Showing papers in "Pramana in 1999"

Quasi-one dimensional electrical conductivity and thermoelectric power studies on a discotic liquid crystal

Abstract: We have studied the electrical conductivity of well aligned samples of hexahexylthiotriphenylene (HHTT) in the pure as well as doped states. The dopant used was a small concentration (0.62 mole %) of the electron acceptor trinitrofluorenone (TNF). In the columnar phases, doping causes the AC(1 kHz) conductivity along the columnar axis (σ ‖) to increase by a factor of 107 or more relative to that in undoped samples; σ ‖ attains a value of 10−2S/m, which was the maximum measurable limit of our experimental set up. On the other hand, in the isotropic phase doping makes hardly any difference to the conductivity. The frequency dependence of the conductivity has been investigated. The DC conductivity of doped samples exhibits an enormous anisotropy, σ ‖/σ ⊥ ≥ 1010, which is 7 orders higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor. We also report the first thermoelectric power studies on these ‘molecular wires’. The sign of the thermoelectric power is in conformity with the expected nature of the charge carriers, namely, holes.

Anisotropic fluid distributions on pseudo-spheroidal spacetimes

Abstract: An exact solution of Einstein’s field equations for anisotropic fluid distribution on the background of a pseudo-spheroidal spacetime has been reported. The models based on this solution are found to accommodate density variation of high degree from the centre to the boundary of the distribution and admit a subclass for which both the radial and tangential pressures vanish at the boundary of the configuration.

Molecular structure and chiral liquid crystalline phases

Abstract: We describe briefly some results obtained on both chiral and achiral compounds exhibiting chiral mesophases. We report the first example of a single component system exhibiting the undulated twist grain boundary C* or UTGB C* phase. Preliminary results concerning a few achiral compounds composed of banana-shaped molecules exhibiting a mesophase is reported. They have been investigated by polarized light optical microscopy, differential scanning calorimetry and x-ray diffraction studies.

New phase modulated solutions for a higher-order nonlinear Schr¨ odinger equation

Abstract: We present exact stationary phase modulated solitary wave and kink solutions for a higher-order nonlinear Schrodinger equation. These solutions differ from the known solutions in the literature both in phase and amplitude. The relevance of these new solutions in the context of propagation of light pulses in optical fibres is also discussed. In particular, we have shown that higher order nonlinear dispersion terms in the nonlinear Schrodinger equation can lead to shock wave solutions.

An x-ray biplanar photodiode and the x-ray emission from magnetically confined laser produced plasma

Abstract: Development of a single and multichannel biplanar vacuum photodiode for x-ray detection is reported, which has been used to study the x-ray emission from laser produced plasma expanding across an externally applied magnetic field. Two to three times enhancement in x-ray emission has been observed which was found correlated with decrease in size of the x-ray emitting plasma plume (expansion velocity of plasma). Experimental observations were found in close agreement with the analytical model based on an increase in plasma density as a result of plasma confinement in magnetic field. Temporal evolution of x-ray emission indicates that recombination radiation seems to be playing an important role in x-ray enhancement.

Liquid water: A very complex fluid

Abstract: Although H2O has been the topic of considerable research since the beginning of the century, the peculiar physical properties are still not well understood First we discuss some of the anomalies of this ‘complex fluid’ Then we describe a qualitative interpretation in terms of percolation concepts Finally, we discuss recent experiments and simulations relating to the hyothesis that, in addition to the known critical point in water, there exists a ‘second’ critical point at low temperatures In particular, we discuss very recent measurements of the compression-induced melting and decompression-induced melting lines of high-pressure forms of ice We show how knowledge of these lines enables one to obtain an approximation for the Gibbs potential G(P, T) and the equation of state V(P,T) for water, both of which are consistent with the possible continuity of liquid water and the amorphous forms of solid water

On the `Listeria' propulsion mechanism

Abstract: We give a short overview of three different aspects of the work done in our laboratory on the propulsion mechanism of the bacterium named ‘listeria’. They concern (1) the mechanical properties of the comet of the native bacterium, (2) the polymerization/crosslinking process of actin gels in spherical geometry, and (3) a theoretical analysis of the mesoscopic aspects of the propulsion.

