% enhancement in magnetization compared with thin polar PZT (5 nm) films, probably due to excess hole transfer from the ferroelectric to the ferromagnetic layers. Core-level X-ray photoelectron spectroscopy studies revealed the presence of larger Mn 3s exchange splitting and higher Mn3+/Mn4+ ion ratio in the LMSO with 7 nm polar capping.

cond-mat.mtrl-sci↗

arXiv2014

Characterization of 3 mm Glass Electrodes and Development of RPC Detectors for $INO-ICAL$ Experiment

India-based Neutrino Observatory (INO) is a multi-institutional facility, planned to be built up in South India. The INO facility will host a 51 kton magnetized Iron CALorimeter (ICAL) detector to study atmospheric muon neutrinos. Iron plates have been chosen as the target material whereas Resistive Plate Chambers (RPCs) have been chosen as the active detector element for the ICAL experiment. Due to the large number of RPCs needed ($\sim$ 28,000 of $2~m \times 2~m$ in size) for ICAL experiment and for the long lifetime of the experiment, it is necessary to perform a detailed $R\&D$ such that each and every parameter of the detector performance can be optimized to improve the physics output. In this paper, we report on the detailed material and electrical properties studies for various types of glass electrodes available locally. We also report on the performance studies carried out on the RPCs made with these electrodes as well as the effect of gas composition and environmental temperature on the detector performance. We also lay emphasis on the usage of materials for RPC electrodes and the suitable enviormental conditions applicable for operating the RPC detector for optimal physics output at INO-ICAL experiment.

physics.ins-det↗

arXiv2014

Characterisation of Glass Electrodes and RPC Detectors for $INO-ICAL$ Experiment

India-based Neutrino Observatory (INO) is a planned neutrino experiment to be build up in southern part of India.The INO observatory will host a 51 kton Iron Calorimeter (ICAL) detector to detect atmospheric neutrinos. Resistive Plate Chamber (RPC) has been chosen as the active detector element for the ICAL experiment. The ICAL experiment will consist of about 28,000 RPC's of dimension $2~m\times 2~m$, divided into three modules. The experiment is planned to take data at least for 20 years from its start date. Due to the large number of RPC needed for ICAL experiment and the long lifetime of the experiment, it is necessary to carry out detailed $R\&D$ to optimise each and every parameter of the detector performance. We report on the performance studies carried out on the RPC's made with these electrodes, and finally compare the detector performance with that of the material properties to optimise the detector parameters.

physics.ins-det↗

arXiv2014

Study of RPC bakelite electrodes and detector performance for INO-ICAL

The Resistive Plate Chambers (RPCs) are going to be used as the active detectors in the India-based Neutrino Observatory (INO)-Iron Calorimeter (ICAL) experiment for the detection and study of atmospheric neutrinos. In this paper, an extensive study of structural and electrical properties for different kind of bakelite RPC electrodes is presented. RPCs fabricated from these electrodes are tested for their detector efficiency and noise rate. The study concludes with the variation of efficiency, leakage current and counting rate over the period of operation with different gas compositions and operational conditions like temperature and relative humidity.

physics.ins-det↗

arXiv2014

Impedance Spectroscopy Study in the vicinity of Ferroelectric Phase Transition

An impedance spectroscopy (IS) is a versatile tool to study the effect of grains (bulk), grain boundaries and electrode-electrolyte interface on dielectric and electrical properties of electro-ceramics. This study only focuses the high frequency (1 kHz to 10 MHz) probe of bulk ferroelectric capacitance near ferroelectric phase transition temperature (FPTT). The PZTFW single phase and PZTFW-CFO composites, respectively, have been investigated to understand the microstructure-property relation. Keeping in mind the complex microstructure of both systems, low frequency ( less than 1 kHZ ) impedance investigation, which basically deals with grain boundaries and electrode-interfaces, has been ignored. X-ray diffraction (XRD) patterns, microstructures, dielectric spectra, and impedance plots revealed two distinct phases, inbuilt compressive strain, small shift in dielectric maximum temperature, and two activation energy regions, respectively, in PZTFW-CFO composite compare to PZTFW ceramic. Addition of CFO in PZTFW medium purified the impurity phases present in the PZTFW matrix. The PZTFW-CFO composite shows flat dielectric behavior and high dielectric constant near FPTT at high frequency may be useful for tunable dielectric capacitors. The changes in bulk capacitance, relaxation time and constant phase element parameters have probed in the proximity of FPTT regions. Nyquist plot and modulus formalism show a poly-dispersive nature of relaxation, relate the activation energy (Ea) of oxygen vacancies, mainly responsible for the bulk capacitive conduction. A spiral kind of modulus spectra was observed at elevated temperatures and frequencies (greater than 2 MHz) suggests the possible experimental artifacts, have no physical reasons to explain.

