Showing papers by "Indian Institute of Science published in 1995"

Routing and wavelength assignment in all-optical networks

Abstract: Considers routing connections in a reconfigurable optical network using WDM. Each connection between a pair of nodes in the network is assigned a path through the network and a wavelength on that path, such that connections whose paths share a common link in the network are assigned different wavelengths. The authors derive an upper bound on the carried traffic of connections (or equivalently, a lower bound on the blocking probability) for any routing and wavelength assignment (RWA) algorithm in such a network. The bound scales with the number of wavelengths and is achieved asymptotically (when a large number of wavelengths is available) by a fixed RWA algorithm. The bound can be used as a metric against which the performance of different RWA algorithms can be compared for networks of moderate size. The authors illustrate this by comparing the performance of a simple shortest-path RWA (SP-RWA) algorithm via simulation relative to the bound. They also derive a similar bound for optical networks using dynamic wavelength converters, which are equivalent to circuit-switched telephone networks, and compare the two cases. Finally, they quantify the amount of wavelength reuse achievable in large networks using the SP-RWA via simulation as a function of the number of wavelengths, number of edges, and number of nodes for randomly constructed networks as well as de Bruijn networks. They also quantify the difference in wavelength reuse between two different optical node architectures. >

The Real Symplectic Groups in Quantum Mechanics and Optics

Abstract: We present a utilitarian review of the family of matrix groups $Sp(2n,\Re)$, in a form suited to various applications both in optics and quantum mechanics. We contrast these groups and their geometry with the much more familiar Euclidean and unitary geometries. Both the properties of finite group elements and of the Lie algebra are studied, and special attention is paid to the so-called unitary metaplectic representation of $Sp(2n,\Re)$. Global decomposition theorems, interesting subgroups and their generators are described. Turning to $n$-mode quantum systems, we define and study their variance matrices in general states, the implications of the Heisenberg uncertainty principles, and develop a U(n)-invariant squeezing criterion. The particular properties of Wigner distributions and Gaussian pure state wavefunctions under $Sp(2n,\Re)$ action are delineated.)

The effect of grain size on the high-strain, high-strain-rate behavior of copper

Abstract: Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-μm grain-size specimens and virtually absent for the 9.5-μm grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shockinduced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.

Investigation of tetragonal distortion in the PbTiO 3 –BiFeO 3 system by high-temperature x-ray diffraction

Abstract: Compositions in the (Pb1−xBix (Ti1−xFex)O3 solid solution system for x ⋚ 0.7 show unusually large tetragonal distortion. High-temperature x-ray diffraction was used to study the tetragonal distortion as a function of temperature (25–700 °C) for compositions (x = 0–0.7) using powders prepared by solid-state reaction in the above system. Large changes in the lattice parameters were observed over a narrow temperature range near Curie temperature (TC) for compositions near the morphotropic phase boundary (MPB) (x ≃ 0.7). Compositions near MPB showed a c/a ratio of 1.18 at room temperature. Polar plots of lattice constants at different temperatures indicated strong anisotropic thermal expansion with zero thermal expansion along the [201] direction.

Band theory for ground-state properties and excitation spectra of perovskite LaMO3 (M=Mn, Fe, Co, Ni).

Abstract: It is shown that the local spin-density approximation describes well electronic structures of perovskite $\mathrm{La}M{\mathrm{O}}_{3}$ ( $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}$ Mn, Fe, Co, Ni) for the ground-state and single-electron excited-state properties. Inclusion of details of crystal structures distorted from the cubic structure is crucial to reproduce different magnetic structures and insulating states. Calculated x-ray photoemission spectra are in very good agreement with experimental ones, and its angular momentum characters are in conformity with ultraviolet photoemission spectra.