Ultrasonic velocity of binary systems at elevated pressures

Abstract: Various empirical theories of ultrasonic velocity have been applied to three binary liquid mixtures, under pressures up to 200 MPa and their validity have been tested A pressure dependent study of ultrasonic velocities has been made at 30315 K The agreement between theory and experiment is found to be quite satisfactory

Linear and nonlinear rheology of wormlike micelles

Abstract: Several surfactant molecules self-assemble in solution to form long, cylindrical, flexible wormlike micelles. These micelles can be entangled with each other leading to viscoelastic phases. The rheological properties of such phases are very interesting and have been the subject of a large number of experimental and theoretical studies in recent years. We shall report our recent work on the macrorheology, microrheology and nonlinear flow behaviour of dilute aqueous solutions of a surfactant CTAT (Cetyltrimethylammonium Tosilate). This system forms elongated micelles and exhibits strong viscoelasticity at low concentrations (∼0.9 wt%) without the addition of electrolytes. Microrheology measurements of G(θ) have been done using diffusing wave spectroscopy which will be compared with the conventional frequency sweep measurements done using a cone and plate rheometer. The second part of the paper deals with the nonlinear rheology where the measured shear stress σ is a nonmonotonic function of the shear rate $$\dot \gamma $$ . In stress-controlled experiments, the shear stress shows a plateau for $$\dot \gamma $$ larger than some critical strain rate, similar to the earlier reports on CPyCl/NaSal system. Cates et al have proposed that the plateau is a signature of mechanical instability in the form of shear bands. We have carried out extensive experiments under controlled strain rate conditions, to study the time-dependence of shear stress. The measured time series of shear stress has been analysed in terms of correlation integral and Lyapunov exponent to show unambiguously that the behaviour is typical of low dimensional dynamical systems.

Nonequilibrium noise and instabilities in membranes with active pumps

Abstract: We study the statistical properties and dynamics of flexible fluid membranes containing active transmembrane proteins and find that such active membranes can be either stable or unstable to small disturbances, depending on the signs of certain crucial parameters characterising the protein-membrane interaction. A major finding is that the shape-fluctuation spectrum in the stable case for a zero-tension active membrane has the same form over a large range of length scales as at equilibrium, but with a nonequilibrium "noise temperature" determined by the activity.

The analysis of reactions leading to synthesis of super heavy elements within the dinuclear system concept

Abstract: The dinuclear system concept of complete fusion of nuclei has been applied to the analysis of superheavy elements synthesis. The optimal excitation energy of compound nuclei and production cross sections in the cold synthesis of heavy elements with charge Z=102–112 have been calculated. The possibility of synthesizing the element with magic number Z=114 in cold and hot fusion reactions has been considered.

Phase separated composite films of liquid crystals

Abstract: Phase separation of liquid crystals from a solution with polymers has long been studied and used to prepare polymer stabilized and polymer dispersed structures. They are formed by spatially isotropic phase separation. A new mode, in which the phase separation proceeds anisotropically, has recently been discovered. Known as phase separated composite films (PSCOF), the resultant structures are made of adjacent parallel layers of liquid crystal and solidified polymer. PSCOFs have been made with nematic, ferroelectric (FLC), and antiferroelectric (AFLC) liquid crystals. Liquid crystals in PSCOFs exhibit electro-optical properties not observed in devices prepared by conventional methods, polymer dispersion, or polymer stabilization methods. Devices incorporating FLCs possess grey scale and switch 100 times faster at low fields than conventional surface stabilized devices. This method makes it possible to prepare very flexible devices and devices with liquid crystal film thickness comparable to optical wavelengths with great ease.

Strong curvature naked singularities in generalized vaidya space-times

Abstract: In this paper, following recent results on generalized Vaidya solutions by Wang, we prove that under certain conditions on generalized mass function, strong curvature naked singularities exist in radiation collapse in monopole-Vaidya space-times and also in charged-Vaidya space-times. We thus unify and generalize results of Dwivedi-Joshi and Lake-Zannias. The general case also covers de Sitter-Vaidya space-time recently treated by Wagh-Maharaj with a view to study existence of naked singularities.