cond-mat.mtrl-sci↗

arXiv2014

Faceting Oscillations in Nano-Ferroelectrics

We observe periodic faceting of 8-nm diameter ferroelectric disks on a 10 s time-scale when thin Pb(Zr0.52Ti0.48)O3 (PZT) film is exposed to constant high-resolution transmission electron microscopy (HRTEM) beams. The oscillation is between circular disk geometry and sharply faceted hexagons. The behavior is analogous to that of spin structure and magnetic domain wall velocity oscillations in permalloy [A. Bisig et al., Nature Commun. 4, 2328 (2013)], involving overshoot and de-pinning from defects [C. P. Amann, et al., J. Rheol. 57, 149-175 (2013)].

cond-mat.mtrl-sci↗

arXiv2014

Higher order optical vortices and formation of speckles

We have experimentally generated higher order optical vortices and scattered them through a ground glass plate that results in speckle formation. Intensity autocorrelation measurements of speckles show that their size decreases with increase in the order of the vortex. It implies increase in angular diameter of the vortices with their order. The characterization of vortices in terms of their annular bright ring also helps us to understand these observations. The results may find applications in stellar intensity interferometry and thermal ghost imaging.

physics.optics↗

arXiv2014

Spatial distribution of Spontaneous Parametric Down-Converted Photons for higher order Optical Vortices

We make a source of entangled photons (SEP) using spontaneous parametric down-conversion (SPDC) in a non-linear crystal and study the spatial distribution of photon pairs obtained through the down-conversion of different modes of light including higher order vortices. We observe that for the Gaussian pump, the thickness of the SPDC ring varies linearly with the radius of pump beam. However, in case of vortex carrying beams, two concentric SPDC rings are formed for beams above a critical radius. The full width at half maximum (FWHM) of SPDC rings increase with increase in the order of optical vortex beams. The presence of a critical beam width for the vortices as well as the observed FWHM of the SPDC rings are supported with our numerical results.

quant-ph↗

arXiv2014

Photoconductivity and photo-detection response of multiferroic bismuth iron oxide

We report visible light detection with in-plane BiFeO3 (BFO) thin films grown on pre-patterned inter-digital electrodes. In-plane configured BFO film displayed photocurrents with a 40:1 photo-to-dark-current ratio and improved photo-sensing ability for >15000 s (4 hrs) under small bias voltage (42V). Nearly sixty percent of the photo-induced charge carriers decay in 1.0 s and follow a double-exponential decay model. At 373 K the effect of light does not significantly increase the dark current, probably due to reduced mobility. Sub-bandgap weak monochromatic light (1 mw/cm2) shows one fold increase in photo-charge carriers.

cond-mat.mtrl-sci↗

arXiv2014

Multifunctional Magnetoelectric Materials for Device Applications

Mutiferroics are a novel class of next generation multifunctional materials, which display simultaneous magnetic spin, electric dipole, and ferroelastic ordering, and have drawn increasing interest due to their multi-functionality for a variety of device applications. Since single-phase materials exist rarely in nature with such cross-coupling properties, an intensive research activity is being pursued towards the discovery of new single-phase multiferroic materials and the design of new engineered materials with strong magneto-electric (ME) coupling. This review article summarizes the development of different kinds of multiferroic material: single-phase and composite ceramic, laminated composite, and nanostructured thin films. Thin-film nanostructures have higher magnitude direct ME coupling values and clear evidence of indirect ME coupling compared with bulk materials. Promising ME coupling coefficients have been reported in laminated composite materials in which signal to noise ratio is good for device fabrication. We describe the possible applications of these materials.

cond-mat.mtrl-sci↗

arXiv2014

Ferroelectric and Photovoltaic Properties of Transition Metal doped Pb(Zr0.14Ti0.56Ni0.30)O3-delta Thin Films

We report nearly single phase Pb(Zr0.14Ti0.56Ni0.30)O3-delta(PZTNi30) ferroelectric having large remanent polarization (15-30 μC/cm2), 0.3-0.4 V open circuit voltage (VOC), reduced band gap (direct 3.4 eV, and indirect 2.9 eV), large ON and OFF photo current ratio, and the fast decay time. Reasonably good photo current density (1-5 μA/cm2) was obtained without gate bias voltage which significantly increased with large bias field. Ferroelectric polarization dictates the polarity of VOC and direction of short circuit current (ISC), a step forward towards the realization of noncentrosymmetric ferroelectric material sensitive to visible light.

cond-mat.mtrl-sci↗

arXiv2013

Measuring Mueller matrix of an arbitrary optical element with a fixed set of polarization optics

We have described a novel way to determine the Mueller matrix of any optical element by using projection method. For this purpose, we have used two universal SU(2) gadgets for polarization optics to obtain projection matrix directly from the experiment. Mueller matrix has been determined using the experimentally obtained projection matrix for three known optical elements namely free space, half wave plate and quarter wave plate. Experimental matrices are in good agreement with the corresponding theoretical matrices. The error is minimized as the experimental conditions remains same for all measurements since we have used a fixed set of polarization optics i.e. there is no removal or insertion of an optical component during the experiment.