Comparison of the assumed modes and finite element models for flexible multilink manipulators

Abstract: Compares two discretization models-namely, the assumed modes and finite element models-to efficiently represent the link flexibility of robot manipulators. We present a systematic modeling procedure based on homogeneous transformation matrices for spatial multilink flexible manipulators with both revolute and prismatic joints. The Lagrangian formulation of dynamics and computer algebra are employed to derive closed-form equations of motion. We show that fewer mathematical operations are required for inertia matrix computation in the finite element model compared with the assumed modes formulation; however, because the number of state-space equations is larger, the numerical simulation time may be greater for finite element models. Use of the finite element model to approximate flexibility usually gives rise to an overestimated stiffness matrix. We analytically show that overestimation of structure stiffness may lead to unstable closed-loop response of the original manipulator system, using a model-based control law. We illustrate the complexity owing to the time-dependent frequency equation of the assumed modes model arising in a prismatic jointed flexible link with payload and in manipulators with more than one link with revolute joints. We describe a novel method based on the differential form of the frequency equation to simulate such systems. A model-based decoupling control law is used to compare the dynamic responses of the manipulator system. The results are illustrated by numerical simulation of a flexible spatial RRP (revolute/revolute/prismatic) configuration robot.

Density-matrix renormalization-group studies of the spin-1/2 Heisenberg system with dimerization and frustration

Abstract: Using the density-matrix renormalization-group technique, we study the ground-state phase diagram and other low-energy properties of an isotropic antiferromagnetic spin-1/2 chain with both dimerization and frustration, i.e., an alternation delta of the nearest-neighbor exchanges and a next-nearest-neighbor exchange J(2). For delta = 0, the system is gapless for J(2) J(2c) where J(2c) is about 0.241. For J(2) = J(2c) the gap above the ground state grows as delta to the power 0.667 +/- 0.001. In the J(2)-delta plane, there is a disorder line 2J(2) + delta = 1. To the left of this line, the peak in the static structure factor S(q) is at q(max) = pi (Neel phase), while to the right of the line, q(max) decreases from pi to pi/2 as J(2) is increased to large values (spiral phase). For delta = 1, the system is equivalent to two coupled chains as on a ladder and it is gapped for all values of the interchain coupling.

Modeling of slip for wheeled mobile robots

Abstract: Wheeled mobile robots (WMRs) are known to be non-holonomic systems, and most dynamic models of WMRs assume that the wheels undergo rolling without slipping. This paper deals with the problem of modeling and simulation of motion of a WMR when the conditions for rolling are not satisfied at the wheels. The authors use a traction model where the adhesion coefficient between the wheels of a WMR and a hard flat surface is a function of the wheel slip. This traction model is used in conjunction with the dynamic equations of motion to simulate the motion of the WMR. The simulations show that controllers which do not take into account wheel slip give poor tracking performance for the WMR and path deviation is small only for large adhesion coefficients. This work shows the importance of wheel slip and suggests use of accurate traction models for improving tracking performance of a WMR. >

Multi-installment load distribution in tree networks with delays

Abstract: This paper presents a new strategy for load distribution in a single-level tree network equipped with or without front-ends. The load is distributed in more than installment in an optimal manner to minimize the processing time. This is a deviation and an improvement over earlier studies in which the load distribution is done in only one installment. Recursive equations for the general case, and their closed-form solutions for a special case in which the network has identical processors and identical links, are derived. An asymptotic analysis of the network performance with respect to the number of processors and the number of installments is carried out. Discussions of the results in terms of some practical issues like the tradeoff relationship between the number of processors and the number of installments are also presented. >

Voxel-based modeling for layered manufacturing

Abstract: Layered manufacturing (LM) technologies have revolutionized the prototyping of complex geometric designs, but still employ traditional CAD tools. A voxel-based approach is under development in a modeling tool called G-WoRP (Geometric Workbench for Rapid Prototyping), which is tuned toward fabrication with LM equipment. The user interacts with the workbench through an input layer that provides two major primitives-the slice and the voxel-and the operations that support them. An import facility also permits designs from other CAD systems. Central to the internal layer is V-Rep, a new representation scheme that provides an efficient interface among the various G-WoRP modules. The output layer gives the part description in a form suitable to the actual LM technology employed. It also supports a process description for manufacturing the part using traditional processes. The design and manufacturing phases do not require an explicit process-planning step because the design description of the part closely resembles the input description needed by the LM equipment. >