Magnetic structure of (Fe0.97Cr0.03)2P

Abstract: Magnetic behaviour of di-metal iron phosphide with a small substitution of iron by chromium, (Fe0.97Cr0.03)2P, has been studied using SQUID magnetometry and powder neutron diffraction. It is paramagnetic at temperatures above ∼180 K with persisting short range ferromagnetic (FM) order. At lower temperatures three different regions of magnetic behaviour are identified. FM order evolves in the region 180 K-120 K but much more slowly and with much less magnetic moments than in Fe2P. In the region 120 K-50 K negative exchange interactions gain some importance leading to a loss of FM order. Below 50 K FM interactions again dominate. Pinning centres influence the behaviour at low temperature up to ∼100 K.

Energy spectrum of ejected electrons in ionization of hydrogen atoms by electrons

Abstract: Energy spectrum of ejected electrons in ionization of hydrogen atoms has been calculated following a multiple scattering theory of Das and Seal [15]. The results show peaks around two to three Rydbergs of energies of the ejected electrons, for incident electron energy of 250 eV and 500 eV, considered here, and for different combinations of the angular variables of the scattered and the ejected electrons, for scattering in a plane. The peaks are very similar to those observed in relativistic K-shell ionization of Ag atoms by electrons at 500 KeV energy [6]. The physical origin of these peaks may be traced to the second order scatterings, scattering first by the atomic nucleus (or the atomic electron) and then a second time by the atomic electron. These peaks are, however, absent in the first Born results. Experimental verification of the present results and theoretical calculation by some other well-known methods will be interesting.

Interesting normal state and superconducting properties of the intermediate valence compound CeRu2

Abstract: Superconductivity in CeRu2 was discovered 40 years ago, and was extensively studied because alloying with magnetic elements showed the coexistence of superconductivity and magnetic order. The normal state of CeRu2 has been of interest because of its intermediate valuence character. The superconducting state has been studied extensively because of its paramagnetic nature and anomalous pinning properties. This review presents the present status of knowledge, and discusses the puzzling features of CeRu2.

Debye-Waller factors and Debye temperatures of alkali halide mixed crystals

Abstract: The Debye-Waller factors of KxRb(1-x)I and NaClxBr(1-x) mixed crystals have been determined from X-ray diffraction intensities. The mean Debye-Waller factor is found to vary nonlinearly with the composition with positive deviations from linearity. The Debye temperatures (calculated from the Debye-Waller factors) are found to vary slightly non-linearly with composition with negative deviations from linearity. The Debye temperatures of seven alkali halide mixed crystal systems are critically compared with values predicted from six laws for composition variation. Using the estimated standard deviation as the criterion for the goodness of the fit, it is shown that the inverse cube (Kopp-Neumann) law provides the best description for the composition dependence of Debye temperatures of mixed crystals.

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

Abstract: The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (T C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for fNRM, δeNRM and distribution parameter αNRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.

Exact scalar field cosmologies in a higher derivative theory

Abstract: We obtain exact cosmological solutions of a higher derivative theory described by the Lagrangian L=R+2αR 2 in the presence of interacting scalar field. The interacting scalar field potential required for a known evolution of the FRW universe in the framework of the theory is obtained using a technique different from the usual approach to solve the Einstein field equations. We follow here a technique to determine potential similar to that used by Ellis and Madsen in Einstein gravity. Some new and interesting potentials are noted in the presence of R 2 term in the Einstein action for the known behaviours of the universe. These potentials in general do not obey the slow rollover approximation.

