• Volume 89, Issue 2

August 2017

• Design of multistable systems via partial synchronization

Many researchers introduce schemes for designing multistable systems by coupling two identical systems. In this paper, we introduce a generalized scheme for designing multistable systems by coupling two different dynamical systems. The basic idea of the scheme is to design partial synchronization of states betweenthe coupled systems and finding some completely initial condition-dependent constants of motion. In our scheme, we synchronize $i$ number $(1 \leq i \leq m − 1)$ of state variables completely and keep constant difference between $j$ $(1 \leq j \leq m −1, i + j = m)$ number of state variables of two coupled m-dimensional different dynamical systems to obtain multistable behaviour.We illustrate our scheme for coupled Lorenz and Lu systems. Numerical simulation results consisting of phase diagram, bifurcation diagram and maximum Lyapunov exponents are presented to show the effectiveness of our scheme.

• Influence of DC arc current on the formation of cobalt-based nanostructures

The synthesis of cobalt-based magnetic nanostructures using DC arc discharge technique with varying arc current is reported here. The structural, morphological, compositional and magnetic properties of thesenanostructures were studied as a function of applied arc current. Various techniques like X-ray diffraction, transmission electron microscopy, EDAX and vibrating sample magnetometry were used to carry out this studyand the results are reported here. The results clearly indicate that for a given oxygen partial pressure, an arc current of 100A favours the formation of unreacted cobalt atomic species. Also change in arc current leads to variationin phase, diversity in morphology etc. Other property changes such as thermal changes, mechanical changes etc. are not addressed here. The magnetic characterization further indicates that the anisotropy in shape plays a crucial role in deciding the magnetic properties of the nanostructured materials.We have quantified an interesting result in our experiment, that is, for a given partial pressure, 100A arc current results in unique variation in structural and magnetic properties as compared to other arc currents.

• Electro-optic response of metal halide $\rm{C_{s}PbI_{3}}$: A first-principles study

A theoretical study of electronic and optical properties of metal-halide cubic perovskite, $\rm{C_{s}PbI_{3}}$, is presented, using first-principles calculations with plane-wave pseudopotential method as implemented in the PWSCF code. In this approach, local density approximation (LDA) is used for exchange-correlation potential. A strong ionic bonding is observed between Cs and I orbitals and a weak covalent bonding is found between Pb-I and Cs-Pb orbitals. The optical properties of this compound are interesting and it has many applications in optoelectronic devices.

• $\it{Ab-initio}$ calculations of electric field gradient in Ru compounds and their implication on the nuclear quadrupole moments of $^{99}$Ru and $^{101}$Ru

The nuclear quadrupole moments, $\mathcal{Q}$, for the ground and first excited states in $^{99}$Ru and ground state of $^{101}$Ru have been determined by comparing the experimentally observed quadrupole interaction frequencies $\mathcal{ν_{Q}}$ with calculated electric field gradient (EFG) for a large number of Ru-based compounds. The $\it{ab-initio}$ calculations of EFG were performed using the all-electron augmented plane wave + local orbital (APW + lo) method of the density functional theory (DFT). From the slope of the linear correlation between theoretically calculated EFGs and experimentally observed $\mathcal{ν_{Q}}$, we obtain the quadrupole moment for the $(5/2^{+})$ ground state in $^{99}$Ru and $^{101}$Ru as 0.0734(17) b and 0.431(14) b respectively, showing excellent agreement with the values reported in literature. For $3/2^{+}$, the quadrupole moment of the first excited state in $^{99}$Ru is obtained as +0.203(3) b, which is considerably lower than the commonly accepted literature value of +0.231(12) b. The results presented in this paper would be useful for the precise determination of quadrupole moment of high spin states in other Ru isotopes and is likely to stimulate further shell model calculations for an improved understanding of nuclear shape in these nuclei.