Fullerenes, nanotubes, onions and related carbon structures

Abstract: Fullerenes, containing five- and six-membered carbon rings, of which C 60 and C 70 are the prominent members, exhibit phase transitions associated with orientational ordering. When C 60 is suitably doped with electrons, it shows novel superconducting and magnetic properties. We review these and other properties of fullerenes in bulk or in film form along with the preparative and structural aspects. Carbon nanotubes and onions (hyperfullerenes) are the other forms of carbon whose material properties have aroused considerable interest. Besides discussing these new forms of carbon, we briefly introduce other possible forms, such as those involving five-, six- and seven-membered rings and hybrids between diamond and graphite.

Peristaltic transport of two immiscible viscous fluids in a circular tube

Abstract: Peristaltic motion of two immiscible viscous incompressible fluids in a circular tube is studied in pumping and copumping ranges under long-wavelength and low-Reynoldsnumber assumptions. The effect of the peripheral-layer viscosity on the time-averaged flux and the mechanical efficiency is studied. The formation and growth of the trapping zone in the core and the peripheral layer are explained. It is observed that the bolus volume in the peripheral layer increases with an increase in the viscosity ratio. The limits of the time-averaged flux but never disappears. The development of the complete trapping of the core fluid by the peripheral-layer fluid with an increase in the time-averaged flux is demonstrated. The effect of peripheral-layer viscosity on the reflux layer is investigated. It is also observed that the reflux occurs in the entire pumping range for all viscosity ratios and it is absent in the entire range of copumping.

Biosorption of metals using nonliving biomass-a review

Abstract: A review of published literature on the biosorption of metals using nonliving biomass is presented. Factors such as pH, temperature, initial metal concentration, biomass loaning, the presence of co-ions and the pretreatment of biomass influence the metal uptake by biomass. Although few generalizations are possible, unified theor ies regarding the mechanism of uptake are not available. Therefore, the above aspects of metal biosorption have to be defined individually for each biomass and metal-ion pair.

Use of microwaves for the synthesis and processing of materials

Abstract: An overview of the synthesis of materials under microwave irradiation has been presented based on the work performed recently. A variety of reactions such as direct combination, carbothermal reduction, carbidation and nitridation have been described. Examples of microwave preparation of glasses are also presented. Great advantages of fast, clean and reduced reaction temperature of microwave methods are emphasized. The example of ZrO2-CeO2 ceramics has been used to show the extraordinarily fast and effective sintering which occurs in microwave irradiation.

Persistent currents in a one-dimensional ring for a disordered Hubbard model

Abstract: We consider a one-dimensional Hubbard model in the presence of disorder. We compute the charge stiffness for a mesoscopic ring as a function of the size L, which is a measure of the persistent currents. We find that for finite disorder the persistent currents of the system with repulsive interactions are larger than those of the system with attractive interactions. This counterintuitive result is due to the fact that local-density fluctuations are reduced in the presence of repulsive interactions.

NUPARM and NUCGEN: software for analysis and generation of sequence dependent nucleic acid structures

Abstract: Software packages NUPARM and NUCGEN, are described, which can be used to understand sequence directed structural variations in nucleic acids, by analysis and generation of non-uniform structures. A set of local inter basepair parameters (viz. tilt, roll, twist, shift, slide and rise) have been defined, which use geometry and coordinates of two successive basepairs only and can be used to generate polymeric structures with varying geometries for each of the 16 possible dinucleotide steps. Intra basepair parameters, propeller, buckle, opening and the C6...C8 distance can also be varied, if required, while the sugar phosphate backbone atoms are fixed in some standard conformation ill each of the nucleotides. NUPARM can be used to analyse both DNA and RNA structures, with single as well as double stranded helices. The NUCGEN software generates double helical models with the backbone fixed in B-form DNA, but with appropriate modifications in the input data, it can also generate A-form DNA ar rd RNA duplex structures.

