Transient electric field distribution within the working volume of an NEMP simulator

Nuclear electro-magnetic pulse (NEMP) simulators which are used in the simulation of transient electromagnetic fields due to a high altitude nuclear detonation are generally excited with a double exponential high voltage pulse. This results in a current distribution on the wires of the simulator and hence a transient electric field in the working volume of the simulator where the test object is kept. It is found that for the simulator under study, the current distribution is non-uniform and so is the field distribution along the width of the simulator in the working volume. To make the current distribution uniform, several methods have been suggested and the results of these methods are analyzed and suitable conclusions are arrived at from those results.

Symmetry in Scalar Field Topology

Study of symmetric or repeating patterns in scalar fields is important in scientific data analysis because it gives deep insights into the properties of the underlying phenomenon. Though geometric symmetry has been well studied within areas like shape processing, identifying symmetry in scalar fields has remained largely unexplored due to the high computational cost of the associated algorithms. We propose a computationally efficient algorithm for detecting symmetric patterns in a scalar field distribution by analysing the topology of level sets of the scalar field. Our algorithm computes the contour tree of a given scalar field and identifies subtrees that are similar. We define a robust similarity measure for comparing subtrees of the contour tree and use it to group similar subtrees together. Regions of the domain corresponding to subtrees that belong to a common group are extracted and reported to be symmetric. Identifying symmetry in scalar fields finds applications in visualization, data exploration, and feature detection. We describe two applications in detail: symmetry-aware transfer function design and symmetry-aware isosurface extraction.

Nature of Electronic States in Atomically Thin MoS(2) Field-Effect Transistors

We present low-temperature electrical transport experiments in five field-effect transistor devices consisting of monolayer, bilayer, and trilayer MoS(2) films, mechanically exfoliated onto Si/SiO(2) substrate. Our experiments reveal that the electronic states In all films are localized well up to room temperature over the experimentally accessible range of gate voltage. This manifests in two-dimensional (2D) variable range hopping (VRH) at high temperatures, while below similar to 30 K, the conductivity displays oscillatory structures In gate voltage arising from resonant tunneling at the localized sites. From the correlation energy (T(0)) of VRH and gate voltage dependence of conductivity, we suggest that Coulomb potential from trapped charges In the substrate is the dominant source of disorder in MoS(2) field-effect devices, which leads to carrier localization, as well.

Characterization of Clean and Fouled Rail Track Ballast Subsurface Using Seismic Surface Survey Method: Model and Field Studies

The efficiency of track foundation material gradually decreases due to insufficient lateral confinement, ballast fouling, and loss of shear strength of the subsurface soil under cyclic loading. This paper presents characterization of rail track subsurface to identify ballast fouling and subsurface layers shear wave velocity using seismic survey. Seismic surface wave method of multi-channel analysis of surface wave (MASW) has been carried out in the model track and field track for finding out shear wave velocity of the clean and fouled ballast and track subsurface. The shear wave velocity (SWV) of fouled ballast increases with increase in fouling percentage, and reaches a maximum value and then decreases. This character is similar to typical compaction curve of soil, which is used to define optimum and critical fouling percentage (OFP and CFP). Critical fouling percentage of 15 % is noticed for Coal fouled ballast and 25 % is noticed for clayey sand fouled ballast. Coal fouled ballast reaches the OFP and CFP before clayey sand fouled ballast. Fouling of ballast reduces voids in ballast and there by decreases the drainage. Combined plot of permeability and SWV with percentage of fouling shows that after critical fouling point drainage condition of fouled ballast goes below acceptable limit. Shear wave velocities are measured in the selected location in the Wollongong field track by carrying out similar seismic survey. In-situ samples were collected and degrees of fouling were measured. Field SWV values are more than that of the model track SWV values for the same degree of fouling, which might be due to sleeper's confinement. This article also highlights the ballast gradation widely followed in different countries and presents the comparison of Indian ballast gradation with international gradation standards. Indian ballast contains a coarser particle size when compared to other countries. The upper limit of Indian gradation curve matches with lower limit of ballast gradation curves of America and Australia. The ballast gradation followed by Indian railways is poorly graded and more favorable for the drainage conditions. Indian ballast engineering needs extensive research to improve presents track conditions.

