Showing papers in "Pramana in 1984"

A hermitian open-shell many-body perturbation theory for treating intruder states

Abstract: In this paper we have formulated an open-shell many-body perturbation theory (mbpt) that applies to an incomplete model space. The effective HamiltonianH eff generated in our theory is hermitian. We follow the resolvent-operator based time-dependent formulation ofmbpt of Banerjeeet al, and show quite generally that, by classifying the various determinants spanning the Hilbert space as model valence spaceP, virtual valence spaceR and virtual spaceQ, a diagrammaticmbpt satisfying size consistency can be developed. The chief new features of the theory are (i) manifest hermiticity ofH eff; (ii) presence of disconnected diagrams no part of which is a legitimate diagram (irreducible disconnected diagrams); (iii) presence of folded diagrams with components that may be irreducible disconnected diagrams; (iv) a consistent treatment of the variousn-valence sectors of the Hilbert space that may be interacting throughH eft if valence-holes are present—in particular a modified treatment of the core-valence separation. The generalisation afforded by the theory offers useful conceptual as well as computational advantages because the convergence difficulty encountered in a complete valence space formulation may be bypassed here by keeping the offending valence space determinants out of the model space. A brief critique of the developmentvis-a-vis the only other general model space development leading to a nonhermitianH eff is also given.

Thermal conduction in a homogeneous two-phase system

Abstract: Assuming a regular geometry of dispersed phase (λ 2) an integrated theory for the effective thermal conductivityλ e of all kind of two-phase materials (including loose materials) is developed. The flux modification is carried out by considering the effective neighbouring interactions in the solution of Poisson’s equation. A comparison of calculatedλ e values with the reported experimental results over a wide range of two-phase materials shows a good agreement.

Mechanism of phase transformations in ZnS

Abstract: X-ray diffraction of the 2H-3C transformation in ZnS crystals has been studied to determine the mechanism of the phase transformation. Single crystals of 2H ZnS were annealed in vacuum at different temperatures to induce the phase transformation and then quenched to arrest it at different intermediate stages. The transformation is found to occur by the non-random nucleation of stacking faults in the 2H structure which produce characteristic diffuse steaks along reciprocal lattice rows parallel toc* for whichH−K ≠ 0 (mod 3). All the crystals finally transform to a disordered twinned 3C structure. A study of the broadening of the x-ray diffraction maxima reveals that the stacking faults involved in the transformation are basal plane deformation faults. Initially these nucleate at random producing a random distribution of cubic nuclei within the 2H structure. As the transformation proceeds these 3C nuclei grow into thick 3C regions by a preferential nucleation of the faults at 2-layer separations. Since the 3C nuclei can have twin orientations the resulting 3C structure invariably contains a random distribution of twin faults. This is confirmed by comparing the experimentally observed intensity profile of the 10.L reflections as recorded on a single crystal diffractometer, with those calculated theoretically for a randomly twinned cubic structure.

Lattice imperfection studies in polycrystalline materials by x-ray diffraction line-profile analysis

Abstract: This review concerns our recent investigations with a series of binary fcc Ag- and Cu-base alloys (viz Ag-Ga, Ag-Ge, Ag-Al and Cu-Ga, Cu-Ge) from detailed analyses of x-ray diffraction line profiles, the importance of which has been briefly summarized. The theoretical formulations of the Warren-Averbach’s method of Fourier analysis of peak-shapes along with the methods of peak-shift and peak-asymmetry have been outlined. A preview on the significant studies carried out earlier with Ag- and Cu-base (fcc) binary alloys has been made in short. A detailed analyses on the recorded profiles in the present considerations revealed, in general, quantitative estimates of several microstructural parameters characterising the deformed state of the materials namely, propensity of stacking faults (intrinsic, extrinsic and twin faults), rms microstrains, coherent domain sizes, long-range residual stresses, lattice parameter changes, dislocation density and stacking fault energy. The results indicate a general trend of increase in the concentrations of stacking faults, primarily, of intrinsic character, with increase in solute concentrations; which are solely responsible for the observed peak-shifts as well as domain size broadening. Small asymmetry in the profiles is due to the presence of extrinsic stacking faults, relatively less in magnitude compared to the intrinsic ones while the deformation twin faults are almost absent—an observation with significance. The dislocation density, quite appreciable in magnitude, has been evaluated from the anisotropic values of the coherent domain sizes and rms microstrains. The stacking fault energies of pure Ag and Cu, an important parameter have also been estimated and compared with those obtained from electron microscopy. Annealing experiments with a Ag-5·8% Al alloy, aluminium being a precipitating solute, do not reveal any detectable evidence of solute segregation at the stacking faults. The occurrence of stacking faults in the alloy systems has been correlated with a number of physical factors involving solvent-solute types.