physics.optics↗

arXiv2013

Experimental generation of ring-shaped beams with random sources

We have experimentally reproduced ring shaped beams from the scattered Laguerre-Gaussian and Bessel-Gaussian beams. A rotating ground glass plate is used as a scattering medium and a plano convex lens collects the scattered light to generate ring shaped beams at the Fourier plane. The obtained experimental results are supported with the numerical results and are in good agreement with the theoretical results proposed by Wang, Cai and Korotkova (Opt. Exp. 17, 22366 (2009)).

physics.optics↗

arXiv2012

In-plane Dielectric and Magnetoelectric Studies of BiFeO3

In-plane temperature dependent dielectric behavior of BiFeO3 (BFO) as-grown thin films show diffuse but prominent phase transitions near 450 (+/-10) K and 550 K with dielectric loss temperature dependences that suggest skin layer effects. The 450 K anomalies are near the "transition" first reported by Polomska et al. [Phys. Stat. Sol. 23, 567 (1974)]. The 550 K anomalies coincide with the surface phase transition recently reported [Xavi et al. PRL 106, 236101 (2011)]. In addition, anomalies are found at low temperatures: After several experimental cycles the dielectric loss shows a clear relaxor-like phase transition near what was previously suggested to be a spin reorientation transition (SRT) temperature (~ 201 K) for frequencies 1 kHz < f < 1MHz which follow a nonlinear Vogel-Fulcher (V-F) relation; an additional sharp anomaly is observed near ~180 K at frequencies below 1 kHz. As emphasized recently by Cowley et al. [Adv. Phys. 60, 229 (2011)], skin effects are expected for all relaxor ferroelectrics. Using the interdigital electrodes, experimental data and a theoretical model for in-plane longitudinal and transverse direct magnetoelectric (ME) coefficient are presented.

cond-mat.mtrl-sci↗

arXiv2012

High-temperature spin-wave propagation in BiFeO3: relation to the Polomska transition

In bismuth ferrite thin films the cycloidal spiral spin structure is suppressed, and as a result the spin-wave magnon branches of long wavelength are reduced from a dozen to one, at ω= 19.2 cm-1 (T=4K). This spin wave has not been measured previously above room temperature, but in the present work we show via Raman spectroscopy that it is an underdamped propagating wave until 455 K. This has important room-temperature device implications. The data show that ω(T) follows an S=5/2 Brillouin function and hence its Fe+3 ions are in the high-spin 5/2 state and not the low-spin S=1/2 state. The spin wave cannot be measured as a propagating wave above 455 K. We also suggest that since this temperature is coincident with the mysterious "Polomska transition" (M. Polomska et al., Phys. Stat. Sol. A 23, 567, (1974)) at 458+/-5 K, that this may be due to overdamping.

cond-mat.mtrl-sci↗

arXiv2011

Systematic study of projectile structure effect on fusion barrier distribution

Quasielastic excitation function measurement has been carried out for the $^{4}$He + $^{232}$Th system at $θ_{lab}$=160$^\circ$ with respect to the beam direction, to obtain a representation of the fusion barrier distribution. Using the present data along with previously measured barrier distribution results on $^{12}$C, $^{16}$O, and $^{19}$F + $^{232}$Th systems a systematic analysis has been carried out to investigate the role of target and/or projectile structures on fusion barrier distribution. It is observed that for $^{4}$He, $^{12}$C, and $^{16}$O + $^{232}$Th, reactions the couplings due to target states only are required in coupled channel fusion calculations to explain the experimental data, whereas for the $^{19}$F+ $^{232}$Th system along with the coupling of target states, inelastic states of $^{19}$F are also required to explain the experimental results on fusion-barrier distribution. The width of the barrier distribution shows interesting transition behavior when plotted with respect to the target-projectile charge product for the above systems.

nucl-ex↗

arXiv2010

Novel room temperature Multiferroics for Random Access Memory Elements

We have fabricated a variety of "PZT-PFW" (PbZr0.52Ti0.48O3)1-x(PbFe2/3W1/3O3)x [PZTFWx; 0.2 < x < 0.4] single-phase tetragonal ferroelectrics via chemical solution deposition (CSD) [polycrystalline] and pulsed laser deposition (PLD) [epitaxial] onto Pt/Ti/SiO2/Si(100) and SrTiO3/Si substrates. These exhibit ferroelectricity and (weak) ferromagnetism above room temperature with strain coupling via electrostriction and magnetostriction. Application of modest magnetic field strength ((mu)0H < 1.0 Tesla) destabilizes the long-range ferroelectric ordering and switches the polarization from ca. 22 (mu)C/cm2 (0.22 C/m2) to zero (relaxor state). This offers the possibility of three-state logic (+P, 0, -P) and magnetically switched polarizations. Because the switching is of large magnitude (unlike the very small nC/cm2 values in terbium manganites) and at room-temperature, commercial devices should be possible.

cond-mat.mtrl-sci↗