arXiv subjects

Ashok Kumar

Publications and source records attributed to Ashok Kumar.

At least 73 records · Page 4Linked to original sources

arXiv2018

Development, Characterization and Qualification of first GEM foils produced in India

The increasing demand for Gas Electron Multiplier (GEM) foils has been driven by their application in many current and proposed high-energy physics experiments. Micropack, a Bengaluru-based company, has established and commercialized GEM foils for the first time in India. Micropack used the double-mask etching technique to successfully produce 10 cm $\times$ 10 cm GEM foil. In this paper, we report on the development as well as the geometrical and electrical properties of these foils, including the size uniformity of the holes and leakage current measurements. Our characterization studies show that the foils are of good quality and satisfy all the necessary quality control criteria.

physics.ins-det↗

arXiv2018

Stability and Carrier Transport Properties of Phosphorene Based Polymorphic Nanoribbons

A few-layer black phosphorene has recently gained significant interest in the scientific community. In this paper, we consider several polymorphs of phosphorene nanoribbons (PNRs) and employ deformation potential theory within the effective mass approximation together with density functional theory to investigate their structural, mechanical and electronic properties. The results show that stability of PNRs strongly depends on the direction along which they can be cut from 2D counterpart. PNRs also exhibit a wide range of line stiffness ranging from 6x10^10 eV/m to 18x10^11 eV/m which has little dependence on the edge passivation. Likewise, the calculated electronic properties of PNRs display them to be either narrow-gap semiconductor (Eg < 1 eV) or wide-gap semiconductor (Eg > 1 eV). The carrier mobility of PNRs is found to be comparable to that of the black phosphorene. Some of the PNRs show n-type (p-type) semiconducting character owing to their higher electron (hole) mobility. Passivation of the edges leads to n-type <-> p-type transition in many of the PNRs considered. The predicted novel characteristics of PNRs with a wide range of mechanical and electronic properties make PNRs to be potentially suitable for the use in nanoscale devices.

cond-mat.mtrl-sci↗

arXiv2018

Quantum-Enhanced Plasmonic Sensing

Quantum resources can enhance the sensitivity of a device beyond the classical shot noise limit and, as a result, revolutionize the field of metrology through the development of quantum-enhanced sensors. In particular, plasmonic sensors, which are widely used in biological and chemical sensing applications, offer a unique opportunity to bring such an enhancement to real-life devices. Here, we use bright entangled twin beams to enhance the sensitivity of a plasmonic sensor used to measure local changes in refractive index. We demonstrate a 56% quantum enhancement in the sensitivity of state-of-the-art plasmonic sensor with measured sensitivities on the order of $10^{-10}$RIU$/\sqrt{\textrm{Hz}}$, nearly 5 orders of magnitude better than previous proof-of-principle implementations of quantum-enhanced plasmonic sensors. These results promise significant enhancements in ultratrace label free plasmonic sensing and will find their way into areas ranging from biomedical applications to chemical detection.

quant-ph↗

arXiv2017

Timing and Charge measurement of single gap Resistive Plate Chamber Detectors for INO-ICAL Experiment

The recently approved India-based Neutrino Observatory will use the world's largest magnet to study atmospheric muon neutrinos. The 50 kiloton Iron Calorimeter consists of iron alternating with single-gap resistive plate chambers. A uniform magnetic field of $\sim$1.5 T is produced in the iron using toroidal-shaped copper coils. Muon neutrinos interact with the iron target to produce charged muons, which are detected by the resistive plate chambers, and tracked using orthogonal pick up strips. Timing information for each layer is used to discriminate between upward and downward traveling muons. The design of the readout electronics for the detector depends critically on an accurate model of the charge induced by the muons, and the dependence on bias voltages. In this paper, we present timing and charge response measurements using prototype detectors under different operating conditions. We also report the effect of varying gas mixture, particularly $SF_6$, on the timing response.

physics.ins-det↗

arXiv2017

Van der Waals Heterostructures Based on Allotropes of Phosphorene and MoSe2

The van der Waals heterostructures of allotropes of phosphorene ($α$- and $β-P$) with MoSe2 (H-, T-, ZT- and SO-MoSe2) are investigated in the framework of state-of-the-art density functional theory. The semiconducting heterostructures, $β$-P /H-MoSe2 and $α$-P / H-MoSe2, forms anti-type structures with type I and type II band alignments, respectively, whose bands are tunable with external electric field. $α$-P / ZT-MoSe2 and $α$-P / SO-MoSe2 form ohmic semiconductor-metal contacts while Schottky barrier in $β$-P / T-MoSe2 can be reduced to zero by external electric field to form ohmic contact which is useful to realize high-performance devices. Simulated STM images of given heterostructures reveal that $α$-P can be used as a capping layer to differentiate between various allotropes of underlying MoSe2. The dielectric response of considered heterostructures is highly anisotropic in terms of lateral and vertical polarization. The tunable electronic and dielectric response of van der Waals phosphorene/MoSe2 heterostructure may find potentials applications in the fabrication of optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2017

