Showing papers in "Pramana in 1990"
TL;DR: In this paper, some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel, which correspond to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of stateρ=(1+W)λ.
Abstract: Some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel. A class of solutions corresponds to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of stateρ=(1+W)λ.
146 citations
TL;DR: In this paper, the effect of variation of dielectric constant on the relative magnetic field effect in singlet luminescence has been studied using a typical exciplex system at a saturating field.
Abstract: The effect of variation of dielectric constant on the relative magnetic field effect in singlet luminescence has been studied using a typical exciplex system at a saturating field. The study indicates strong specificity in the perturbation of the magnetic field effect by alcoholic solvents. In contrast to alcohols where relative singlet magnetic field effect is of the order of 2% only, the magnetic field effect in non-alcoholic medium reaches as high as 9%. Moreover, dielectric constant variation in alcohols yields curves which are distinctly different from those in non-alcoholic media. It turns out that this dependence of magnetic field effect on dielectric constant is similar in all non-alcoholic solvent mixtures. An analytical study based on Hong and Noolandi’s solution of Smoluchowski equation has been made. Derived expressions can interpret experimental curves reasonably well.
43 citations
TL;DR: In this paper, a diamond anvil cell and its X-ray diffraction pattern studied by the energy-dispersive technique were compressed to pressures of 16 GPa and the compound became amorphous at ∼ 12 GPa.
Abstract: AlPO4 has been compressed to pressures of 16 GPa in a diamond anvil cell and its X-ray diffraction pattern studied by the energy-dispersive technique. The compound is observed to become amorphous at ∼ 12 GPa. This explains the loss of Raman spectrum of AlPO4 reported by Jayaraman and coworkers (1987).
24 citations
TL;DR: In this article, the angular dependence of emission intensity of L shell X-rays induced by 59.57 keV photons in Pb and U is investigated by measuring the normalized intensities of the resolved L X-ray peaks at different angles varying from 40° to 120°.
Abstract: The angular dependence of emission intensity of L shell X-rays induced by 59.57 keV photons in Pb and U is investigated by measuring the normalized intensities of the resolved L X-ray peaks at different angles varying from 40° to 120°. It is observed that while L
l
and Lα X-ray peaks (originating fromJ = 3/2 state) show some anisotropic angular distribution, the emission of Lβ and Lγ X-ray peaks is isotropic. The present results contradict the calculations of Co-oper and Zare (1969) that after photoionization of inner shell, the vacancy state has equal population of magnetic substates and the subsequent X-ray emission is isotropic but confirm the predictions of Fluggeet al (1972) that the atomic inner shell vacancies produced after photoionization are aligned and the x-ray emission from the filling of vacancies in state withJ ⩾ 3/2 is anisotropic.
21 citations
TL;DR: In this article, the Sogami pair potential has been used to explain features of a homogeneous liquid as well as phase separated states, whereas the screened Coulomb repulsive Yukawa potential does not.
Abstract: Static structure factorsS(Q) are obtained for dilute charged colloidal systems using Brownian dynamics simulation method for the widely used screened Coulomb repulsive Yukawa potential and the recently proposed Sogami pair potential. The latter potential has, in addition to the usual repulsive part, an attractive term which is necessary to understand the reentrant phase transition reported in these colloids. It is shown for the first time thatS(Q) obtained using the Sogami potential for parameters favourable for liquid-like order agrees well with that measured experimentally. Thus it appears that the Sogami potential explains features of a homogeneous liquid as well as phase separated states, whereas Yukawa potential does not.
18 citations
TL;DR: In this article, a definition for the intrinsic time co-ordinate is proposed, using the phase of the wave function of the universe, which generalizes the notion of time coordinate which arises in the semiclassical cosmology and leads to acceptable results for the evaluation of expectation values of physical variables.
Abstract: A definition for the intrinsic time co-ordinate is proposed, using the phase of the wave function of the universe. This definition generalizes the notion of time co-ordinate which arises in the semiclassical cosmology. It also leads to acceptable results for the evaluation of expectation values of physical variables.
