• Volume 86, Issue 4

April 2016,   pages  723-946

• A physics-based potential and electric field model of a nanoscale rectangular high-K gate dielectric HEMT

In this paper, we have developed a physics-based model for surface potential, channel potential, electric field and drain current for AlGaN/GaN high electron mobility transistor with high-K gate dielectric using two-dimensional Poisson equation under full depletion approximation with the inclusion of effect of polarization charges. The accuracy of the model has been verified and is found to be in good agreement with the simulated results.

• Moving potential for Dirac and Klein–Gordon equations

Using the Lorentz transformation, the Klein–Gordon and Dirac equations with moving potentials are reduced to one standard where the potential is time-independent. As application, the reflection and transmission coefficients are determined by considering the moving step with a constant velocity $v$. It has been found that $R \pm T = 1$ only at $x = vt$. The problem of massless (2+1) Dirac particle is also considerered.

• Deriving relativistic Bohmian quantum potential using variational method and conformal transformations

In this paper we shall argue that conformal transformations give some new aspects to a metric and changes the physics that arises from the classical metric. It is equivalent to adding a new potential to relativistic Hamilton–Jacobi equation. We start by using conformal transformations on a metric and obtain modified geodesics. Then, we try to show that extra terms in the modified geodesics are indications of a background force. We obtain this potential by using variational method. Then, we see that this background potential is the same as the Bohmian non-local quantum potential. This approach gives a method stronger than Bohm’s original method in deriving Bohmian quantumpotential. We do not use any quantum mechanical postulates in this approach.

• Decoherence of quantum excitation of even/odd coherent states in thermal environment

In this paper, we study the decoherence of quantum excitation (photon-added)even/odd coherent states, $((\^{a})^m|\alpha_{\pm}\rangle)$, in a thermal environment by investigating the variation of negative part of the Wigner quasidistribution function vs. the rescaled time. For this purpose, at first we obtain the time-dependent Wigner function corresponding to the mentioned states in the framework of standard master equation. Then, the time evolution of the Wigner function associated with photon-added even/odd coherent states, as well as the number of added photons $m$ are analysed. It is shown that, in both states, the negative part of the Wigner function decreases with time. By deriving the threshold value of the rescaled time for single photon-added even/odd coherent states, it is also found that, if the rescaled time exceeds the threshold value, the associated Wigner function becomes positive, i.e., the decoherence occurs completely.

• Simulation of a quantum NOT gate for a single qutrit system

A three-level system based an a three-level atom interacting with a detuned cavity is considered. Because of the fact that the three-level atom defines a total normalized state composed of superposition of three different single-level states, it is assumed that such a system implements a qutrit. In order to achieve a quantum NOT gate for a single qutrit, the respective Schrödinger equation is solved numerically within a two-photon rotating wave approximation. For small values of one-photon detuning, there appear decoherence effects. Meanwhile, for large values of onephoton detuning, an ideal quantum NOT gate for a single qutrit is achieved. An expression for the execution time of the quantum NOT gate for a single qutrit as a function of the one-photon detuning is found.

• Economic scheme for remote preparation of an arbitrary five-qubit Brown-type state

A scheme for remotely preparing an arbitrary five-qubit Brown state by using three three-qubit GHZ states as the quantum channel is proposed. It is shown that, after the sender performs two different three-qubit projective measurements, the receiver should introduce two auxiliary qubits and employ suitable C-NOT gates, Toffoli gate and unitary operations on his qubits, theoriginal state can be recovered with unit probability. Compared with the previous scheme, the advantage of the present scheme is that the entanglement resource can be reduced.

• Charged analogue of Vlasenko–Pronin superdense star with variable cosmological term

The set of three static spherically symmetric solutions of the Einstein–Maxwell field equations by Maurya and Gupta, {\it Astrophys. Space Sci.} 333, 149 (2011) are modified by introducing the variable cosmological term. Motivated by Tiwari et al, {\it Indian J. Pure Appl. Math.} 31, 1017 (2000), some particular values of the cosmological term are taken to obtain well-behaved solutionsof the Einstein–Maxwell field equations. All the results given by Maurya and Gupta can be obtained as particular cases of our solutions by choosing a cosmological term equal to zero.

• A novel four-wing non-equilibrium chaotic system and its circuit implementation

In this paper, we construct a novel, 4D smooth autonomous system. Compared to the existing chaotic systems, the most attractive point is that this system does not display any equilibria, but can still exhibit four-wing chaotic attractors. The proposed system is investigated through numerical simulations and analyses including time phase portraits, Lyapunov exponents, bifurcation diagram, and Poincaré maps. There is little difference between this chaotic system withoutequilibria and other chaotic systems with equilibria shown by phase portraits and Lyapunov exponents. But the bifurcation diagram shows that the chaotic systems without equilibria do not have characteristics such as pitchfork bifurcation, Hopf bifurcation etc. which are common to the normal chaotic systems. The Poincaré maps show that this system is a four-wing chaotic system with more complicated dynamics. Moreover, the physical existence of the four-wing chaotic attractor without equilibria is verified by an electronic circuit.