Tunnelling Characteristics of Stone-Wales Defects in Monolayers of Sn and Group-V Elements

Topological defects in ultrathin layers are often formed during synthesis and processing, thereby, strongly influencing their electronic properties . In this paper, we investigate the role of Stone-Wales (SW) defects in modifying the electronic properties of the monolayers of Sn and group-V elements. The calculated results find the electronic properties of stanene (monolayer of Sn atoms) to be strongly dependent on the concentration of SW-defects e.g., defective stanene has nearly zero band gap (~ 0.03 eV) for the defect concentration of 2.2 x 10^13 cm^-2 which opens up to 0.2 eV for the defect concentration of 3.7 x 10^13 cm^-2. In contrast, SW-defects appear to induce conduction states in the semiconducting monolayers of group-V elements. These conduction states act as channels for electron tunnelling, and the calculated tunnelling characteristics show the highest differential conductance for the negative bias with the asymmetric current-voltage characteristics. On the other hand, the highest differential conductance was found for the positive bias in stanene. Simulated STM topographical images of stanene and group-V monolayers show distinctly different features in terms of their cross-sectional views and distance-height profiles which can serve as fingerprints to identify the topological defects in the monolayers of group-IV and group-V elements in experiments.

cond-mat.mtrl-sci↗

arXiv2017

Optically Controlled Polarization in Highly Oriented Ferroelectric Thin Films

The out-of-plane and in-plane polarization of (Pb0.6Li0.2Bi0.2)(Zr0.2Ti0.8)O3(PLBZT) thin film has studied in the dark and under illumination of a weak light source of a comparable bandgap. A highly oriented PLBZT thin film was grown on LaNiO3 (LNO)/LaAlO3(LAO) substrate by pulsed laser deposition system which illustrates well-saturated polarization and its significant enhancement under illumination of light. We have employed two configurations for polarization characterization; first deals with out of plane polarization with single capacitor under investigation, whereas second demonstrates the two capacitors connected in series via the bottom electrode. Two different configurations were illuminated using different energy sources and their effects were studied. The latter configuration shows a significant change in polarization under illumination of light that may provide an extra degree of freedom for device miniaturization. The polarization was also tested using positive-up & negative-down (PUND) measurements which confirm robust polarization and their switching under illumination.

cond-mat.mtrl-sci↗

arXiv2017

Giant Enhancement in Ferroelectric Polarization under Illumination

We report optical enhancement in polarization and dielectric constant near room temperature in Pb0.6Li0.2Bi0.2Zr0.2Ti0.8O3 (PLBZT) electro-ceramics; these are doubly substituted members of the most important commercial ferroelectric PbZr0.2Ti0.8O3 (PZT:20/80). Partial (40%) substitution of equal amounts of Li+1 and Bi+3 in PZT: 20/80 retains the PZT tetragonal structure with space group P4mm. Under illumination of white light and weak 405-nm near-ultraviolet laser light (30 mW), an unexpectedly large (200-300%) change in polarization and displacement current was observed. Light also changes the dc conduction current density by one to two orders of magnitude with a large switchable open circuit voltage (Voc ~ 2 V) and short circuit current (Jsc ~ 5x10-8 A). The samples show a photo-current ON/OFF ratio of order 6:1 under illumination of weak light.

cond-mat.mtrl-sci↗

arXiv2017

Physics Potential of the ICAL detector at the India-based Neutrino Observatory (INO)

The upcoming 50 kt magnetized iron calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) is designed to study the atmospheric neutrinos and antineutrinos separately over a wide range of energies and path lengths. The primary focus of this experiment is to explore the Earth matter effects by observing the energy and zenith angle dependence of the atmospheric neutrinos in the multi-GeV range. This study will be crucial to address some of the outstanding issues in neutrino oscillation physics, including the fundamental issue of neutrino mass hierarchy. In this document, we present the physics potential of the detector as obtained from realistic detector simulations. We describe the simulation framework, the neutrino interactions in the detector, and the expected response of the detector to particles traversing it. The ICAL detector can determine the energy and direction of the muons to a high precision, and in addition, its sensitivity to multi-GeV hadrons increases its physics reach substantially. Its charge identification capability, and hence its ability to distinguish neutrinos from antineutrinos, makes it an efficient detector for determining the neutrino mass hierarchy. In this report, we outline the analyses carried out for the determination of neutrino mass hierarchy and precision measurements of atmospheric neutrino mixing parameters at ICAL, and give the expected physics reach of the detector with 10 years of runtime. We also explore the potential of ICAL for probing new physics scenarios like CPT violation and the presence of magnetic monopoles.