DNA- and chromatin-condensing properties of rat testes H1a and H1t compared to those of rat liver H1bdec: H1t is a poor condenser of chromatin

Abstract: Histones H1a and H1t are two major linker histone variants present at the pachytene interval of mammalian spermatogenesis. The DNA- and chromatin-condensing properties of these two variants isolated from rat testes were studied and compared with those from rat liver. For this purpose, the histone H1 subtypes were purified from the respective tissues using bath acid and salt extraction procedures, Circular dichroism studies revealed that acid exposure during isolation affects the alpha-helical structure of both the globular domain (in the presence of 1 M NaCl) and the C-terminal lambda-tail (in the presence of 60% trifluoroethanol). The condensation of rat oligonucleosomal DNA, as measured by circular dichroism spectroscopy, by the salt-extracted histone H1 was at least 10 times more efficient than condensation by the acid-extracted histone H1. A site size of 16-20 base pairs was calculated for the salt-extracted histone H1. Among the different histone H1 subtypes, somatic histone H1bdec had the highest DNA-condensing property, followed by histone H1a and histone H1t. All the salt-extracted histones condensed rat oligonucleosomal DNA more efficiently than linear pBR-322 DNA, Histones H1bdec and H1a condensed histone H1-depleted chromatin, prepared from rat liver nuclei, with relatively equal efficiency. On the other hand, there was no condensation of histone H1-depleted chromatin with the testes specific histone H1t. A comparison of the amino acid sequences of histone H1d (rat) and histone H1t (rat) revealed several interesting differences in the occurrence of DNA-binding motifs at the C-terminus. A striking observation is the presence of a direct repeat of an octapeptide motif K(A)T(S)PKKA(S)K(T)K(A) in histone H1d that is absent in histone H1t.

Milling maps and amorphization during mechanical alloying

Abstract: The effect of various milling parameters such as, milling intensity, ball:powder weight ratio and number of balls on the glass forming ability of an elemental blend of composition Ti50Ni50 has been studied by mechanical alloying. In order to understand the results, all the milling parameters have been converted into two energy parameters, namely, impact energy of the ball and the total energy of milling. In a milling map of these two parameters, the conditions for amorphous phase formation have been isolated. A similar exercise has been carried out for Ti50Cu50 as a function of milling time at two milling intensities. The results indicate that a minimum impact energy of the ball and a minimum total energy are essential for amorphization by mechanical alloying.

The line transect method for estimating densities of large mammals in a tropical deciduous forest: An evaluation of models and field experiments

Abstract: We have evaluated techniques of estimating animal density through direct counts using line transects during 1988-92 in the tropical deciduous forests of Mudumalui Sanctuary in southern India for four species of large herbivorous mammals, namely, chital (Axis axis). sambar (Cervus unicolor). Asian elephant (Elephas maximus) and gaur (Bos gaurus) Density estimates derived from the Fourier Series and the Half-Normal models consistently had the lowest coefficient of variation. These two models also generated similar mean density estimates. For the Fourier Series estimator, appropriate cut-off widths for analyzing line transect data for the four species are suggested. Grouping data into various distance classes did not produce any appreciable differences in estimates of mean density or their variances, although model fit is generally better when data arc placed in fewer groups. The sampling effort needed to achieve a desired precision (coefficient of variation) in the density estimate is derived. A sampling effort of 800 km of transects returned a 10% coefficient of variation on estimate for ehital; for the other species a higher effort was needed to achieve this level of precision. There was no statistically significant relationship between detectability of a group and the size of the group for any species. Density estimates along roads were generally significantly different from those in the interior of the forest, indicating that road-side counts many not be appropriate for most species.

Chemical ordering in Ge20Se80−χInχ glasses

Abstract: Bulk glasses of Ge-Se-In have been prepared by quenching the melt and have been subjected to thermal crystallization and X-ray structure analysis of devitrified materials. These glasses show a transformation from a single-stage to a double-stage crystallization with respect to composition. The present investigations indicate that these materials are structurally inhomogeneous at a molecular level, in which In 2 Se 3 microclusters are dispersed in the Ge-Se matrix. These results are discussed using the chemical threshold in glasses.