Density wave theory of freezing and the solid

Abstract: The theory of the liquid to solid transition, in three as well as in two dimensions, is reviewed. The transition can be viewed either as the melting of the solid due to phonon or defect proliferation instabilities or alternatively as freezing of the liquid into a density wave state with crystalline symmetry. A theory due to Yussouff and the author, based on the latter idea, is discussed and its predictions are compared with experiment. It is shown that the theory leads to a new approach to the properties of a deformed (e.g., sheared) solid and of defects such as grain boundaries and dislocations in a solid. The approach brings out explicitly the structural nature of these properties, and is not restricted to small deviations from perfect periodicity (harmonic approximation) since the solid, the liquid and anything in between can be handled theoretically.

Absorption spectrum of the HCCO radical

Abstract: In the flash-photolysis of oxazole and isoxazole, new transient absorption bands are observed in the region 3080–3670 A. Vibrational analysis of these bands showed that they can be arranged into two electronic systems with their origin bands at 3333.60 A and 3367.01 A. Based on the experimental conditions under which the bands are produced and on the analysis of their vibrational and gross-rotational structure, these bands are assigned to a new carrier, the HCCO free radical.

Velocity dependent inertial induction: An extension of Mach’s principle

Abstract: In this article a model of inertial induction has been presented. According to this model the magnitude of the acceleration dependent inertia force comes out exactly as the product of the acceleration and inertial mass. The model also indicates that even uniform velocity gives rise to inertia force. However, the magnitude of the velocity dependent inertia force is exceedingly small but it causes a cosmological red shift whose order of magnitude is same as that of the observed values.

Analysis and merit of the constrained-component variation in dirac theory

Abstract: It is shown that the constrained-component variation generally suggested by Rosicky and Mark is very fundamental, has consistent variational features and reproduces, as a special case, earlier variational results for atomic systems obtained by Drake and Goldman. Numerical merits and demerits of this method are qualitatively assessed.

Thermal conduction through loose and granular two-phase materials at normal pressure

Abstract: The integrated theory derived for the lattice-type dispersions is modified and extended to estimate the effective thermal conductivity of loose and granular two-phase materials at normal pressure assuming an effective continuous media approximation. A comparison of calculated values ofλ e with the reported experimental results over a wide range of loose and granular two-phase materials shows a good agreement.

Studies on the valence state of Ce and Eu in some new boron and silicon containing rare earth intermetallic compounds