physics.ins-det↗

arXiv2017

Observation of Spatial Quantum Correlations in the Macroscopic Regime

Spatial quantum correlations in the transverse degree of freedom promise to enhance optical resolution, image detection, and quantum communications through parallel quantum information encoding. In particular, the ability to observe these spatial quantum correlations in a single shot will enable such enhancements in applications that require real time imaging, such as particle tracking and in-situ imaging of atomic systems. Here, we report on measurements in the far-field that show spatial quantum correlations in single images of bright twin-beams with $10^8$ photons in a 1~$μ$s pulse using an electron-multiplying charge-coupled device camera. A four-wave mixing process in hot rubidium atoms is used to generate narrowband-bright pulsed twin-beams of light. Owing to momentum conservation in this process, the twin-beams are momentum correlated, which leads to spatial quantum correlations in the far field. We show around 2~dB of spatial quantum noise reduction with respect to the shot noise limit. The spatial squeezing is present over a large range of total number of photons in the pulsed twin-beams.

physics.optics↗

arXiv2017

Electron Transport and Thermoelectric Performance of Defected Monolayer MoS2

Electronic and thermoelectric properties of a two-dimensional MoS2 monolayer containing atomic defects are investigated using density functional theory. All the atomic defects have been found to exhibit endothermic nature. Electronic structure of MoS2 shows tuneability of band gap with the atomic defects. The MoS2 vacancy in pristine monolayer makes it magnetic and narrow band gap semiconductor. The spin-polarized character of the monolayer with defects is clearly captured by the tunneling current calculated in the STM-like setup. A relatively low thermal conductivity has been observed in monolayers with defects as compared to pristine form resulting in enhanced room temperature figure of merit as high as 6.24 and 1.30 respectively. We believe that our results open up a new window for the use of monolayer MoS2 in electronic devices, thermal management and thermoelectric devices.

cond-mat.mes-hall↗

arXiv2017

Unipolar resistive switching in cobalt titanate thin films

We report giant resistive switching of an order of 104, long-time charge retention characteristics up to 104 s, non-overlapping SET and RESET voltages, ohmic in low resistance state (LRS) and space charge limited current (SCLC) mechanism in high resistance state (HRS) properties in polycrystalline perovskite Cobalt Titanate (CoTiO3 ~ CTO) thin films. Impedance spectroscopy study was carried out for both LRS and HRS states which illustrates that only bulk resistance changes after resistance switching, however, there is a small change (<10% which is in pF range) in the bulk capacitance value in both states. These results suggest that in LRS state current filaments break the capacitor in many small capacitors in a parallel configuration which in turn provides the same capacitance in both states even there was 90 degree changes in phase-angle and an order of change in the tangent loss.

cond-mat.mtrl-sci↗

arXiv2016

Development and Commissioning of the HARDROC based Readout for the INO-ICAL Experiment

Glass based Resistive Plate Chambers (RPCs) are going to be used as an active element in the Iron Calorimeter (ICAL) experiment at the India based Neutrino Observatory (INO), which is being constructed to study atmospheric neutrinos. Though the RPC detector operational parameters are more or less finalized, the readout electronics is being developed using various technologies. The ICAL experiment will consist of about 29,000 RPC detectors of 2 m $\times$ 2 m in size with each detector having 64 readout channels both in the X and Y directions. The present study focusses on multi-channel electronics based on SiGe 350 nm technology as an option for the INO-ICAL RPC detectors. The study includes commissioning and usage of frontend application specific integrated circuit (ASIC) HARDROC chip in which 64 channels are handled independently to perform zero suppression. We present first testbench results using the HARDROC chip with the aim to use it finally in the ICAL experiment.

physics.ins-det↗

arXiv2016

Search for Dark Matter with LHC proton Beam Dump

Dark Matter (DM) comprising particles in the mass range of a few MeV to GeV is waiting to be explored, given the many theoretical models accommodating cosmological abundance. We hereby propose an experiment with the LHC proton beam of 7 TeV striking onto the beam dump target, emitting neutrinos and possibly, Dark Matter candidates. This experiment would also permit to observe signatures involving elastic and inelastic processes involving DM candidates, electrons and strongly interacting particles present in nuclei of the dump target. There will be residual neutrino background present in each of these signatures, hence the proposed experimental detector sub-systems would be such that they would involve as final states, elastically or inelasticity scattered, standard model particles. The bump or the excess in the tail of the kinematic distributions will eventually give us glimpse of presence of new particles which could possibly be Dark Matter candidates. Given the parameters of the LHC machine, the sensitivity of such an experiment is unique in the energy region it is supposed to exploit.

hep-ex↗

arXiv2016

Giant Magnetoelectric coupling in Single Phase Pb(Zr0.20Ti0.80)0.70Pd0.30O3-δ Multiferroics