Extremes of nuclear structure

Abstract: With the advent of medium and large gamma detector arrays, it is now possible to look at nuclear structure at high rotational forces. The role of pairing correlations and their eventual breakdown, along with the shell effects have showed us the interesting physics for nuclei at high spins — superdeformation, shape co-existence, yrast traps, alignments and their dramatic effects on nuclear structure and so on. Nuclear structure studies have recently become even more exciting, due to efforts and possibilities to reach nuclei far off from the stability valley. Coupling of gamma ray arrays with ‘filters’, like neutron wall, charged particle detector array, gamma ray total energy and multiplicity castles, conversion electron spectrometers etc gives a great handle to study nuclei produced online with ‘low’ cross-sections. Recently we studied, nuclei in mass region 80 using an array of 8 germanium detectors in conjunction with the recoil mass analyser, HIRA at the Nuclear Science Centre and, most unexpectedly came across the phenomenon of identical bands, with two quasi-particle difference. The discovery of magnetic rotation is another highlight. Our study of light In nucleus, 107In brought us face to face with the ‘dipole’ bands. I plan to discuss some of these aspects. There is also an immensely important development — that of the ‘radioactive ion beams’. The availability of RIB, will probably very dramatically influence our ‘conventional’ concept of nuclear structure. The exotic shapes of these exotic nuclei and some of their expected properties will also be touched upon.

Detection of gravitational waves using a network of detectors

Abstract: We formulate the data analysis problem for the detection of the Newtonian coalescing-binary signal by a network of laser interferometric gravitational wave detectors that have arbitrary orientations, but are located at the same site. We use the maximum likelihood method for optimizing the detection problem. We show that for networks comprising of up to three detectors, the optimal statistic is just the matched network-filter. Alternatively, it is simply a linear combination of the signal-to-noise ratios of the individual detectors. This statistic, therefore, can be interpreted as the signal-to-noise ratio of the network. The overall sensitivity of the network is shown to increase roughly as the square-root of the number of detectors in the network. We further show that these results continue to hold even for the restricted post-Newtonian filters. Finally, our formalism is general enough to be extended, in a straightforward way, to address the problem of detection of such waves from other sources by some other types of detectors, eg., bars or spheres, or even by networks of spatially well-separated detectors.

The cosmological constant revisited.

Abstract: I briefly review the observational evidence for a small cosmological constant at the present epoch. This evidence mainly comes from high redshift observations of Type 1a supernovae, which, when combined with CMB observations strongly support a flat Universe with . Theoretically a cosmological constant can arise from zero point vacuum fluctuations. In addition ultra-light scalar fields could also give rise to a Universe which is accelerating driven by a time de- pendent -term induced by the scalar field potential. Finally a dominated Universe also finds support from observations of galaxy clustering and the age of the Universe. The cosmological constant was introduced into cosmology by Einstein in 1917. Ein- stein, under the influence of Mach's principle, constructed a Universe which was static and closed - a configuration that could only arise under the joint influence of both mat- ter and a cosmological constant. Subsequently Friedmann derived expanding solutions to the Einstein equations and Einstein acknowledged that the introduction of was probably unnecessary, particularly in view of Hubble's discovery that the Universe was expanding. However, interest in the cosmological constant remained, partly due to the rich variety of new solutions which arise in the Einstein equations with a cosmological constant includ- ing: the static Einstein Universe, singularity free 'bouncing' models, quasi-static 'loitering' models etc. Interest in was reignited in the late 1960's, when it was felt than an excess of QSO's was being observed at redshift 1.95. This observation was difficult to explain in the framework of standard FRW cosmology, but easier to account for if the Universe loitered at that redshift. More recently a large cosmological constant at an early epoch is the basis of the inflationary model, and a much smaller cosmological constant at a much later epoch is suggested by current observations.

Theoretical study of structural energy, phonon spectra, and elastic constants of Rh and Ir

Abstract: The transition metal pair potential (TMPP) is used to study band structure energy of Rh and Ir. Both metals are found to be most stable in fcc structure down to atomic volume 0.5V 0. The pressure at 0.5V 0 is found to be 5.235 Mbar and 9.216 Mbar in Rh and Ir, respectively. The TMPP is also used to study other properties of these metals like cohesive energy, phonon frequencies at observed volume. The bulk moduli and elastic constants of these metals at observed volume are calculated by including the volume contribution.

Calculation of magnetic multipole moment integrals using translation formulas for slater-type orbitals

Abstract: Using translation formulas for Slater type orbitals (STO’s) the infinite series through the overlap integrals are derived for the magnetic multipole moment integrals. By the use of the derived expressions the magnetic multipole moment integrals, therefore, the magnetic properties of molecules can be evaluated most efficiently and accurately. The convergence of the series is tested by calculating concrete cases. An accuracy of 10−5 for the computer results is obtained in the case 2 p -pole magnetic moment integrals for 1 ≤ v ≤ 5, and for the arbitrary values of internuclear distances and screening constants of atomic orbitals.

