Effect of the crystal potential on electronic polarizabilities of ions in alkaline earth chalcogenides

The electronic polarizabilities of ions in alkaline earth chalcogenides are estimated by taking account of the effect of the crystalline potential. The polarizabilities thus obtained are found to present a good agreement with experimental data. It has been shown that the polarizabilities and radii of alkaline earth and chalcogenide ions follow the polarizability-radius cube relation approximately well.     

Hydromagnetic instability of streaming fluids in porous medium

The instability of the plane interface between two uniform, superposed, electrically conducting and counter-streaming fluids through a porous medium is considered in the presence of a horizontal magnetic field. In the absence of surface tension, perturbations transverse to the direction of streaming are found to be unaffected by the presence of streaming if perturbations in the direction of streaming are ignored. For perturbations in all other directions there exists instability for a certain wavenumber range. The instability of this system is postponed by the presence of magnetic field. The magnetic field and surface tension are able to suppress this Kelvin-Helmholtz instability for small wavelength perturbations and the medium porosity reduces the stability range given in terms of a difference between the streaming velocities and the Alfvén velocity.This research forms a part of the research project awarded to the first author (R.C.S.) by the University Grants Commission.     

Finite Larmor radius effect on thermosolutal instability of a plasma in porous medium

The thermosolutal instability of a plasma in porous medium is considered in the presence of finite Larmor radius effect. The finite Larmor radius, stable solute gradient and magnetic field introduce oscillatory modes in the systems which were nonexistent in their absence. For stationary convection, the finite Larmor radius and stable solute gradient have stabilizing effects on the thermosolutal instability in porous medium. In presence of finite Larmor radius effect, the medium permeability has a destabilizing (or stabilizing) effect and the magnetic field has a stabilizing (or destabilizing) effect under certain condition whereas in the absence of finite Larmor radius effect, the medium permeability and the magnetic field have destabilizing and stabilizing effects, respectively, on thermosolutal instability of a plasma in porous medium. The sufficient conditions for nonexistence of overstability are obtained.The financial assistance to Mr. Sunil in the form of Senior Research Fellowship of the Council of Scientific and Industrial Research (CSIR), New Delhi is gratefully acknowledged.     

Search for a precursor crystal-to-crystal phase transition to amorphization in α-GeO2 and α-AlPO4 under pressure

Recent X-ray diffraction studies on α-quartz (SiO₂) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced amorphization, α-GeO₂ and α-AlPO₄ (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO₂.     

Contact hypersurfaces of Kaehler manifolds

Contact hypersurfaces of a Kaehler manifold have been characterized and classified, assuming the second fundamental form to be Codazzi (in particular, parallel). We have also discussed the special cases when the ambient space is a (i) Calabi-Yau manifold and (ii) a complex space-form.     

Pressure dependence of the compressibility of alkali halides

The parameterC ₁=[(1/K _T )/P] _T , which describes the pressure variation of the compressibility, has been examined correlating the thermodynamical and interatomic potential approaches employing fewer approximations than has been usual heretofore. General expressions have been derived forC ₁ by including the thermal correction terms, which have generally been ignored in previous studies concerning thermal properties of ionic crystals. The parameterC ₁ has also been related to the Grüneisen parameter, , using a relation given earlier. The applicability of the derived equations is investigated and discussed for alkali halides employing few realistic potential forms. A good general accord is found with the available experimental data, which exhibits an essential improvement over other theoretical determinations.     

On the non-BoltzmannA-X emission in Na2 following laser excitation of theB state

A ¹Σ _u ⁺ -X ¹Σ _g ⁺ emission in Na₂ is observed following excitation ofB ¹π _u by various lines of an argon ion laser. The excitation energy ofB ¹π _u is collisionally transferred to the (2)¹Σ _g ⁺ which then radiatively populates theA ¹Σ _u ⁺ state. The Na vapour is contained in a stainless steel crossed heat pipe with Ar buffer gas and temperature around 600°C. For all laser lines except 4579 Å, the coarse features ofA-X emission are independent of the laser wavelength. However, at high resolution the finer differences between different laser line excitation are explained. Variousv′-v″ transitions in this emission are identified. Computer simulation is presented to help explain some features of this emission.     

Effect of pressure on the behavior of nontransition metallic superconductors

An explicit expression has been derived for the volume dependence of electron-phonon coupling strength () by using a recently proposed quadratic form of the screened pseudopotential and by considering the variation of Fermi momentum with volume. Variation of Coulomb pseudopotential ( ^*) has also been explicitly considered by varying the Fermi momentum. Dependence of transition temperatureT _c and the logarithm of the effective interaction strength (N ₀ V) on the volume has been studied for eight nontransition metallic superconductors. A linear drop inT _c for small values of (–/ ₀) as well as the calculated values of= InN ₀ V/ In, exhibits better matching with the experimental data, as compared to the earlier theoretical results.     

Analysis of surface and guided wave plasmon polariton modes in insulator–metal–insulator planar plasmonic waveguides

Theoretical and experimental study of the surface plasmon–polariton and guided wave plasmon polariton modes is presented for the Sapphire/Ag/Polycarbonate/Air structure. Theoretical results are obtained by solving complex multilayer eigenvalue equations as well as the reflectivity equation for this structure. It is proposed that the mode attenuation can be significantly reduced by inserting a low index dielectric buffer between the metal and the guiding dielectric layer. The dispersion and attenuation curves are generated. Both the surface plasmon and guided wave plasmon polariton modes are studied experimentally. The experimental values of the effective refractive indices agree well with the theoretical values. The electric field profiles are generated and used to examine the nature of modes. After optimization of various parameters the condition for low loss single mode guiding is obtained for the proposed structure. Effect of metal thickness on surface plasmon mode is also discussed. It is inferred that in a properly optimized plasmonic waveguide, the losses can be reduced by a factor of 4.