Abstract: Compounds of the formula RPd3B x (R=rare earth with 0⩽x⩽1) and RPd3Si x (R=La, Ce, Eu with 0⩽x⩽0.3) can be prepared by alloying boron or silicon with parent RPd3 compounds. Addition of boron (silicon) does not change the structure but results in lattice expansion. The valence state of Ce in CePd3 and that of Eu in EuPd3 is strongly influenced by boron and silicon. Ce is known to be in a valence fluctuating state in CePd3 while Eu is trivalent (J=0) in EuPd3. The increase in the lattice parameter as a function of boron concentration is observed to be larger in CePd3B x and EuPd3B x compared to that in other RPd3B x alloys giving the first indication of the change in the valence state of Ce and Eu. This is confirmed from susceptibility measurements. With the addition of boron, susceptibility increases and the effective paramagnetic moments approach the values corresponding to Ce3+ (J=5/2, μeff=2.54 μ B ) and Eu2+ (J=7/2, μeff=7.94 μ B ) in the two alloy systems CePd3B x and EuPd3B x respectively. In the case of europium alloys,151Eu Mossbauer studies point out the importance of near-neighbour environment effects. Further, in EuPd3B, where all the europium ions are crystallographically equivalent, a single Mossbauer line, with an isomer shift characteristic of europium ions in valence-fluctuating state, is observed at 300 K. However, at 88 K the Mossbauer absorption splits into two lines corresponding to europium ions in two valence states,e.g. divalent- and trivalent-like. Such a behaviour indicates thermally-induced charge ordering of europium ions. Addition of silicon to CePd3, like boron, results in unusual lattice expansion and changes the valency of cerium towards 3+. the valence change is further corroborated by susceptibility measurements. In EuPd3Si x alloys, susceptibility and Mossbauer studies indicate that in the limiting single phase alloy EuPd3Si0.25 the europium ions are on the verge of valence instability. Susceptibility results on CeRh3B x alloys are also presented.

Ga 0.47 In 0.53 As -- The material for high-speed devices

Abstract: Electron transport properties of Ga047In053As are reviewed The available physical constants of the material and results on electron mobility in bulk materials, 2deg systems and under hot-electron conditions are presented Applications of the material in the construction offet’s and photo-conductive detectors are briefly discussed

An explanation of the phenomenon of polytypism

Abstract: Based on the analogy between polytypes and spin-half Ising chains, polytypes can be considered as different phases of a spin-half Ising system with competing nearest neighbour and next nearest neighbour interactions operating in a single direction It is known that such an Ising system exhibits extremely rich and complicated phase behaviour This behaviour is shown to be very similar to the phase behaviour exhibited by polytypes

Gauge fields, space-time geometry and gravity

Abstract: A general framework for the gauge theory of the affine group and its various subgroups in terms of connections on the bundle of affine frames and its subbundles is given, with emphasis on the correct gauging of groups including space-time translations. For consistency of interpretation, the appropriate objects to be identified with gravitational vierbeins in such theories are not the translational gauge fields themselves, but their pull backs,via appropriate bundle homomorphisms, to the bundle of frames. This automatically solves the problems usually encountered in constructing a gauge theory of the conventional sort for groups containing translations. We give a consistent formulation of the Poincare gauge theory and also of the theory based on translational gauge invariance which, in the absence of matter fields with intrinsic spin, gives a local Lorentz invariant theory equivalent to Einstein gravity.

Photoacoustic studies on excitation transfer in Rh6G and RhB dye mixture

Abstract: The photoacoustic spectra of Rhodamine 6G (Rh6G), Rhodamine B (RhB) dyes and their mixtures were recorded at different concentrations. It is observed that in the mixture the photoacoustic signal increases due to excitation transfer from Rh6G to RhB in the wavelength region 480–530 nm. The excitation transfer rate estimated from these data show that resonance is dominant rather than the collisional transfer process.

Flip angle dependence in two-dimensional multiple quantum coherence NMR spectroscopy

Abstract: The multiple-quantum pathway description developed earlier for two-dimensional single-quantum correlation spectroscopy (cosy), is generalized and applied to the two-dimensional multiple-quantum transitions (2Dmqt) spectroscopy. The connectivity classes ofcosy are also generalized tomqt spectroscopy. The pathway description allows a straightforward method of computation of the flip angle dependence of the intensity of various peaks in two-dimensional correlation spectroscopy. It is shown that a variation of flip angle allows distinction between various classes and types of transitions, as well as optimization of experiment for selective detection of certain classes of peaks in 2D spectroscopy.

The electron-electron interaction in doubly-excited states of atoms

Abstract: Doubly-excited states of the isoelectronic sequence of H−, in which both electrons are in high principal quantum numbers, are examined on the basis of alternative pictures of the correlations between the two electrons. Restricting ourselves to the lowest1 S states with both electrons in principal quantum numbern, we parametrise the electron-electron interaction on the basis of these pictures and compare the resulting simple expressions with more elaborate numerical calculations. This provides further understanding of the nature of correlations in such states.