The foraging ecology of dhole (Cuon alpinus) in Mudumalai Sanctuary, southern India

Abstract: Two dhole (Cuon alpinus) packs were monitored in Mudumalai Sanctuary, southern India, during 1989-93 to look at population dynamics, movement pattern, and foraging strategy and their inter-relationship with the maintenance of social groups. Pack size fluctuated substantially (4-18 and 4-25 in the two packs) owing to dispersal and demographic factors such as females not breeding in a given year. Both packs killed a much higher proportion of chital (Axis axis) and sambar (Cervus unicolor) fawns (< one year old) than their availability in the population. There was no correlation between pack size and body weight of prey killed, while per capita consumption of meat declined with increasing pack size. Home-range area (83.3 km(2) and 54.2 km(2) for the two packs) was not correlated with pack size. Pack movement from one resource patch (consisting of resting sites and aggregations of prey species) to another was not random or based on factors such as inter-patch distance or relative prey densities. There was no difference in mean residence time of the pack across the four resource patches; the pack moved across these in a sequential manner in one direction. We conclude that dholes live in groups not because of any advantages accruing from enhanced group sizes through increased per capita yield of food, but as a consequence of the dispersion of resources.

Non-Commutative Arithmetic Circuits: Depth Reduction and Size Lower Bounds

Abstract: We investigate the phenomenon of depth-reduction in commutative and non-commutative arithmetic circuits. We prove that in the commutative setting, uniform semi-unbounded arithmetic circuits of logarithmic depth are as powerful as uniform arithmetic circuits of polynomial degree (and unrestricted depth); earlier proofs did not work in the uniform setting. This also provides a unified proof of the circuit characterizations of the class LOGCFL and its counting variant #LOGCFL. We show that AC1 has no more power than arithmetic circuits of polynomial size and degree nO(log log n) (improving the trivial bound of nO(log n)). Connections are drawn between TC1 and arithmetic circuits of polynomial size and degree. Then we consider non-commutative computation. We show that over the algebra ( ∑ ∗ , max, concat), arithmetic circuits of polynomial size and polynomial degree can be reduced to O(log2 n) depth (and even to O(log n) depth if unbounded-fanin gates are allowed). This establishes that OptLOGCFL is in AC1. This is the first depth-reduction result for arithmetic circuits over a non-commutative semiring, and it complements the lower bounds of Kosaraju and Nisan showing that depth reduction cannot be done in the general non-commutative setting. We define new notions called “short-left-paths” and “short-right-paths” and we show that these notions provide a characterization of the classes of arithmetic circuits for which optimal depth reduction is possible. This class also can be characterized using the AuxPDA model. Finally, we characterize the languages generated by efficient circuits over the semiring ( 2 ∑ ∗ , union, concat) in terms of simple one-way machines, and we investigate and extend earlier lower bounds on non-commutative circuits.

Reactive magnetron co-sputtered antiferroelectric lead zirconate thin films

Abstract: Antiferroelectric lead zirconate thin films were formed on platinum coated silicon substrates by a reactive magnetron co‐sputtering method. The films showed (240) preferred orientation. The crystallization temperatures and the preferred orientation were affected by the lead content in the films. The electric field forced transformation from the antiferroelectric phase to the ferroelectric phase was observed at room temperature with a maximum polarization value of 36 μC/cm2. The average field to excite the ferroelectric state and that for the reversion to the antiferroelectric state were 267 and 104 kV/cm respectively.

Blind separation of multiple co-channel BPSK signals arriving at an antenna array

Abstract: In this letter, we propose a method for blind separation of d co-channel BPSK signals arriving at an antenna array. Our method involves two steps. In the first step, the received data vectors at the output of the array is grouped into 2/sup d/ clusters. In the second step, we assign the 2/sup d/ d-tuples with /spl plusmn/1 elements to these clusters in a consistent fashion. From the knowledge of the cluster to which a data vector belongs, we estimate the bits transmitted at that instant. Computer simulations are used to study the performance of our method. >