Showing papers by "Tata Institute of Fundamental Research published in 1998"

Helioseismic Studies of Differential Rotation in the Solar Envelope by the Solar Oscillations Investigation Using the Michelson Doppler Imager

Abstract: The splitting of the frequencies of the global resonant acoustic modes of the Sun by large-scale flows and rotation permits study of the variation of angular velocity Ω with both radius and latitude within the turbulent convection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft positioned at the L1 Lagrangian point in continuous sunlight, yield oscillation power spectra with very high signal-to-noise ratios that allow frequency splittings to be determined with exceptional accuracy. This paper reports on joint helioseismic analyses of solar rotation in the convection zone and in the outer part of the radiative core. Inversions have been obtained for a medium-l mode set (involving modes of angular degree l extending to about 250) obtained from the first 144 day interval of SOI-MDI observations in 1996. Drawing inferences about the solar internal rotation from the splitting data is a subtle process. By applying more than one inversion technique to the data, we get some indication of what are the more robust and less robust features of our inversion solutions. Here we have used seven different inversion methods. To test the reliability and sensitivity of these methods, we have performed a set of controlled experiments utilizing artificial data. This gives us some confidence in the inferences we can draw from the real solar data. The inversions of SOI-MDI data have confirmed that the decrease of Ω with latitude seen at the surface extends with little radial variation through much of the convection zone, at the base of which is an adjustment layer, called the tachocline, leading to nearly uniform rotation deeper in the radiative interior. A prominent rotational shearing layer in which Ω increases just below the surface is discernible at low to mid latitudes. Using the new data, we have also been able to study the solar rotation closer to the poles than has been achieved in previous investigations. The data have revealed that the angular velocity is distinctly lower at high latitudes than the values previously extrapolated from measurements at lower latitudes based on surface Doppler observations and helioseismology. Furthermore, we have found some evidence near latitudes of 75° of a submerged polar jet which is rotating more rapidly than its immediate surroundings. Superposed on the relatively smooth latitudinal variation in Ω are alternating zonal bands of slightly faster and slower rotation, each extending some 10° to 15° in latitude. These relatively weak banded flows have been followed by inversion to a depth of about 5% of the solar radius and appear to coincide with the evolving pattern of torsional oscillations reported from earlier surface Doppler studies.

Destruction of the Fermi surface in underdoped high- T c superconductors

Abstract: The Fermi surface—the set of points in momentum space describing gapless electronic excitations—is a central concept in the theory of metals. In this context, the normal ‘metallic’ state of the optimally doped high-temperature superconductors is not very unusual: above the superconducting transition temperature, Tc, there is evidence for a large Fermi surface1,2,3, despite the absence of well-defined elementary excitations. In contrast, the normal state of underdoped high-temperature superconductors differs in that there is evidence for a ‘pseudogap’ above Tc (4–7). Here we examine, using angle-resolved photoemission spectroscopy, the temperature dependence of the Fermi surface in underdoped Bi2Sr2CaCu2O8+δ. We find that, on cooling the sample, the pseudogap opens up at different temperatures for different points in momentum space. This leads to an initial breakup of the Fermi surface, at a temperature T*, into disconnected arcs, which then shrink with decreasing temperature before collapsing to the point nodes of the superconducting ground state below Tc. This unusual behaviour, where the Fermi surface does not form a continuous contour in momentum space as in conventional metals, is unprecedented in that it occurs in the absence of long-range order. Moreover, although the superconducting gap below Tc evolves smoothly into the pseudogap above Tc, the pseudogap differs in its unusual temperature-dependent anisotropy, implying an intimate but non-trivial relationship between the pseudogap and the superconducting gap.