Trapped fermions in a synthetic non-Abelian gauge field

On increasing the coupling strength (lambda) of a non-Abelian gauge field that induces a generalized Rashba spin-orbit interaction, the topology of the Fermi surface of a homogeneous gas of noninteracting fermions of density rho similar to k(F)(3) undergoes a change at a critical value, lambda(T) approximate to k(F) [Phys. Rev. B 84, 014512 ( 2011)]. In this paper we analyze how this phenomenon affects the size and shape of a cloud of spin-1/2 fermions trapped in a harmonic potential such as those used in cold atom experiments. We develop an adiabatic formulation, including the concomitant Pancharatnam-Berry phase effects, for the one-particle states in the presence of a trapping potential and the gauge field, obtaining approximate analytical formulas for the energy levels for some high symmetry gauge field configurations of interest. An analysis based on the local density approximation reveals that, for a given number of particles, the cloud shrinks in a characteristic fashion with increasing.. We explain the physical origins of this effect by a study of the stress tensor of the system. For an isotropic harmonic trap, the local density approximation predicts a spherical cloud even for anisotropic gauge field configurations. We show, via a calculation of the cloud shape using exact eigenstates, that for certain gauge field configurations there is a systematic and observable anisotropy in the cloud shape that increases with increasing gauge coupling lambda. The reasons for this anisotropy are explained using the analytical energy levels obtained via the adiabatic approximation. These results should be useful in the design of cold atom experiments with fermions in non-Abelian gauge fields. An important spin-off of our adiabatic formulation is that it reveals exciting possibilities for the cold-atom realization of interesting condensed matter Hamiltonians by using a non-Abelian gauge field in conjunction with another potential. In particular, we show that the use of a spherical non-Abelian gauge field with a harmonic trapping potential produces a monopole field giving rise to a spherical geometry quantum Hall-like Hamiltonian in the momentum representation.

Growth and characterization of excimer laser-ablated BaBi2Nb2O9 thin films

The pulsed-laser ablation technique has been employed to deposit polycrystalline thin films of layered-structure ferroelectric BaBi2Nb2O9 (BBN). Low-substrate-temperature growth (Ts = 400 °C) followed by ex situ annealing at 800 °C for 30 min was performed to obtain a preferred orientation. Ferroelectricity in the films was verified by examining the polarization with the applied electric field and was also confirmed from the capacitance–voltage characteristics. The films exhibited well-defined hysteresis loops, and the values of saturation (Ps) and remanent (Pr) polarization were 4.0 and 1.2 μC/cm2, respectively. The room-temperature dielectric constant and dissipation factor were 214 and 0.04, respectively, at a frequency of 100 kHz. A phase transition from a ferroelectric to paraelectric state of the BBN thin film was observed at 220 °C. The dissipation factor of the film was observed to increase after the phase transition due to a probable influence of dc conduction at high temperatures. The real and imaginary part of the dielectric constant also exhibited strong frequency dispersion at high temperatures.

Effect of gate-drain/source overlap on the noise in 90 nm N-channel metal oxide semiconductor field effect transistors

In this paper, we have studied the effect of gate-drain/source overlap (LOV) on the drain channel noise and induced gate current noise (SIg) in 90 nm N-channel metal oxide semiconductor field effect transistors using process and device simulations. As the change in overlap affects the gate tunneling leakage current, its effect on shot noise component of SIg has been taken into consideration. It has been shown that “control over LOV” allows us to get better noise performance from the device, i.e., it allows us to reduce noise figure, for a given leakage current constraint. LOV in the range of 0–10 nm is recommended for the 90 nm gate length transistors, in order to get the best performance in radio frequency applications.

Nonlinear dielectric behavior in three-component ferroelectric superlattices

Three-component ferroelectric superlattices consisting of alternating layers of SrTiO3, BaTiO3, and CaTiO3 (SBC) with variable interlayer thickness were fabricated on Pt(111)/TiO2/SiO2/Si (100) substrates by pulsed laser deposition. The presence of satellite reflections in x-ray-diffraction analysis and a periodic concentration of Sr, Ba, and Ca throughout the film in depth profile of secondary ion mass spectrometry analysis confirm the fabrication of superlattice structures. The Pr (remnant polarization) and Ps (saturation polarization) of SBC superlattice with 16.4-nm individual layer thickness (SBC16.4) were found to be around 4.96 and 34 μC/cm2, respectively. The dependence of polarization on individual layer thickness and lattice strain were studied in order to investigate the size dependence of the dielectric properties. The dielectric constant of these superlattices was found to be much higher than the individual component layers present in the superlattice configuration. The relatively higher tunability ( ∼ 55%) obtained around 300 K indicates that the superlattice is a potential electrically tunable material for microwave applications at room temperature. The enhanced dielectric properties were thus discussed in terms of the interfacial strain driven polar region due to high lattice mismatch and electrostatic coupling due to polarization mismatch between individual layers.

Improved ferroelectric and leakage properties in symmetric BiFeO3/SrTiO3 superlattice

Symmetric BiFeO3/SrTiO3 superlattices were fabricated by pulsed laser deposition on SrTiO3 (100) substrates. Frequency independent and near saturated P-E hysteresis was observed in the case of a superlattice (periodicity of ∼ 11 nm) as compared to leaky hysteresis observed in epitaxial BiFeO3. Room temperature leakage current density of the superlattice was almost two orders of magnitude lower than that of BiFeO3. Observed leakage current behavior in case of both BiFeO3 and superlattice indicates the dominance of space charge limited conduction. Improvement in ferroelectric property was discussed in connection to enhanced epitaxial strain, reduced leakage current, and electrostatic interaction between BiFeO3 and SrTiO3.