17 citations
TL;DR: In this article, the ionization energies and polarizabilities of a donor in an isolated well of a quasi two dimensional (Q2D) GaAs/Ga1−x Al x As heterostructure have been obtained for different well widths including electron-lattice coupling.
Abstract: The ionization energies and the polarizabilities of a donor in an isolated well of a quasi two dimensional (Q2D) GaAs/Ga1−x Al x As heterostructure have been obtained for different well widths including electron-lattice coupling A wave function that properly reduces to the hydrogenic function in the limiting case has been used For fields of the order of 105 V/m, the ionization energies decrease slightly with electric fields for all well widths (10 nm to 50 nm) studied Also for a given electric field, as the well width increases, the ionization energy decreases For fields of the order of 107 V/m and for smaller well widths (<10 nm), the ionization energy generally increases with electric field The results also show that for electric fields of this order, no donor bound state associated with the lowest subband is possible for well widths greater than 20 nm The polarizabilities estimated using the expression for the dipole operator show that as the well width increases, the polarizability values also increase and do not show any abnormal behaviour
13 citations
TL;DR: In this paper, the static properties such as magnetic moments, charge radii and axial vector coupling constant ratios of the quark core of baryons in the nucleon octet have been calculated in an independent quark model based on the Dirac equation with equally mixed scalar-vector potential in linear form in the current quark mass limit.
Abstract: The static properties such as magnetic moments, charge radii and axial vector coupling constant ratios of the quark core of baryons in the nucleon octet have been calculated in an independent-quark model based on the Dirac equation with equally mixed scalar-vector potential in linear form in the current quark mass limit. The results obtained with appropriate corrections due to centre-of-mass motion are in reasonable agreement with experimental data. The magnetic moments of the quark core of baryons in the charmed andb-flavoured sectors have also been calculated with this model and the overall predictions so obtained compare very well with other model predictions.
13 citations
TL;DR: In this paper, the effect of acceptor concentration on the energy transfer from Coumarin 30 (donor) to Rhodamine 6G (acceptor) has been studied.
Abstract: The effect of acceptor concentration on the energy transfer from Coumarin 30 (donor) to Rhodamine 6G (acceptor) has been studied. The nature of energy transfer reaction has been studied through lifetime measurements by recording the time-resolved fluorescence decay curves. The energy transfer parameters calculated were used to confirm the occurrence of energy transfer on the basis of the emission-reabsorption effect.
12 citations
TL;DR: In this article, the Koster-Kohn variational principle was used for direct calculation of the Wannier function of the electron crystal. But the results of the calculation support Wigner's prediction of electron crystallization.
Abstract: The character of the ground state of the electron crystal—an electron gas with periodic density and/or spin density is investigated. Calculations for non-magnetic, ferromagnetic and anti-ferromagnetic electron crystals based on the Koster-Kohn variational principle for direct calculation of Wannier functions are presented. The Wannier function is approximated by a symmetrically orthonormalized Gaussian. The orbital exponent of the Gaussian is used as a variational parameter. The effect of the positive background is suitably taken into account. The results of our calculation support Wigner’s prediction of electron crystallization.
11 citations
TL;DR: The fusion cross section for the system 6Li+28Si has been measured atE∼36 MeV as discussed by the authors, and the nucleus-nucleus real potentials have been determined for a range of interaction distances.
Abstract: The fusion cross section for the system6Li+28Si has been measured atE∼36 MeV. Combining this with the data available at lower energies, the nucleus-nucleus real potentials have been determined for a range of interaction distances.
TL;DR: In this paper, an experimental method for measuring photoacoustic (PA) signals generated by a pulsed laser beam in liquids is described and the pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions.
Abstract: Experimental method for measuring photoacoustic(PA) signals generated by a pulsed laser beam in liquids is described. The pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions. Concentration dependence of quantum yield of rhodamine 6G in water is studied using the above method. The results indicate that the quantum yield decreases with increase in concentration in the quenching region in agreement with the existing reports based on radiometric measurements.
TL;DR: In this paper, the Ali-Bodmerα-α potential between two 12C nuclei in linear chain configuration has been calculated microscopically using the Ali Bower potential.