Energy dependence of photon-inducedL shell x-ray intensity ratios in Ta and W

Abstract: TheL shell x-ray intensity ratios have been measured for the elements Ta and W by photoionization ofL shell electrons in the photon energy region 14⩽E⩽44 keV. The experimental results are compared with those calculated at the photon energies used in the present measurements. The measured values show fairly good agreement with the calculated values within the experimental uncertainties.

Pressure-induced electronic and structural transformations in bulk GeSe 2 glass

Abstract: The pressure dependence of the electrical resistivity of bulk GeSe2 glass shows a semiconductor-to-metal transition at 7 GPa pressure. The high pressure phase is examined using the x-ray diffractometer and is found to be crystalline, with a face-centred cubic structure havinga = 4·06 A. The electrical conductivity has also been studied as a function of temperature at various pressures.

High pressure transport behaviour of AgI-Ag2O-MoO3 glasses and the cluster model

Abstract: The effect of pressure on the conductivity of fast ion conducting AgI-Ag2O-MoO3 glasses has been investigated down to 150 K. The observed variation of conductivities appears to support the application of cluster model to the ionic glasses.

Potential double layers in double plasma device

Abstract: Results of the investigation on the formation of double layers in double plasma device are presented. By appropriate modifications in the biasing conditions, we have been able to obtain both weak (eΔφ 10kTe) in the device. Unlike previous experiments, we have not been limited to potential jumps equal to ionisation potential of the neutral gas. A detailed investigation has been carried out to find out why earlier experiments in similar devices were limited to only weak double layers.

Symmetry groups of mathematical physics

Abstract: Recent work on Lie’s method of extended groups to obtain symmetry groups and invariants of differential equations of mathematical physics is surveyed. As an essentially new contribution one-parameter Lie groups admitted by three-dimensional harmonic oscillator, three-dimensional wave equation, Klein-Gordon equation, two-component Weyl’s equation for neutrino and four-component Dirac equation for Fermions are obtained.

Superconducting transition temperature of a paramagnetic material close to magnetic ordering

Abstract: An explicit expression for the superconducting transition temperatureTc in a paramagnetic material is derived, when the transition occurs just before a possible magnetic ordering. As first noted by Uspenskii, such a transition may arise from electronic mechanism itself, without the necessary role played by the usual phonon-exchange mechanism. The result is discussed in terms of some recent experimental observations on the binary alloy Y9Co7.

Lattice dynamical study of alkali metals: An unified approach based on CGW model

Abstract: The phonon dispersion relations for lithium, sodium, potassium, rubidium and cesium along the principal symmetry directions as well as their lattice specific heats have been deduced using Clark, Gazis and Wallis angular force model This model which conforms to the translational symmetry of the lattice, reproduces the observed crossover in lithium along [ζ00] direction at ζ = 0·49, without producing any crossovers in other alkali metals Besides, the theoretical dispersion curves of all alkali metals are in excellent agreement with the corresponding experimental or homologous dispersion relations and theirϑ D values compare well with the experimental values over a wide temperature range It is shown that the strength of electron-ion interactions plays a significant role in the success of any unified lattice dynamical study of alkali metals while the three-body interactions of thecgw model do not The importance of umklapp processes, failure of the earlier models to produce a crossover and the experimentalϑ D-T curve in lithium as well as the apparent variation in the nature and range of atomic interactions of alkali metals are discussed

Electrical transport and high pressure studies on bulk Ge20Te80 glass

Abstract: The electrical resistivity of bulk Ge20Te80 has been measured as a function of pressure and temperature. At 5 GPa, an amorphous semiconductor-to-crystalline metal transition has been observed. The sample recovered from the high pressure cell, after the application of 7 GPa, has a face-centred cubic structure with a lattice constant of 6·42 A. In crystalline sample, the semiconductor-to-metal transition occurs at 7 GPa. The thermoelectric power has also been measured for glassy samples in the temperature range 300–240 K.