During the last fifteen years, multiferroic (MF) research communities have been searching for an alternative room temperature MF material with large magnetoelectric (ME) coupling for possible applications in high density electronic components, low heat dissipation memory and logic devices. We have studied Pb(Zr0.20Ti0.80)0.70Pd0.30O3-δ (PZTP30) system with an unusually large (30%) palladium occupancy in B site of PZT. This material exhibited a giant ME coupling coefficient ~0.36 mV/cm.Oe. Interestingly, this is the first time any room temperature single phase compound that showed ME trends, and magnitude similar to those in the well established mechanical strain-mediated ferroelectric and ferromagnetic composites; the latter ones are already in the commercial stage as nT/pT magnetic field sensors due to their large ME values. The presence of Pd in PZTP30 has been confirmed by XPS and XRF studies and assigned with related binding energies of Pd+2 and Pd+4 ions as 336.37 eV, 342.9 eV, and 337.53 eV, 343.43 eV, respectively, which may be the origin of room temperature magnetism in Pd substituted PZT ceramics. A sharp first order ferroelectric phase transition was observed at ~569 K (+/-5 K) that is confirmed from dielectric, Raman, and thermal analysis. Both ferromagnetic and ferroelectric orderings with large ME coupling were found above room temperature, a significant step forward in the development of single phase ME material with enhanced functionalities.

cond-mat.mtrl-sci↗

arXiv2016

Novel optically active lead-free relaxor ferroelectric (Ba0.6Bi0.2Li0.2)TiO3

We discovered a near room temperature lead-free relaxor-ferroelectric (Ba0.6Bi0.2Li0.2)TiO3 (BBLT) having A-site compositional disordered ABO3 perovskite structure. Microstructure-property relations revealed that the chemical inhomogeneities and development of local polar nano regions (PNRs) are responsible for dielectric dispersion as a function of probe frequencies and temperatures. Rietveld analysis indicates mixed crystal structure with 80% tetragonal structure (space group P4mm) and 20% orthorhombic structure (space group Amm2) which is confirmed by the high resolution transmission electron diffraction pattern. Dielectric constant and tangent loss dispersion with and without illumination of light obey nonlinear Vogel-Fulture relation. It shows slim polarization-hysteresis (P-E) loops and excellent displacement coefficients (d33 ~ 233 pm/V) near room temperature, which gradually diminish near the maximum dielectric dispersion temperature (Tm). The underlying physics for light-sensitive dielectric dispersion was probed by X-ray photon spectroscopy (XPS) which strongly suggests that mixed valence of bismuth ions, especially Bi5+ ions, are responsible for most of the optically active centers. Ultraviolet photoemission measurements showed most of the Ti ions are in 4+ states and sit at the centers of the TiO6 octahedra, which along with asymmetric hybridization between O 2p and Bi 6s orbitals appears to be the main driving force for net polarization. This BBLT material may open a new path for environmental friendly lead-free relaxor-ferroelectric research.

cond-mat.mtrl-sci↗

arXiv2015

Pyroelectric Control of Spin Polarization Assisted Coexistence of Giant Positive and Negative Magnetocaloric Effects

Electric field control of magnetism is the key to many next generation spintronics applications1. Ferroelectric control of spin polarization2,3 followed by the electrically driven repeatable magnetization reversal in the absence of applied magnetic field4 have proven to be the milestones in this direction. This article propose how these phenomena could be utilized in a reverse manner to gain control over magnetization even in the absence of electric field. In turn, coexistence of positive5 as well as negative6 magnetocaloric effect (MCE) is attained in tri-layered PbZr0.53Ti0.47O3-CoFe2O4-PbZr0.53Ti0.47O3 (PZT/CFO/PZT) nanostructures for identical temperature ranges when subjected to different applied magnetic fields. Unlike conventional approaches5-9 the present study demonstrate that it is possible to obtain giant MCE merely by magneto-pyroelectric coupling. Consequently, the MCE entropy changes calculated using Maxwell equations are found to be as large as reported for existing giant MCE values6,9,10.

cond-mat.mtrl-sci↗

arXiv2015

Anomalous change in leakage and displacement currents after electrical poling on lead-free ferroelectric ceramics

We report the polarization, displacement current and leakage current behavior of a trivalent nonpolar cation Al cation substituted lead free ferroelectric NBT-BT electroceramics with tetragonal phase and P4mm space group symmetry. Nearly three orders of magnitude decrease in leakage current were observed under electrical poling, which significantly improves microstructure, polarization, and displacement current. Effective poling neutralizes the domain pinning, traps charges at grain boundaries and fills oxygen vacancies with free charge carriers in matrix, thus saturated macroscopic polarization in contrast to that in upoled samples. E-poling changes bananas type polarization loops to real ferroelectric loops.

cond-mat.mtrl-sci↗