Abstract: The potential between two12C nuclei in linear chain configuration has been calculated microscopically using the Ali-Bodmerα-α potential. This potential shows a pocket and compares well in the tail region with the phenomenological potential extracted before, from the data on the quasi-molecular resonances of the12C +12C system. This provides support to the diatomic like rotation-vibration picture of quasi-molecular states.
TL;DR: In this article, the RF SQUID behavior due to grain boundary weak links in a bulk YBCO was observed at 77 K using modified commercial rf electronics, where the rf pumping frequency was kept slightly higher than the resonance frequency of the tank circuit.
Abstract: RF SQUID behaviour due to grain boundary weak links in a bulk YBCO is observed at 77 K using modified commercial rf electronics. Porous samples with lowI
c
are found to show this characteristic whereas dense samples with higherI
c
do not show SQUID behaviour.V-B modulation characteristic is found to be better when the rf pumping frequency is kept slightly higher than the resonance frequency of the tank circuit. Designing of coil for tank circuit with appropriateQ has been found to be very crucial for seeing the SQUID behaviour. Estimation of parameters such as coupling constant, mutual inductance, inductance and radius of the SQUID loop, have been made and their significance is discussed. Flux noise spectrum of the bulk rf SQUID in flux locked mode is also reported.
TL;DR: A hole shunted with a microbridge type of geometry is patterned manually for observing rf SQUID behavior at 77 K in YBaCuO thick films prepared by screen printing technique.
Abstract: RF SQUID behaviour has been observed at 77 K in YBaCuO thick films prepared by screen printing technique. A hole shunted with a microbridge type of geometry is patterned manually for observing rf SQUID behaviour. Flux noise spectrum is also studied and it is found to depend on the quality of the film. The spectral density of the flux noise in the white noise region is 1.7×10−3 Φ0/√Hz at 77 K.
TL;DR: In this article, the authors examined the absorption of a laser beam at different wavelengths by a partially ionized vapour during the interaction mechanism, and the applicability of theoretical models developed is discussed in detail.
Abstract: The present study examines the absorption of a laser beam at different wavelengths by a partially-ionized vapour during the interaction mechanism. The applicability of the theoretical models developed is discussed in detail. The interaction of the high- and low-power intensities of a laser beam with plasma is distinguished. It is shown that different metal vapours at similar temperatures and densities have absorption depths which may differ by an order of magnitude. Even more substantial is the difference between the absorption depths of light from different lasers in common use. It is also shown that the free electron temperature becomes significantly different from the heavy particle temperature for power intensities above the critical level which is typically > 1014W/m2. The free electron velocity distribution has an isotropic part which becomes non-Maxwellian for power intensities greater than the critical power intensity.
TL;DR: In this article, the authors review some aspects of conformal field theories on the plane as well as on higher genus Riemann surfaces, and present a survey of the literature on these topics.
Abstract: We review some aspects of conformal field theories on the plane as well as on higher genus Riemann surfaces.
TL;DR: In this article, some new solutions of shear-free imperfect fluid spheres with heat flux in the radial direction were obtained, which have isotropic pressure and could be generalizations of earlier solutions of Nariai and of Banerjee and Banerji.
Abstract: Some new solutions of shear-free imperfect fluid spheres with heat flux in the radial direction are obtained. They have isotropic pressure and could be the generalizations of earlier solutions of Nariai and of Banerjee and Banerji for perfect fluid without dissipation.
TL;DR: In this article, the inner-shell excitation of alkali-metal atoms, which leads to auto-ionization, is investigated, and the basic difficulties in making accurate calculations for inner shell excitation process are discussed.
Abstract: Inner-shell excitation of alkali-metal atoms, which leads to auto-ionization, is investigated. Comparison is made with other available data. Basic difficulties in making accurate calculations for inner-shell excitation process are discussed. Suggestions are made for further study of inner-shell process in atoms and ions.
TL;DR: In this article, the influence of annealing on the structure and opto-electronic properties of Cu0.9In1.0Se2.0 films prepared by solution growth technique has been studied.