J- and H-aggregates of porphyrin-surfactant complexes: time-resolved fluorescence and other spectroscopic studies

Abstract: The interactions of several water-soluble ionic porphyrins with different ionic or neutral surfactants in aqueous solutions were studied as a function of surfactant concentration. The interaction leads to the formation of porphyrin aggregates and/or micelle-encapsulated monomers with the exception of those porphyrin−surfactant pairs for which the interaction is Coulombically repulsive. The premicellar surfactant−porphyrin aggregate is identified by absorption and fluorescence spectroscopy, fluorescence lifetime and anisotropy, and resonance light scattering. The spectroscopic results are used to characterize the premicellar aggregates as J-type, H-type, or nonspecific aggregates. All premicellar surfactant−porphyrin aggregates dissociate to form micelle-encapsulated monomers when the surfactant concentration approaches cmc (critical micellar concentration). The interaction of tetrakis-(4-sulfanatophenyl)porphine dianion (H4TPPS2-) at pH <3.5 with cetyltrimethylammonium cation (CTAB) is described by the fo...

Phenomenology of the low-energy spectral function in high-T c superconductors

Abstract: We introduce a simple phenomenological form for the self-energy which allows us to extract important information from angle-resolved photoemission data on the high-${T}_{c}$ superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ for binding energies of order the spectral gap. First, we find a rapid suppression of the single particle scattering rate below ${T}_{c}$ for all doping levels. Second, we find that in the overdoped materials the gap $\ensuremath{\Delta}$ at all k points on the Fermi surface has significant temperature dependence and vanishes near ${T}_{c}.$ In contrast, in the underdoped samples such behavior is found only at k points close to the diagonal. Near $(\ensuremath{\pi},0)$, $\ensuremath{\Delta}$ is essentially $T$ independent in the underdoped samples. The filling-in of the pseudogap with increasing $T$ is described by a broadening proportional to $T\ensuremath{-}{T}_{c},$ which is naturally explained by pairing correlations above ${T}_{c}.$

Role of Spatial Amplitude Fluctuations in Highly Disordered s-Wave Superconductors

Abstract: The effect of non-magnetic impurities on 2D s-wave superconductors is studied beyond the weak disorder regime. Within the Bogoliubov-de Gennes (BdG) framework, the local pairing amplitude develops a broad distribution with significant weight near zero with increasing disorder. Surprisingly, the density of states continues to show a finite spectral gap. The persistence of the spectral gap at large disorder is shown to arise from the break up of the system into superconducting "islands". Superfluid density and off-diagonal correlations show a substantial reduction at high disorder.A simple analysis of phase fluctuations in the highly inhomogeneous BdG state is shown to lead to a transition to a non-superconducting state.

Role of the scalar field in gravitational lensing

Abstract: A static and circularly symmetric lens characterized by mass and scalar charge parameters is constructed. For the small values of the scalar charge to the mass ratio, the gravitational lensing is qualitatively similar to the case of the Schwarzschild lens; however, for large values of this ratio the lensing characteristics are significantly different. The main features are the existence of two or nil Einstein ring(s) and a radial critical curve, formation of two or four images and possibility of detecting three images near the lens for sources located at relatively large angular positions. Such a novel lens may also be treated as a naked singularity lens.

Oral submucous fibrosis in India: A new epidemic?

Abstract: Bilckground. Oral submucousfibrosis(OSF)is a precancerousconditioncausedby useof the arecanut. The reported prevalenceof OSF in Bhavnagardistrict during 1967 was 0.16%. Weinvestigated whetherthe impression of an increase inthe incidenceof the diseasewasreal. Methods. A house-to-housesurvey was conducted in Bhavnagar district,Gujaratstate.Theuseof arecanut-containing productsandtobaccowasassessed throughaninterviewer administeredquestionnaire. Theoralexamination wasdonebydentists. The diagnosticcriteriafor OSF was the presenceof palpable

Driven lattice gases with quenched disorder: Exact results and different macroscopic regimes