• Reduction of the Bethe–Salpeter wave function: Fermion–scalar case and scalar–scalar case

In this paper, the general forms of the nonrelativistic Bethe–Salpeter wave functions for fermion–scalar bound state and scalar–scalar bound state are presented. Using the obtained normalization conditions and the corresponding Schrödinger equations for these bound states, the nonrelativistic Bethe–Salpeter wave functions can be calculated and can be used to compute the amplitude for the process involving these bound states.

• Cluster decay of $^{112−122}$Ba isotopes from ground state and as an excited compound system

The decay properties of various even-even isotopes of barium in the range $112 \le A \le 122$ is studied by modifying the Coulomb and proximity potential model for both the ground and excited state decays, using recent mass tables. Most of the values predicted for ground state decays are within the experimental limit for measurements $(T_{1/2}$less than $10^{30}$s). The minimum $T_{1/2}$ value refers to doubly magic or nearly doubly magic Sn $(Z = 50)$ as the daughter nuclei. A comparison of log$_{10}(T_{1/2})$ value reveals that the exotic cluster decay process slows down due to the presence of excess neutrons in the parent nuclei. The half-lives are also computed using the Universal formulafor cluster decay (UNIV) of Poenaru et al and the Universal decay law (UDL) of Qi et al, and are compared with CPPM values and found to be in good agreement. A comparison of half-life for ground and excited systems reveals that probability of decay increases with a rise in temperature or otherwise, inclusion of excitation energy decreases the $T_{1/2}$ values.

• Ignition curves for deuterium/helium-3 fuel in spherical tokamak reactor

In this paper, ignition curve for deuterium/helium-3 fusion reaction is studied. Four fusion reactions are considered. Zero-dimensional model for the power balance equation has been used. The closed ignition curves for $\rho$ = constant (ratio of particle to energy confinement time) have been derived. The results of our calculations show that ignited equilibria for deuterium/helium-3 fuel in a spherical tokamak is only possible for $\rho$ = 5.5 and 6. Then, by using the energy confinement scaling and parameters of the spherical tokamak reactor, the plasma stability limits have been obtained in $n_e, T$ plane and, to determine the thermal instability of plasma, the time dependent transport equations have been solved.

• Enhancing the accelerated beam current in the booster synchrotron by optimizing the transport line beam propagation

In this paper, we present the results of transverse beam emittance and twiss parameter measurement of an electron beam, delivered by a 20 MeV microtron which is used as a pre-injector system for a booster synchrotron in the Indus Accelerator Facility at RRCAT Indore. Based on these measured beam parameters, beam optics of a transport line was optimized and its results are alsodiscussed in this paper. This beam transport line is used to transport the electron beam from the 20MeV microtron to the booster synchrotron. The booster synchrotron works as a main injector for Indus-1 and Indus-2 synchrotron radiation facilities. To optimize the beam optics of a transport linefor proper beam transmission through the line as well as to match the beam twiss parameters at the beam injection point of another accelerator, it is necessary to know the transverse beam emittance and twiss parameters of the beam coming from the first one. A MATLAB-based GUI program has been developed to calculate the beam emittance and twiss parameters, using quadrupole scanmethod. The measured parameters have been used for beam transport line optimization and twiss parameters matching at booster injection point. After this optimization, an enhancement of ∼50% beam current has been observed in the booster synchrotron.

• Ionization of Rydberg atoms by the kicks of half-cycle pulses

We present a quantum mechanical model to study the ionization of quasione dimensional Rydberg atoms interacting with half-cycle pulses (HCPs) and use it to demonstrate the inadequacy of semiclassical approaches to calculate ionization probabilities of such atoms subject to the impact of more than one HCP. For a single-kicked atom both models correctly reproduce the experimentally observed ‘s-curve’ as can be seen by plotting the ionization probability $P$ as a function of momentum transfer $q_1$.We demonstrate that for a twice-kicked atom, the semiclassical model yields numbers for $P$ which are not physically realizable. For fixed values of momentum transfers $q_1$and $q_2$, in a twice-kicked atom, the ionization probability as a function of time delay between the kicks exhibits periodic decay and revival. The results of the semiclassical approach appear to agree with the quantum mechanical values at the times of revival of P, else these show considerable deviation. We attempt to provide a physical explanation for the limitation of the semiclassical approach.

• Two-soliton and three-soliton interactions of electron acoustic waves in quantum plasma

The overtaking collision between electron acoustic multisolitons in an unmagnetized quantum plasma consisting of ions, and both hot and cold electrons has been investigated. The Hirota bilinear method is employed to study phase shifts and trajectories during the overtaking collision of multisolitons. It is observed that phase shifts are significantly affected by the quantum parameter $H$. The phase shifts are proportional to $B^{1/3}$ (dispersion coefficient) and are functions of their respective amplitudes. It is also seen that the soliton structure occurs only if $H$ less than 2.