• New interior solution describing relativistic fluid sphere

Anewexact solution of embedding class I is presented for a relativistic anisotropicmassive fluid sphere. The new exact solution satisfies Karmarkar condition, is well-behaved in all respects, and therefore is suitable for the modelling of superdense stars. Consequently, using this solution, we have studied in detail two compact stars, namely, XTE J1739-289 (strange star 1.51$M_{\odot}$, 10.9 km) and PSR J1614-2230 (neutron star 1.97$M_{\odot}$, 14 km). The solution also satisfies all energy conditions with the compactness parameter lying within the Buchdahl limit.

• A novel technique for active vibration control, based on optimal tracking control

In the last few decades, researchers have proposed many control techniques to suppress unwanted vibrations in a structure. In this work, a novel and simple technique is proposed for the active vibration control. In this technique, an optimal tracking control is employed to suppress vibrations in a structure by simultaneously tracking zero references for modes of vibration. To illustrate the technique, a two-degrees of freedom spring-mass-dampersystem is considered as a test system. The mathematical model of the system is derived and then converted into a state-space model. A linear quadratic tracking control law is then used to make the disturbed system track zero references.

• Effect of temperature on $In_{x}Ga_{1−x}As/GaAs$ quantum dots

In this paper, the strain, band-edge, and energy levels of pyramidal $In_{x}Ga_{1−x}As/GaAs$ quantum dots are investigated by 1-band effective mass approach. It is shown that while temperature has no remarkable effect on the strain tensor, the band gap lowers and the radiation wavelength elongates by increasing temperature. Also, band gap and energy do not linearly decrease by temperature rise. Our results appear to agree with former researches. This can be used in designing laser devices and sensors when applied in different working temperatures. Furthermore, when the device works for a long time, self-heating occurs which changes the characteristics of the output.

• Charge and/or spin limits for black holes at a non-commutative scale

In the commutative geometrical background, one finds the total charge $\mathcal{(Q)}$ and/or the total angular momentum $\mathcal{(J)}$ of a generalized black hole of mass $M$ to be bounded by the condition $\mathcal{Q^{2} + (J/M)^{2} \leq M^{2}}$, whereas the inclusion of the concept of non-commutativity in geometry leads to a much more richer result. It predicts that the upper limit to $\mathcal{Q}$ and/or $\mathcal{J}$ is not fixed but depends on the mass/length scale of black holes; it (the upper limit to $\mathcal{Q}$ and/or $\mathcal{J}$ ) goes towards a ‘commutative limit’ when $M \gg \sqrt{\vartheta} (\sqrt{\vartheta}$ characterizes the minimal length scale) and rapidly diminishes towards zero with $M$ decreasing in the strongly non-commutative regime, until approaching a perfect zero value for $M \simeq 1.904\sqrt{\vartheta}$. We have performed separate calculations for a pure Kerr or a pure Reissner–Nordström black hole, and briefly done it for a generalized black hole.

• Recent observational constraints on EoS parameters of a class of emergent Universe

Emergent Universe (EU) model is investigated here using the recent observational data of thebackground tests. The background test data comprise observed Hubble data, baryon acoustic oscillation data, cosmic microwave background shift data and Union compilation(2.1) data. The flat EU model obtained by Mukherjee $\it{et al}$ is permitted with a non-linear equation of state (in short, EoS) $(p = Aρ − Bρ^{1/2})$, where $A$ and $B$ are constants. The best-fit values and permitted range of values of the EoS parameters are determined in general EU model and in specific EU model $(A = 0)$ by using chi-square minimization technique. The best-fit values of the EoS parameters are used to study the evolution of the squared adiabatic sound speed $c^{2}_{s}$ , state parameter $\omega$ anddeceleration parameter $q$ for different red-shifts $z$. The present age of the Universe $t_0$ has been determined in general EU model as well as for EU model with $A = 0$. The late accelerating phase of the Universe in the EU model is accommodated satisfactorily.