On quark matter in a strong magnetic field

Abstract: The effect of strong magnetic field on the bulk properties of quark matter is reinvestigated takingu, d ands-quarks as well as electrons in the presence of magnetic field. Here the bag pressure is chosen such that in the absence of magnetic field and at zero temperature the binding energy of theuds-system is <930 MeV while that ofud-system is greater than 940 MeV. It is observed that the equation of state changes significantly in a strong magnetic field. At finite temperature the electron chemical potential varies between 6 and 50 MeV. Thus the expansion of thermodynamical quantities in powers ofT/(Μi2-Mv(i)2)1/2 is valid only up to few MeV. For high temperatures ∼40 MeV the exact integral expressions are to be taken.

Effect of collimator size and absorber thickness on gamma ray attenuation measurements for bakelite and perspex

Abstract: The mass attenuation coefficient (μ m) of 662 keV gamma rays have been measured in the extended media of bakelite and perspex under different collimation conditions. The increase in attenuation coefficient is seen with increase in sample thickness as well as with collimator size due to the contribution of multiple scattered photons in the uncollided beam of 662 keV gamma rays.

Thermal lens measurement technique in end-pumped solid state lasers: Application to diode-pumped microchip lasers

Abstract: A simple derivation, which relates the thermal lens focal length in solid state lasers to pump power and a method for direct estimation of thermal lens focal length, is reported. This method is applicable to any type of stable resonator. The method is used for the measurement of the thermal lens focal length with an accuracy of 8% in an axially pumped microchip laser. The variation of focal length with pump power is also measured.

An alternative approach to determine the spot-size of a multi-mode laser beam and its application to diode laser beams

Abstract: An alternative approach is suggested to determine the spot-size of a multi-mode laser beam. It has been shown by simulations that the suggested approach can give the beam quality factor and characteristic radius with less than 5% error. Unlike the power content method, the proposed method is applicable to the beams even with diameter one tenth of the CCD size. The new approach has been applied to a multi-mode diode laser output and it is shown that the ABCD matrix analysis can be used for beam propagation, with the measured parameters of the laser.

Some new four-parameter potentials and their use in the study of vibrational thermodynamical quantities of diatomic molecules

Abstract: Three four-parameter potentials,U I,U II andU III have been proposed and their accuracy has been demonstrated by finding the mean square deviation from the true RKR potential curve for 15 electronic states of 12 diatomic molecules. Their percentage average mean square deviations from RKR curve have been found to be 1.45, 1.86 and 2.89, respectively. These compare favourably with the value 2.67 for the recently suggested four-parameter potential of Wei Hua which itself yields better results than the commonly employed three-parameter potentials. The superiority of the new potentials (especially ofU I andU II has been further established by using these potentials to calculate the molecular constants αe and ωexe following Dunham’s method. The corresponding percentage average mean deviations, for αe, turn out to be 3.75, 5.13 and 15.43 and for ωexe 8.73, 17.23 and 27.49, respectively, against the respective values of 7.97 and 18.88 with Wei Hua’s four-parameter potential. Also included are the values of dissociation energy determined with these potentials and these too corroborate the better performance ofU I andU II. The relative worth of various potential functions has been further tested by carrying out numerical study of vibrational partition function (evaluated by sum over states method), entropy and thermal capacity for the ground state of 7 molecules and comparing these with the corresponding findings based on the RKR data.

Genericity and stability of naked singularities arising in an inhomogeneous dust collapse

Abstract: In this paper, we consider an inhomogeneous dust collapse, and extend earlier works of Jhingan, Joshi, and Singh to the case where initial density and velocity distributions are finitely differentiable functions of co-moving coordinate r. We study the occurrence of naked singularities under various conditions on the derivatives of initial density and velocity distributions in marginally as well as non-marginally bound case. We then study their stability and genericity with respect to perturbations in the initial data in an appropriate topological sense.