Abstract: The influence of annealing on the structure and opto-electronic properties of Cu0.9In1.0Se2.0 films prepared by solution growth technique has been studied. The films annealed at 500–520°C in air, vacuum (10−4 torr), In-vapour and Se-vapour show polycrystalline chalcopyrite structure with orientation perpendicular to the (220) plane. Films annealed in Se-vapour at 500°C for 30 min have maximum grain size (560 A), minimum optical energy gap, maximum absorption coefficient, lowest resistivity, maximum photosensitivity and thus are suitable for photovoltaic applications. Annealing in In-vapour or in vacuum changesp-type CuInSe2 inton-type which possibly arises due to the increase in Se vacancies.
TL;DR: In this article, a brief account for the higher order wave function in Hartle-Hawking (H-H) proposal is given which is compared with the tunneling wave function due to Vilenkin.
Abstract: A brief account for the higher order wave function in Hartle-Hawking (H-H) proposal is given which is compared with the tunneling wave function due to Vilenkin. The probability distributions are determined for both types of wave functions. Also a class of solutions are evaluated using H-H approach for Kantowski-Sachs metric with a scalar field and inflation is observed.
TL;DR: In this article, diffraction patterns from Fibonacci quasilattices have been calculated, and small shifts in peak-positions are unexpectedly found, the shifts being proportional to $N^{-3/2}$ for a large number of scatterers.
Abstract: The diffraction patterns from Fibonacci quasilattices have been calculated. Finite-size effects are evaluated for weak and strong peaks. For a smaller number of scatterers (<100) there are fluctuations in the intensities of weak and strong peaks. The fluctuations in weak peaks are greater than that in strong peaks. The fluctuations in intensities of weak and strong peaks near the origin are larger than in the corresponding cases of weak and strong peaks far away from the origin. Small shifts in peak-positions are unexpectedly found, the shifts being proportional to $N^{-3/2}$ for a large number of scatterers. The diffraction pattern of a qne-dimensional crystal and random structure is compared with that of the Fibonacci quasilattice. The strong peaks observed in the diffraction pattern of l-d crystal show negligible peak-shifts, they being comparable with computational errors even when the number of scatterers is as small as 5. The implications for analysing the experiments are briefly indicated.
TL;DR: An array of seven hexagonal NaI(Tl) detectors has been set up for measuringγ-ray spectra in the energy region 5 MeV ≤E ≥ 40 MeV with good accuracy as discussed by the authors.
Abstract: An array of seven hexagonal NaI(Tl) detectors has been set up for measuringγ-ray spectra in the energy region 5 MeV ≤E
γ ≤ 40 MeV with good accuracy. This is in contrast to earlier set ups which mostly used one large sized (about 10 inchesφ × 15 inches long)NaI(Tl) detector. This set up has been made for the study ofγ decay of GDR based on high spin states and ultra-dipole radiations. The array has been provided with the following features: a) TOF discrimination against neutrons, b) pile up detection and elimination, c) active and passive shielding to cut down background and d) an array of trigger counters for multiplicity dependence measurements. The well known program EGS4 has been used to determine the response of the array forγ-rays in the energy region 5–40 MeV and several test measurements have been carried out to confirm the validity of the calculated response functions. Some typicalγ-ray spectra fromα and16O induced reactions measured at VECC, Calcutta and Pelletron accelerator at TIFR are also shown.
TL;DR: In this article, a theoretical analysis of a downstream-mixing 16-µm CO2 gas-dynamic laser revealed the possibility of utilizing the downstream mixing scheme for the generation of 9.4 µm radiation.
Abstract: A theoretical analysis of a downstream-mixing 16-µm CO2 gasdynamic laser revealed the possibility of utilizing the downstream-mixing scheme for the generation of 9.4-µm radiation using a CO2 gasdynamic laser. The flow-field has been analyzed using complete two-dimensional, unsteady laminar form of Navier-Stokes equations coupled with the finite rate vibrational kinetic equations. The analysis showed that integrated small-signal gain of 11.5m−1 for Lorentzian broadening and 4.8m−1 considering Voigt function can be obtained for N2 reservoir temperature of 2000°K and velocity ratio 1:1 between the CO2 and N2 mixing streams. These results (presented in graphs) clearly highlight the large potential of downstream-mixing CO2 gasdynamic laser for 9.4-µm laser generation.
