• Sanjay

Articles written in Pramana – Journal of Physics

• Computer calculation of positron drift velocities in He, Ne and Ar

The drift velocities of positrons in rare gases, He, Ne and Ar have been calculated at various temperatures. The drift velocity depends quite sensitively on the strength of the electric field and temperature.

• Cosmological models consistent with supersymmetric compactiflcation of superstring theories

The compactification of 10-dimensional supergravity, coupled to super Yang-Mills theory, to curved 4-dimensional spacetimes is investigated. The requirement of unbroken supersymmetry leads to a set of consistency conditions. These are fairly restrictive, but nevertheless permit some nontrivial solutions, including the Milne universe. More general time-dependent metrics are also not ruled out.

• Some recent applications of cell dynamical modelling to phase ordering dynamics

We study experimentally relevant effects in phase ordering dynamics using Cell Dynamical System (CDS) models. In particular, we present representative numerical results for phase ordering in random magnets and phase separation in binary fluids.

• Surface adsorption and collapse transition of linear polymer chains

The critical behaviour of surface adsorption and collapse transition of a flexible self-attracting self-avoiding polymer chain is examined. Depending upon the underlying lattice and space dimensionality, phase diagrams that exhibit many different universality domains of critical behavior are found. We discuss these phase diagrams and the values of the critical exponents found from different theoretical methods.

• Effect of polarization on population transfer in H2 by stimulated Raman transition with partially overlapping laser pulses

Polarization effects on population transfer by stimulated Raman transition using overlapping time dependent pump and Stokes laser pulses from the ground X1Σg/+(vg=0, Jg=1) level of H2 to the final X1Σg/+(vf=1, Jf=1) level via the intermediate B1Σu/+(vi=14, Ji=0,2), C1Πu/+(vi=3, Ji=2) and C1Πu/−(vi=3, Ji=1) levels have been theoretically investigated by applying the density matrix formalism. We have studied in detail the dependence of the population transfer on time delay between two pulses for the cases of on-resonance excitations considering linear parallel and same-sense circular polarizations of the fields. The pump and Stokes fields are taken as having Gaussian pulse shapes with peak intensities IP/0(IS/0)=2 × 106 and 1 × 107 W/cm2. Density matrix equations have been solved for each value of the magnetic quantum number Mg(0, ±1) of the initial ground level taking into account the Mg dependence of the Rabi frequencies. Mg — averaged population transfer to the final level has also been calculated. For resonance excitations to the B(14, 0) or C(3, 1) levels, appreciable population transfer is achieved for intuitive pulse order for some particular values of Mg and Mi (magnetic quantum number of the resonant intermediate level) depending on the nature of polarizations. The calculated values of Mg — averaged population transfer for the two cases of polarizations show that for on-resonance excitation to the B(14, 0) or the C(3, 1) level, linear parallel polarization of the laser fields yield more transfer efficiency whereas for resonance excitation to the B(14, 2) level, larger population transfer results from the same-sense circular polarizations. For resonance excitation to the C(3, 2) level, Mg — averaged population is found to be almost polarization independent. The calculations for the six-level H2 system reveal some interesting features of polarization effects on the population transfer efficiency.

• Multifractal behaviour of n-simplex lattice

We study the asymptotic behaviour of resistance scaling and fluctuation of resistance that give rise to flicker noise in an n-simplex lattice. We propose a simple method to calculate the resistance scaling and give a closed-form formula to calculate the exponent, βL, associated with resistance scaling, for any n. Using current cumulant method we calculate the exact noise exponent for n-simplex lattices.

• Bianchi type I string cosmologies

By making use of Letelier’s form of energy—momentum tensor for a cloud of stringdust we present some classes of solutions of general relativistic field equations which describe cosmological string-dust models in Bianchi type I space-time. Some of the classes of models obey Takabayashi’s equation of state whereas a class of models exhibits inflation in the initial stage. Two of the classes presented here have Kasner’s space-time as past asymptote

• D-branes inpp-wave background

We show the existence of classical solutions ofD-branes as well as a system ofD3-branes oriented at an arbitrary angle with respect to each other, in a six-dimensionalpp-wave background obtained fromAdS3 × S3 ×R4, withR — R andNS — NS 3-from flux. The world volume coordinate of D5-brane lies along the six-dimensional pp-wave directions, whereas thepp-wave direction is transverse to the system of D3-branes. We also present moreD-brane bound state solutions by applyingT-duality symmetries. The system ofD3-branes oriented at an arbitrary angle is shown to preserve 1/16 supersymmetries. Finally a brief discussion of the open string construction is presented for both the cases.