Abstract: We study the effect of quenched spatial disorder on the current-carrying steady states of driven stochastic systems of particles interacting through hard-core exclusion. Two sorts of models are studied: disordered drop-push processes and their generalizations, and the disordered asymmetric simple exclusion process. Quenched disorder enters through spatially random microscopic transition probabilities and the drive is modeled by asymmetry in transition probabilities between sites. Exact steady-state measures are obtained for the drop-push and the generalized drop-push dynamics in $d$ dimensions for arbitrary disorder. This allows us to compute closed form expressions for the steady-state current and site-dependent densities. The steady state of the asymmetric exclusion process with disordered bond strengths is studied in one dimension by numerical simulation and by a mean-field approximation that allows for density variations from site to site. In the totally asymmetric case, we present strong numerical evidence that the current is invariant under reflection. We show that disorder can induce phase separation into macroscopic regions of different densities. We propose approximations, supported by direct numerical simulations, to describe these phenomena and the phase diagram of the model in the current-density plane in terms of macroscopic parameters of the model. We also study the effect of making the direction of easy flow in each bond a random variable and find that the current decreases with system size in this case. We conclude that there are three distinct regimes in disordered driven diffusive systems in one dimension: a homogeneous regime in which the state of the system is characterized by a single macroscopic density and a nonzero current; a segregated-density regime, where the state of the system is characterized by two distinct phase-separated values of density and a nonzero current; a vanishing-current regime, where the state of the system is characterized by two distinct values of the density and the current decreases as the system size increases and vanishes in the thermodynamic limit. Using a mapping from lattice gases to interfaces, these regimes translate into distinct regimes of interface growth in the presence of columnar disorder.

Nonequilibrium phase transitions in models of aggregation, adsorption, and dissociation

Abstract: We study nonequilibrium phase transitions in a mass-aggregation model which allows for diffusion, aggregation on contact, dissociation, adsorption, and desorption of unit masses. We analyze two limits explicitly. In the first case mass is locally conserved, whereas in the second case local conservation is violated. In both cases the system undergoes a dynamical phase transition in all dimensions. In the first case, the steady state mass distribution decays exponentially for large mass in one phase, and develops an infinite aggregate in addition to a power-law mass decay in the other phase. In the second case, the transition is similar except that the infinite aggregate is missing.

Monopoles and Solitons in Fuzzy Physics

Abstract: Monopoles and solitons have important topological aspects like quantized fluxes, winding numbers and curved target spaces. Naive discretizations which substitute a lattice of points for the underlying manifolds are incapable of retaining these features in a precise way. We study these problems of discrete physics and matrix models and discuss mathematically coherent discretizations of monopoles and solitons using fuzzy physics and noncommutative geometry. A fuzzy sigma-model action for the two-sphere fulfilling a fuzzy Belavin-Polyakov bound is also put forth.

Direct measurement of the top quark mass by the DO Collaboration

Abstract: We determine the top quark mass m{sub t} using t{bar t} pairs produced in the DO/ detector by {radical} (s) =1.8thinspTeV p{bar p} collisions in a 125thinsppb{sup {minus}1} exposure at the Fermilab Tevatron. We make a two constraint fit to m{sub t} in t{bar t}{r_arrow}bW{sup +}{bar b}W{sup {minus}} final states with one {ital W} boson decaying to q{bar q} and the other to e{nu} or {mu}{nu}. Likelihood fits to the data yield m{sub t}(l+jets)=173.3{plus_minus}5.6thinsp(stat)thinsp{plus_minus}thinsp5.5thinsp(s st) GeV/c{sup 2}. When this result is combined with an analysis of events in which both {ital W} bosons decay into leptons, we obtain m{sub t}=172.1{plus_minus}5.2thinsp(stat)thinsp{plus_minus}thinsp4.9thinsp(syst) GeV/c{sup 2}. An alternate analysis, using three constraint fits to fixed top quark masses, gives m{sub t}(l+jets)=176.0{plus_minus}7.9thinsp(stat){plus_minus}thinsp4.8thinsp(syst) GeV/c{sup 2}, consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented. {copyright} {ital 1998} {ital The American Physical Society}

Solar internal rotation rate and the latitudinal variation of the tachocline

Abstract: A new set of accurately measured frequencies of solar oscillations is used to infer the rotation rate inside the Sun, as a function of radial distance as well as latitude. We have adopted a regularized least-squares technique with iterative refinement for both 1.5D inversion, using the splitting coefficients, and 2D inversion using individual m splittings. The inferred rotation rate agrees well with earlier estimates showing a shear layer just below the surface and another one around the base of the convection zone. The tachocline or the transition layer where the rotation rate changes from differential rotation in the convection zone to an almost latitudinally independent rotation rate in the radiative interior is studied in detail. No compelling evidence for any latitudinal variation in the position and width of the tachocline is found, although it appears that the tachocline probably shifts to a slightly larger radial distance at higher latitudes and possibly also becomes thicker. However, these variations are within the estimated errors and more accurate data would be needed to make a definitive statement about latitudinal variations.