Realization of biferroic properties in La0.6Sr0.4MnO3/0.7Pb(Mg1/3Nb2/3)O3–0.3(PbTiO3) epitaxial superlattices

A set of symmetric and asymmetric superlattices with ferromagnetic La0.6Sr0.4MnO3 (LSMO) and ferroelectric 0.7Pb(Mg1/3Nb2/3)O3–0.3(PbTiO3) as the constituting layers was fabricated on LaNiO3 coated (100) oriented LaAlO3 substrates using pulsed laser ablation. The crystallinity, and magnetic and ferroelectric properties were studied for all the superlattices. All the superlattice structures exhibited a ferromagnetic behavior over a wide range of temperatures between 10 and 300 K, whereas only the asymmetric superlattices exhibited a reasonably good ferroelectric behavior. Strong influence of an applied magnetic field was observed on the ferroelectric properties of the asymmetric superlattices. Studies were conducted toward understanding the influence of conducting LSMO layers on the electrical responses of the heterostructures. The absence of ferroelectricity in the symmetric superlattice structures has been attributed to their high leakage characteristics. The effect of an applied magnetic field on the ferroelectric properties of the asymmetric superlattices indicated strong influence of the interfaces on the properties. The dominance of the interface on the dielectric response was confirmed by the observed Maxwell-Wagner-type dielectric relaxation in these heterostructures.

Ferroelectric interaction and polarization studies in BaTiO3/SrTiO3 superlattice

Ferroelectric superlattice structures consisting of alternating layers of BaTiO3 and SrTiO3 with variable interlayer thickness were grown on Pt (111)/TiO2/SiO2/Si (100) substrates by pulsed laser deposition. The presence of superlattice reflections in the x-ray diffraction pattern clearly showed the superlattice behavior of the fabricated structures over a range of 6.4–20 nm individual layer thicknesses. Depth profile conducted by secondary ion mass spectrometry analysis showed a periodic concentration of Ba and Sr throughout the film. Polarization hysteresis and the capacitance-voltage characteristics of these films show clear size dependent ferroelectric characteristics. The spontaneous (Ps) and remnant (Pr) polarizations increase gradually with decreasing periodicity, reach a maximum at a finite thickness and then decrease. The competition between the size effect and long-range ferroelectric interaction is suggested as a possible reason for this phenomenon. The temperature dependence of Ps and Pr shows a single ferroelectric phase transition, and the Curie temperature is estimated to be about 316 K. The curve shows that the ferroelectric superlattice tends to form an artificial material, responding as a single structure with an averaged behavior of both the parent systems.

Interface dominated biferroic La0.6Sr0.4MnO3/0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 epitaxial superlattices

Superlattices composed of ferromagnetic La0.6Sr0.4MnO3 and ferroelectric 0.7Pb(Mg1/3Nb2/3)O3–0.3(PbTiO3) layers were fabricated on (100) LaAlO3 substrates by pulsed laser deposition technique. The ferromagnetic and frequency independent ferroelectric hysteresis characteristics established the biferroic nature of the superlattices. Influence of magnetic field was observed in tuning the P-E characteristics of the superlattices. A similar effect was observed on application of a high dc electric field to the samples. The nature of the observed ferroelectric properties and their modulation by applied magnetic and electric fields were thus discussed in connection to the ferroelectric/ferromagnetic interfaces.

Prebreakdown Current and Sparking Potential in a Nonuniform Electric Field with a Crossed Magnetic Field

Prebreakdown currents in a coaxial cylindrical geometry in nitrogen have been measured with and without a crossed magnetic field. The range of parameters used in the investigation are 2.6 ÿ p ÿ 14.5 torr, 50 ÿ (E/p) ÿ 420 V cm-1 torr-1, and 43.0 ÿ H/p ÿ 1185 Oe torr-1 (p is the pressure, E is the electric field, and H is the magnetic field). The initial photoelectric current is obtained by allowing photons produced in an auxiliary glow discharge to strike the cathode. Ions and electrons produced in the auxiliary discharge are prevented from reaching the main gap by suitable shielding. By modifying the Rice equation for back diffusion, the measured ionization current multiplication without a crossed magnetic field is compared with the multiplication predicted by the Townsend growth equation for nonuniform electric fields. It is observed that over the range of 50 Ã�¿ (E/P)max Ã�¿ 250 [(E/P)max is the value of E/p at the central electrode of the coaxial system] measured and calculated multiplication of current agree with each other. With a crossed magnetic field the prebreakdown currents have been measured and compared with the theoretically calculated currents using the equivalent pressure concept. Agreement between the calculated and measured currents is not satisfactory, and this has been attributed more to the uncertainty in the collision frequency data available than nonuniformity of the electric field. Sparking potentials have been measured with and without a crossed magnetic field.

Digital Simulation of Field-Oriented Control of Induction Motor

This paper describes the simulation of a control scheme using the principle of field orientation for the control of a voltage source inverter-fed induction motor. The control principle is explained, followed by an algorithm to simulate various components of the system in the digital computer. The dynamic response of the system for the load disturbance and set-point variations have been studied. Also, the results of the simulation showing the behavior of field coordinates for such disturbances are given.