TL;DR: In this article, a new scale of electronegativity based on X-ray spectroscopic data was presented for most of the heavier elements lying between 55Cs and 92U.
Abstract: A new scale of electronegativity based on X-ray spectroscopic data was earlier presented by Mande and others for the elements3Li to54Xe. The present paper reports an extension of this scale carried out for most of the heavier elements lying between55Cs and92U. The present approach is based on the physical interpretation of electronegativity of an atom as the attractive electrostatic force it exerts at a distance equal to its covalent radius. To estimate this force, experimental spectroscopic data have been employed. The merits of such an approach have been pointed out recently by Mullay and may be of interest in the context of the intimate relation found between electronegativity and superconducting transition temperature.
TL;DR: In this article, two sensors (thermocouples) are mounted parallel to the needle probe at known distances, which makes it a device for simultaneous conductivity and diffusivity determination.
Abstract: An attempt has been made to minimize the sources of error involved in the transient probe method for thermal conductivity determination. Two sensors (thermocouples) are mounted parallel to the needle probe at known distances. This modification makes it a device for simultaneous conductivity and diffusivity determination. Thermal conductivity and diffusivity for glycerine, dune sand and mustard seed are determined by this method. Results obtained are compared with those obtained by a calibrated transient probe for conductivity and by a parallel wire method for diffusivity. Analysis of the results prove it to be a better instrument over the traditional ones. The technique can also be used as a direct reading device for conductivity and diffusivity measurements.
TL;DR: In this article, the dynamic properties of a biased two-level system in contact with a dissipative bath were studied in the weak coupling limit using a resolvent expansion method.
Abstract: The dynamic properties of a biased two-level system in contact with a dissipative bath are studied in the weak coupling limit using a resolvent expansion method. The theory yields consistent results at low temperatures, a regime in which the widely used dilute bounce gas approximation (DBGA) to an underlying functional integral expression breaks down. The present results are however equivalent to a recently adapted functional integral technique that goes beyond the DBGA. The calculated expressions are relevant for analyzing the neutron scattering data on tunneling of light interstitials, e.g., hydrogen, in metals, at very low temperatures.
TL;DR: In this article, it was shown that the assumption of factorizability of the structure function F(x, t) inx andt and the equality of the singlet and gluon distributions are not necessary to obtain the approximate solutions of Altarelli-Parisi equations derived by us.
Abstract: We show that the assumptions of factorizability of the structure functionF(x, t) inx andt and the equality oft evolution of the singlet and gluon distributions are not necessary to obtain the approximate solutions of Altarelli-Parisi equations derived by us recently.
TL;DR: In this article, a spatial light modulator (SLM) using reverse saturable absorber molecules is suggested, and the SLM characteristics are derived using a recently proposed steady-state kinetic analysis.
Abstract: Construction of a spatial light modulator (SLM) using reverse saturable absorber molecules is suggested. The SLM characteristics are derived using a recently proposed steady-state kinetic analysis. Results are presented for the rhodamine 6G dye molecules.
TL;DR: In this paper, the angular distribution of scattered neutrons from 0.002 M CTAB solutions was calculated to distinguish spherical and cylindrical aggregates. But the results showed that SANS spectra are different for the above three types of aggregates, suggesting that technique of SANS can indeed be used to distinguish the three aggregation processes and the size of the aggregate can also be obtained from such studies.
Abstract: Spherical micelles in ionic micellar solutions, often aggregate to form spherical, cylindrical or chain-like aggregates on addition of salt to the solution. It is known that the technique of small angle neutron scattering (SANS) can be used to distinguish spherical and cylindrical aggregates. To examine if SANS can be used to distinguish the latter two aggregation processes, we have calculated the angular distribution of scattered neutrons from 0.002 M CTAB solutions. These calculations show that aggregation of CTAB micelles results in large changes in SANS spectra. The shapes of SANS spectra are different for the above three types of aggregates, suggesting that technique of SANS can indeed be used to distinguish the three aggregation processes. The size of the aggregate can also be obtained from such studies.