• The dust-acoustic mode in two-temperature electron plasmas with charging effects

Dust charging in an unmagnetized collisionless dusty plasma with two-temperature electrons was investigated based on the orbital motion limited theory, where the two-temperature electrons and ions are modelled by the Maxwellian distributions. Then by taking into account the effects of two-temperature electron and the associated charging fluctuations, the dispersion peculiarities of dust-acoustic waves are studied based on dust fluid dynamics. The present results show that the effect will introduce a dissipation on the mode, and the dispersion and the dissipation depend on the temperature ratio and number density ratio of hot and cold electrons.

• Defect characterization of Ga$_4$Se$_3$S layered single crystals by thermoluminescence

Trapping centres in undoped Ga$_4$Se$_3$S single crystals grown by Bridgman method were characterized for the first time by thermoluminescence (TL) measurements carried out in the low temperature range of 15−300 K. After illuminating the sample with blue light (∼470 nm) at 15 K, TL glow curve exhibited one peak around 74 K when measured with a heating rate of 0.4 K/s.The results of the various analysis methods were in good agreement about the presence of one trapping centre with an activation energy of 27 meV. Analysis of curve fitting method indicated that mixed order of kinetics dominates the trapping process. Heating rate dependence and distribution of the traps associated with the observed TL peak were also studied. The shift of peak maximum temperature from 74 to 113 K with increasing rate from 0.4 to 1.2 K/s was revealed. Distribution of traps was investigated using an experimental technique based on cleaning the centres giving emission at lower temperatures. Activation energies of the levels were observed to be increasing from 27 to 40 meV by rising the stopping temperature from 15 to 36 K.

• An outdoor investigation of the absorption degradation of single-junction amorphous silicon photovoltaic module due to localized heat/hot spot formation

This paper investigates the absorbance degradation of single-junction amorphous silicon (a-Si:H) photovoltaic (PV) module, due to the presence of localized heat. The decrease in optical density is a huge challenge due to the long-term degradation of PV modules. The reduction in solar cell optical density causes a decline in its conversion efficiency. This decreases the photogenerating current, hence reduces the effective efficiency of the PV device. An infrared thermography was used for mapping the module temperature profile. Fourier transform infrared spectroscopy (FTIR) was used for the absorption characterization. The rationale behind the outdoor deployment was to deduce a practical effect of hot spot formation on the module’s absorption ability.The results show a direct correlation between localized heat and the absorption degradation.

• A model of evaluating the pseudogap temperature for high-temperature superconductors

We have presented a model of evaluating the pseudogap temperature for high temperature superconductors using paraconductivity approach. The theoretical analysis is based on the crossing point technique of the conductivity expressions. The pseudogap temperature T $^∗$ is found to depend on dimension and is calculated for 2D and 3D superconducting samples. Numerical calculation is given in favour of the YBCO and doped SmFeAsO$_{1−x}$ samples.

• A phenomenological theory for polarization flop in spiral multiferroic TbMnO$_3$

A phenomenological Landau theory has been used to explain magnetic field-driven polarization flop in TbMnO3. The Néel wall-like magnetic structure in spiral multiferroics induces a space-dependent internal magnetic field which exerts a torque on spins to rotate bc-spiral to abspiral. The external magnetic field is argued to be competing with easy axis anisotropy and the system stabilizes when anisotropy is minimum.With the help of Landau free energy with DM magnetoelectric coupling and a general ansatz for magnetization, the phenomenon of polarization flop has been explained. Relation between Tflop and critical magnetic field has been established and found to be in good agreement with the experiment. This could be an indication that anisotropy ofthe system is temperature- and magnetic field-dependent.

• Sensitivity for detection of decay of dark matter particle using ICAL at INO

We report on the simulation studies addressing the possibility of dark matter particle (DMP) decaying into $\mu+\mu$− channel. While not much is known about the properties of dark matter particles except through their gravitational effect, it has been recently conjectured that the so-called ‘anomalous Kolar events’ observed some decades ago may be due to the decay of unstable darkmatter particles. The aim of this study is to see if this conjecture can be verified at the proposed iron calorimeter (ICAL) detector at INO. We study the possible decay to$\mu\pm$ mode which may be seen in this detector with some modifications. For the purposes of simulation, we assume that the channel saturates the decay width for the mass ranging from 1 to 50 GeV/c$^2$. The aim is not only to investigate the decay signatures, but also, more generally, to establish lower bounds on the lifetime of DMP even if no such decay takes place.

• MCNPX and GEANT4 simulation of γ -ray polymeric shields

In this work, the shielding ability of a polymeric compound with gadolinium for gamma radiation has been investigated. The conceptual calculation of radiation attenuation and energy absorption as a function of different Gd percentages and the calculation of total compound density are performed using MCNP and GEANT4. It is found that, 2 mm of the compound can reduce up to 5% and 50% of 1 MeV and 50 keV $\gamma$-rays respectively. Both Monte Carlo tools are in a good agreement.

• Erratum to: Elastic and piezoelectric properties, sound velocity and Debye temperature of (B3) BBi compound under pressure

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