Rotating relativistic electron beam-plasma interaction and formation of a field-reversed configuration

Abstract: Experimental results on interaction of a rotating relativistic electron beam with plasma and neutral gas are presented. The rotating relativistic electron beam has been propagated up to a distance of 150 cm in a plasma. The response of the plasma to the rotating electron beam is found to be of magnetic diffusion type over a plasma density range 1011–1013 cm−3. Excitation of the axial and azimuthal return currents by the rotating beam and subsequent trapping of the azimuthal return current layer by the magnetic mirror field are observed. A field-reversed configuration has been formed by the rotating relativistic electron beam when injected into neutral hydrogen gas. We have observed field reversal up to three times the initial field in an axial length of 100 cm.

Conditions for the relativistic virial theorem for one electron

Abstract: Conditions for the usual form of one-electron relativistic virial theorem to be satisfied are derived and illustrated. These can be applied to generate quality basic spinors for solving a one-electron Dirac equation.

Kerr metric in the deSitter background

Abstract: In addition to the Kerr metric with cosmological constant Λ several other metrics are presented giving a Kerr-like solution of Einstein’s equations in the background of deSitter universe. A new metric of what may be termed as rotating deSitter space-time—a space-time devoid of matter but containing null fluid with twisting null rays, has been presented. This metric reduces to the standard deSitter metric when the twist in the rays vanishes. Kerr metric in this background is the immediate generalization of Schwarzschild’s exterior metric with cosmological constant.

Elastic scattering of 36 MeV alpha particles from 197 Au

Abstract: Angular distribution for the elastic scattering of 36 MeV alpha particles from gold target has been measured fromϑ ∼ 10–56°. The cross-section data have been analyzed in terms of the optical model. The real part of the optical model potential (V R ) has been deduced by two prescriptions: (i) combining the volume integral, the radius at whichV R =2.4 MeV and the slope at this radius (1/V R ) (dV R /dr) (ii) combining the volume integral, the root mean square radius and the equivalent sharp radius systematics. The imaginary potential depth has been searched to fit the data. The prediction using (i) for the real potential fits the data the best.

The crystal fields inα-alums—A high pressure magnetic resonance study

Abstract: The origin of the crystal field and its variation with temperature in severalα-alums have been studied bynmr of27Al andepr of Cr3+ at high hydrostatic pressures and low temperatures. The results lead to an explanation of the anomalous temperature variation of the axial crystal field at the trivalent ion site. The mechanism of the phase transition in ammonium alum is also explained. A correlation between the axial crystal field as determined bynmr (e2qQ/h) and that determined byepr (D) has been obtained.

Complete mass spectra of\(q\bar q\) and\(Q\bar q\) mesons with improved Bethe-Salpeter dynamics of confinementmesons with improved Bethe-Salpeter dynamics of confinement

Abstract: The Bethe-Salpeter (bs) dynamics of harmonic confinement developed byanm and collaborators over the last three years and already applied with considerable experimental success to various hadron spectra and coupling structures has been significantly improved through (i) a more exact treatment of a certain momentum-dependent operator\(\hat Q_q \) appearing in thebs equation, using the techniques of SO (2, 1) Lie algebra, and (ii) a sharpened definition of theqcd Coulomb term, so as to yield unambiguous values for different flavour sectors. The resulting mass spectra of light\((q\bar q)\) meson towers and semi-heavy\((Q\bar q)\) quarkonia which are most sensitive to the improved treatment of\(\hat Q_q \), reveal excellent agreement with experiment, one in which only slight changes in the reduced spring constant\((\tilde \omega )\) and quark masses (mq) over the earlier parametrizations are involved. These changes are however found to have a negligible effect on the (already good) numerical values of the other predictions (electroweak and pionic couplings) depending on the\(q\bar q\) andqqq wave functions. A critical assessment of the strong and weak points of this method is madevis-a-vis other related approaches.