• Propagation characteristics of ion-acoustic double layer in multicomponent inhomogeneous auroral zone plasma

In the present investigation, ion-acoustic double layers in an inhomogeneous plasma consisting of Maxwellian and non-thermal distributions of electrons are studied.We have derived a modified Korteweg–de Vries (mKdV) equation for ion-acoustic double layers propagating in a collisionless inhomogeneous plasma. It is observed that the non-thermal parameters affect the amplitude and width of the double layer which further depend on the density.

• Observational constraints on extended Chaplygin gas cosmologies

We investigate cosmological models with extended Chaplygin gas (ECG) as a candidate for dark energy and determine the equation of state parameters using observed data namely, observed Hubble data, baryon acousticoscillation data and cosmic microwave background shift data. Cosmological models are investigated considering cosmic fluid which is an extension of Chaplygin gas, however, it reduces to modified Chaplygin gas (MCG) andalso to generalized Chaplygin gas (GCG) in special cases. It is found that in the case of MCG and GCG, the best-fit values of all the parameters are positive. The distance modulus agrees quite well with the experimental Union2data. The speed of sound obtained in the model is small, necessary for structure formation. We also determine the observational constraints on the constants of the ECG equation.

• Low-energy electron impact cross-sections and rate constants of $NH_2$

This systematic study reports various electron impact cross-sections, rate constants and transport properties of $NH_2$ radical in the low-energy limit. The collision study is based on $R$-matrix formalism and involves the use of various scattering models employing different active spaces. Both electron excited inelasticcross-sections and resonances are found influenced by correlation and polarization effects. The non-relativistic molecular bremsstrahlung radiation cross-section for soft photons, binary encounter Bethe model-based ionization cross-sections and a few molecular properties of the target radical are also reported. The present calculations are found to be in agreement with the available results. This theoretical study provides a pathway to understand collision dynamics and generates data required in various fields of applied physics.

• Transverse spin with coupled plasmons

We study theoretically the transverse spin associated with the eigenmodes of a thinmetal film embedded in a dielectric. We show that the transverse spin has a direct dependence on the nature and strength of the coupling leading to two distinct branches for the long- and short-range modes. We show that the short-range mode exhibits larger extraordinary spin because of its more ‘structured’ nature due to higher decay in propagation. In contrast to some of the earlier studies, calculations are performed retaining the full lossy character of the metal. In the limit of vanishing losses, we present analytical results for the extraordinary spin for both the coupled modes. The results can have direct implications for enhancing the elusive transverse spin exploiting the coupled plasmon structures.

• Complex dynamics of a particle in an oscillating potential field

In this paper, the classical problem of the motion of a particle in one dimension with an external time dependent field is studied from the point of view of the dynamical system. The dynamical equations of motion of the particle are formulated. Equilibrium points of the non-oscillating systems are found and their local stability natures are analysed. Effect of oscillating potential barrier is analysed through numerical simulations. Phase diagrams,bifurcation diagrams and variations of largest Lyapunov exponents are presented to show the existence of a wide range of nonlinear phenomena such as limit cycle, quasiperiodic and chaotic oscillations in the system. Effects ofnonlinear damping in the model are also reported. Analysis of the physically interesting cases where damping is proportional to higher powers of velocity are presented for the sake of generalizing our findings and establishingfirm conclusion.

• Short-channel drain current model for asymmetric heavily/lightly doped DG MOSFETs

The paper presents a drain current model for double gate metal oxide semiconductor field effect transistors (DG MOSFETs) based on a new velocity saturation model that accounts for short-channel velocity saturation effect independently in the front and the back gate controlled channels under asymmetric front and back gate bias and oxide thickness. To determine the front and the back-channel velocity saturation, drain-induced barrierlowering is evaluated by effective gate voltages at the front and back gates obtained from surface potential at the threshold condition after considering symmetric and asymmetric front and back oxide thickness. The model alsoincorporates surface roughness scattering and ionized impurity scattering to estimate drain current for heavily/lightly doped channel for short-channel asymmetric DG MOSFET and a good agreement has been achieved with TCADsimulations, with a relative error of around 3–7%.

• # Pramana – Journal of Physics

Current Issue
Volume 90 | Issue 3
March 2018