Characterization of a partially unfolded structure of cytochrome c induced by sodium dodecyl sulphate and the kinetics of its refolding

Abstract: The mechanism of unfolding of ferricytochrome c induced by the surfactant sodium dodecyl sulfate has been studied by heme absorption, tryptophan fluorescence, circular dichroism, resonance Raman scattering, stopped-flow and time-resolved resonance energy transfer to obtain a comprehensive view of the whole process. Unfolding occurred at an almost specific molecular ratio of SDS/cytochrome c in the concentration range (20-50 microM) studied here. However there appears to be a point at approximately 0.6 mM SDS where unfolding begins to occur for lower cytochrome c concentrations. The kinetics of unfolding revealed only a single transition with a rate constant of 33 s(-1) (at 298 K, [SDS] = 8.7 mM) and activation energy barrier of approximately 16 kJ/mol, indicating that other associated steps, if any, are too fast to be significantly populated. The free energy change (deltaG(o)) involved with the unfolding transition was estimated to be about 16.8 kJ/mol. The CD spectrum at 220 nm of SDS-unfolded cytochrome c shows only a partial decrease (25%), indicating that a significant amount of helical structure remains folded in contrast to a complete loss of helical structure in GdnHCl-denatured cytochrome c. The heme structure in SDS-unfolded cytochrome c, as deduced from heme absorption and resonance Raman spectra, shows a major population (approximately 95%) of mis-ligated histidine to the heme which acts as a kinetic trap in the folding process. The structural changes associated with cytochrome c unfolding were also monitored by time-resolved resonance energy transfer which shows a drastic increase in tryptophan fluorescence lifetime from 12 ps in the native protein to 0.63 ns in the unfolded one, associated with a movement of Trp59 by 10 A away from heme. The maximum entropy method analysis of fluorescence decay indicated the growth of various conformational substates in SDS-unfolded cytochrome c in contrast to narrowly distributed conformations in the native protein. The refolding was comprised of three kinetic steps; the first was significantly fast (approximately 8 ms) and was assigned to the dissociation of His26 that paves the protein towards correct folding pathway. The other two slower steps probably arise from chain misorganization and prolyl isomerization. The absence of a burst-phase amplitude supports the idea that the burst phase observed in the folding from completely unfolded cytochrome c corresponds to a molecular collapse that produces significant secondary structure. The partially unfolded state represents a unique intermediate state in the folding pathway.

Can the turbulent galactic dynamo generate large-scale magnetic fields?

Abstract: Large-scale magnetic fields in galaxies are thought to be generated by a turbulent dynamo. However, the same turbulence also leads to a small-scale dynamo which generates magnetic noise at a more rapid rate. The efficiency of the large-scale dynamo depends on how this noise saturates. We examine this issue, taking into account ambipolar drift, which obtains in a galaxy with significant neutral gas. We argue as follows. (i) The small-scale dynamo generated field does not fill the volume, but is concentrated into intermittent rope-like structures. The flux ropes are curved on the turbulent eddy scales. Their thickness is set by the diffusive scale determined by the effective ambipolar diffusion. (ii) For a largely neutral galactic gas, the small-scale dynamo saturates, as a result of inefficient random stretching, when the peak field in a flux rope has grown to a few times the equipartition value. (iii) The average energy density in the saturated small-scale field is subequipartition, since it does not fill the volume. (iv) Such fields neither drain significant energy from the turbulence nor convert eddy motion of the turbulence on the outer scale into wave-like motion. The diffusive effects needed for the large-scale dynamo operation are then preserved until the large-scale field itself grows to near equipartition levels.