• Wetting and phase separation at surfaces

We study the problem ofsurfacedirected spinodal decomposition, viz., the dynamical interplay of wetting and phase separation at surfaces. In particular, we focus on the kinetics of wetting-layer growth in a semi-infinite geometry for arbitrary surface potentials and mixture compositions. We also present representative results for phase separation in confined geometries, e.g., cylindrical pores, thin films, etc.

• Thermoelastic properties of minerals at high temperature

The knowledge of elasticity of the minerals is useful for interpreting the structure and composition of the lower mantle and also in seismic studies. The purpose of the present study is to discuss a simple and straightforward method for evaluating thermoelastic properties of minerals at high temperatures. We have extended the Kumar’s formulation by taking into the account the concept of anharmonicity in minerals above the Debye temperature ($\theta_D$). In our present study, we have investigated the thermophysical properties of two minerals (pyrope-rich garnet and MgAl2O4) under high temperatures and calculated the second-order elastic constant ($C_{ij}$) and bulk modulus ($K_T$) of the above minerals, in two cases ﬁrst by taking Anderson–Gruneisen parameter ($\delta_T$) as temperature-independent and then by treating $\delta_T$ as temperature-dependent parameter. The results obtained when $\delta_T$ is temperature-dependent are in close agreement with experimental data.

• CO2 laser-inscribed low-cost, shortest-period long-period fibre grating in B–Ge co-doped fibre for high-sensitivity strain measurement

We have developed high sensitivity long-period fibre gratings (LPGs) in B–Ge codoped fibre for strain sensing application. These LPGs are shortest grating period (180 𝜇m) LPGs inscribed in B–Ge co-doped fibre using CO2 laser-based grating inscription set-up. Strain sensitivity of 1.77 dB/mε has been obtained for attenuation band corresponding to the turnaround point mode. TAP operation of LPG facilitates intensity-based detection using simple optical power meter instead of wavelength-based detection.

• Brief report: Volume dependence of Grüneisen parameter for solids under extreme compression

The Nie expression is amended in such a way that the expression follows the infinite pressure behaviour, i.e., P → ∞or V → 0. A new empirical relationship is developed to predict the values of volume dependence of Grüneisen parameter. NaCl and ε-Fe have been employed to test the suitability of the expression.The results obtained reveal that the relationship is reliable as there is a good agreement between the calculated and the experimental data

• Study of chaos in chaotic satellite systems

In this paper,we study the qualitative behaviour of satellite systems using bifurcation diagrams, Poincaré section, Lyapunov exponents, dissipation, equilibrium points, Kaplan–Yorke dimension etc. Bifurcation diagrams with respect to the known parameters of satellite systems are analysed. Poincaré sections with different sowing axes of the satellite are drawn. Eigenvalues of Jacobian matrices for the satellite system at different equilibrium points are calculated to justify the unstable regions. Lyapunov exponents are estimated. From these studies, chaosin satellite system has been established. Solution of equations of motion of the satellite system are drawn in the form of three-dimensional, two-dimensional and time series phase portraits. Phase portraits and time series display the chaotic nature of the considered system.

• Analysis and time-delay synchronisation of chaotic satellite systems

In this paper, we analyse the chaotic satellite system using dissipativity, equilibrium points, bifurcation diagrams, Poincare section maps, Lyapunov exponents and Kaplan–Yorke dimension. We obtain the equilibrium points of chaotic satellite system and at each equilibrium point, we obtain the eigenvalue of Jacobian matrix of the satellite system to verify the unstable region.We calculate the Kaplan–Yorke dimension, which ensures the strange behaviour of the system. We observe closely the three-dimensional (3D) phase portraits of the satellite system at different parameter values. We plot the Lyapunov exponent graphs corresponding to every 3D phase portrait of satellite systems, to verify the chaoticity of satellite systems. We establish time-delay synchronisation for twoidentical satellite systems. The simulated and qualitative results are in an excellent agreement.

• Real-time stand-off detection of improvised explosive materials using time-gated UV–Raman spectroscopy

A transportable, trolley-mounted stand-off explosive material detection system based on the time-gated Raman spectroscopy was developed and tested in our laboratory. This system is capable of identifying the explosives and improvised explosive materials located up to a distance of 30 m. A frequency tripled Nd:YAG, nanosecond pulsed laser (355 nm, 6 ns) operated at 10 Hz was used as an excitation source to induce Raman spectra of explosive materials under investigation. A reflected type 200 mm aperture telescope designed using Zemax opticaldesign software was used to collect the backscattered Raman signals. Raman signals were recorded using the gated intensified charge coupled device (ICCD) spectrograph. A LabVIEW-based data acquisition and analysis software for real-time identification of materials was developed and used. It gives audio as well as text alarm to the operatorabout threat identification.

• Pramana – Journal of Physics

Volume 94, 2020
All articles
Continuous Article Publishing mode

• Editorial Note on Continuous Article Publication

Posted on July 25, 2019