Ferroelectric behavior in thin films of antiferroelectric materials

Abstract: We show that when a typically antiferroelectric material such as ${\mathrm{PbZrO}}_{3}$ or ${\mathrm{BiNbO}}_{4}$ is deposited as a thin film on a semiconducting substrate (Si), it displays ferroelectric behavior below a critical film thickness characteristic of the system. In an attempt to understand this ``size effect,'' we consider the contributions of residual strain and the self-biasing effect produced by the intrinsic electric field at the semiconductor-insulator interface.

Anomalous peak effect in CeRu 2 and 2 H − NbSe 2 : Fracturing of a flux line lattice

Abstract: ${\mathrm{CeRu}}_{2}$ and $2H\ensuremath{-}{\mathrm{NbSe}}_{2}$ display remarkable similarities in their magnetic response, reflecting the manner in which the weakly pinned flux line lattice (FLL) loses spatial order in the peak-effect (PE) regime. We present evidence for discontinuous changes in the screening response near the onset of the PE in these systems, and demonstrate history-dependent effects. We attribute these features to a disorder-induced fracturing and entanglement of the FLL, as an alternative to the appearance of a spatially modulated ground state for ${\mathrm{CeRu}}_{2}.$

Prevalence of Respiratory symptoms, bronchial hyperreactivity, and asthma in a megacity: Results of the European Community Respiratory Health Survey in Mumbai (Bombay)

Abstract: To estimate adult asthma prevalence in the world's most rapidly growing mega-city, we applied epidemiologic surveillance tools, as a cooperating center of the European Community Respiratory Health Survey, to a randomly selected sample of Mumbai (Bombay) residents in 1992 through 1995. From a metropolitan population of over 10 million, we took a one-in-ten random sample from electoral rolls in a socially diverse residential district, and examined asthma symptoms in adults age 20 to 44 yr. In Phase I, we interviewed 2,313 adults about symptoms, asthma diagnosis, and medications in the previous 12 mo. In Phase II, family and smoking history, socioeconomic data, housing characteristics, serum IgE, allergy skin tests, spirometry, and methacholine challenge tests were obtained in a subset of 20% of those who had completed Phase I. House dust mite was the most common positive skin test (18% prevalence) and the only one of the nine applied that was significantly associated with asthma symptoms and physician-diagnosed asthma. Asthma prevalence was 3.5% by physician diagnosis, and 17% using a very broad definition including those with asymptomatic bronchial hyperreactivity. Asthma prevalence was strongly associated with positive house dust mite skin test, family history of asthma, and total IgE.

Development and testing of a quantitative food frequency questionnaire for use in Gujarat, India

Abstract: Objective: To develop and test a food frequency questionnaire (FFQ) for use in rural areas of Kerala, India. Design: Based on food use and market surveys of the study area, a quantitative 81item interviewer-administered FFQ was developed. A validation study was conducted consisting of 24-h diet recalls (24HR) administered on 8 days randomly selected over an entire year and two administrations of the FFQ, one at the beginning of the l-year period and the other at the end. FFQ and 24HR-derived nutrient scores were compared using correlation and regression analyses and by examining differences in the nutrient scores. Setting: Rural villages in Ernakulum district, Kerala, South India. Subjecrs: In each of 30 households, the male head of household and female food preparer were enrolled. Results: Pearson (parametric) correlation coefficients (Y,) averaged about 0.50 in comparing nutrient scores derived from the 24HR with those from the first FFQ and about 0.55 in comparing the second FFQ. On average, Spearman correlation coefficients (YJ were slightly lower than the Y, in comparing the scores derived from the first FFQ, but virtually identical for the second FFQ. Regression analyses indicated better agreement in the comparison of the 24HR-derived scores with the first FFQ than the second FFQ. Difference scores, however, tended to be larger in comparing the first FFQ scores with the 24HR. Conclusions: This FFQ produces results broadly comparable to those used in Europe and North America, indicating its suitability for comparing exposures within a study population in reference to health-related endpoints.

Pseudogap above T c in a model with d x 2 − y 2 pairing

Abstract: We study the anomalous normal state properties of a simple two-dimensional model whose ground state is a d-wave superconductor. Using a self-consistent, conserving formulation, we show that pairing correlations above T_c lead to the appearance of a highly anisotropic pseudogap in the electronic spectral function and the destruction of the Fermi surface. We discuss the similarities and differences between our results and ARPES experiments on underdoped cuprates.

Polycarbazole-based electrochemical transistor

Abstract: A polycarbazole conducting polymer transistor has been constructed having the dimensions 1 cm × 2 cm × 1 mm. Polycarbazole film used here has a redox potential of 1.30 V. Polymer-coated platinum plates were used as the source and drain. The inter-electrode spacing of the device is typically of the order of 200–500 μm to minimise the internal resistance. The high saturation current region of the transistor in the most positive bias voltage (1.3 V), with negligible hysteresis and greater stability, appears to give a device that is superior to other conducting polymer transistors.

J- and H-Aggregates of Porphyrins with Surfactants: Fluorescence, Stopped Flow and Electron Microscopy Studies

Abstract: The interaction of cationic surfactants such as CTAB (cetyl trimethyl ammonium bromide) with tetrakis-(4-sulfonato phenyl) porphine (H4TPPS2−), a dianion, leads to the formation of two premicellar aggregates of porphyrin when [CTAB] is below CMC (critical micelle concentration) and a micellized monomer when [CTAB] is above CMC. The premicellar aggregates are labeled as J- and H-aggregates because of their characteristic spectroscopic properties. Simple inorganic cations such as K+, Ba2+, Ca2+ and Zr4+ also induce the formation of the J-aggregate but not of the H-aggregate. The formation of the J-aggregate is preceded by kinetic intermediates, while no intermediate was observed prior to the formation of the H-aggregate. The rate of formation of the H-aggregate was faster than that of the J-aggregate. The fluorescence depolarization (anisotropy) properties of the monomer and the H- and J-aggregates were studied and compared. The size and structure of the J-aggregate were examined by transmission electron mi...

Observation of a minimum in the temperature-dependent electrical resistance above the magnetic-ordering temperature in Gd2PdSi3

Abstract: Results on electrical resistivity, magnetoresistance, magnetic susceptibility, heat capacity and 155Gd Mossbauer measurements on a Gd-based intermetallic compound, Gd2PdSi3, are reported. A finding of interest is that the resistivity unexpectedly shows a well-defined minimum at about 45 K, well above the long-range magnetic-ordering temperature (21 K), a feature which gets suppressed by the application of a magnetic field. This observation in a Gd alloy presents an interesting scenario. On the basis of our results, we propose electron localization induced by s-f (or d-f) exchange interaction prior to long-range magnetic order as a mechanism for the electrical resistance minimum.

Low-Mass X-Ray Binaries, Millisecond Radio Pulsars, and the Cosmic Star Formation Rate

Abstract: We report on the implications of the peak in the cosmic star formation rate (SFR) at redshift z ≈ 1.5 for the resulting population of low-mass X-ray binaries (LMXBs) and for that of their descendants, the millisecond radio pulsars (MRPs). Since the evolutionary timescales of LMXBs, their progenitors, and their descendants are thought be significant fractions of the time interval between the SFR peak and the present epoch, there is a lag in the turn-on of the LMXB population, with the peak activity occurring at z~0.5-1. The peak in the MRP population is delayed further, occurring at z0.5. We show that the discrepancy between the birthrate of LMXBs and the birthrate of MRPs, found under the assumption of a steady state SFR, can be resolved for the population as a whole when the effects of a time-variable SFR are included. A discrepancy may persist for LMXBs with short orbital periods, although a detailed population synthesis will be required to confirm this. Furthermore, since the integrated X-ray luminosity distribution of normal galaxies is dominated by X-ray binaries, it should show strong luminosity evolution with redshift. In addition to an enhancement near the peak (z ≈ 1.5) of the SFR due to the prompt turn-on of the relatively short-lived massive X-ray binaries and young supernova remnants, we predict a second enhancement by a factor of ~10 at a redshift between ~0.5 and ~1 due to the delayed turn-on of the LMXB population. Deep X-ray observations of galaxies out to z ≈ 1 by the Advanced X-Ray Astrophysics Facility will be able to observe this enhancement and, by determining its shape as a function of redshift, will provide an important new method for constraining evolutionary models of X-ray binaries.