Showing papers by "Tata Institute of Fundamental Research published in 2001"

Reviving fossil radio plasma in clusters of galaxies by adiabatic compression in environmental shock waves

Abstract: We give for a plasma with a history of several expansion and contraction phases an analytical model of the evolution of a contained relativistic electron population under synchrotron, inverse Compton and adiabatic energy losses or gains. This is applied to different scenarios for evolution of radio plasma inside the cocoons of radio galaxies, after the activity of the central engine has ceased. It is demonstrated that fossil radio plasma with an age of even up to 2 Gyr can be revived by compression in a shock wave of large-scale structure formation, caused during the merging events of galaxy clusters, or by the accretion onto galaxy clusters. We argue, that this is a highly plausible explanation for the observed cluster radio relics, which are the regions of diffuse radio emission found in clusters of galaxies, without any likely parent radio galaxy seen nearby. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

Inhomogeneous pairing in highly disordered s-wave superconductors

Abstract: We study a simple model of a two-dimensional s-wave superconductor in the presence of a random potential as a function of disorder strength. We first use the Bogoliubov--de Gennes (BdG) approach to show that, with increasing disorder, the pairing amplitude becomes spatially inhomogeneous, and the system cannot be described within conventional approaches for studying disordered superconductors that assume a uniform order parameter. In the high-disorder regime, we find that the system breaks up into superconducting islands, with large pairing amplitude, separated by an insulating sea. We show that this inhomogeneity has important implications for the physical properties of this system, such as superfluid density and the density of states. We find that a finite spectral gap persists in the density of states, even in the weak-coupling regime, for all values of disorder, and we provide a detailed understanding of this remarkable result. We next generalize Anderson's idea of the pairing of exact eigenstates to include an inhomogeneous pairing amplitude, and show that it is able to qualitatively capture many of the nontrivial features of the full BdG analysis. Finally, we study the transition to a gapped insulating state driven by quantum phase fluctuations about the inhomogeneous superconducting state.

Time-Resolved Area-Normalized Emission Spectroscopy (TRANES): A Novel Method for Confirming Emission from Two Excited States

Abstract: A model-free method is described for constructing time-resolved area-normalized emission spectra (TRANES) using luminescence decays at all emission wavelengths. An isoemissive point in TRANES indicates that the observed emission from the sample is due to two species only, irrespective of the origin of the two species or the excited-state kinetics. Proof for the existence of an isoemissive point in TRANES is given for various cases involving two emissive species. The isoemissive point in TRANES is qualitatively similar to the isosbestic point in time-resolved absorption spectra (TRAS) in kinetic spectrophotometry involving two species.

An improved Hardy-Sobolev inequality and its application

Abstract: For\Omega \subset $IR^n$,n\geq 2, a bounded domain, and for 1 < p < n, we improve the Hardy-Sobolev inequality, by adding a term with a singular weight of the type \frac{1}{log(1/|x|)}$^2$ . We show that this weight function is optimal in the sense that the inequality fails for any other weight function more singular than this one. Moreover, we show that a series of finite terms can be added to improve the Hardy-Sobolev inequality, which answers a question of Brezis-Vazquez. Finally, we use this result to analyze the behaviour of the first eigenvalue of the operator L\mu\omega := -(div(| abla\upsilon|{p-2} abla\upilson)as \mu increases to \frac{n-p}{p}$^p$ for 1 < p < n.

Renormalization of Spectral Line Shape and Dispersion below T c in Bi 2 Sr 2 CaCu 2 O 8 + δ

Abstract: Angle-resolved photoemission data in the superconducting state of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ show a kink in the dispersion along the zone diagonal, which is related via a Kramers-Kr\"onig analysis to a drop in the low energy scattering rate. As one moves towards $(\ensuremath{\pi},0)$, this kink evolves into a spectral dip. The occurrence of these anomalies in the dispersion and line shape throughout the zone indicates the presence of a new energy scale in the superconducting state.

Depression – emerging insights from neurobiology

Abstract: An emerging hypothesis suggests that the pathogenesis and treatment of depression is likely to involve a plasticity of neuronal pathways. The inability of neuronal systems to exhibit appropriate, adaptive plasticity could contribute to the pathogenesis of depression. Antidepressant treatments may exert their therapeutic effects by stimulating appropriate adaptive changes in neuronal systems. Recent studies have demonstrated that chronic antidepressant administration up-regulates the cAMP signal transduction cascade resulting in an increased expression and function of the transcription factor CREB. Enhanced CREB expression leads to an up-regulation of specific target genes, including the neurotrophin BDNF. Chronic antidepressant treatments enhance BDNF expression within hippocampal and cortical neurons and can prevent the stress-induced decrease in BDNF expression. Stress has been shown to: (i) induce neuronal atrophy/death; and (ii) decrease neurogenesis of hippocampal neurons. Clinical studies indicate significant hippocampal damage in cases of major, recurrent depression. It is possible that antidepressant treatments through enhanced expression of growth and survival promoting factors like BDNF may prevent or reverse the atrophy and damage of hippocampal neurons. Indeed, studies have indicated that chronic antidepressant treatments enhance hippocampal neurogenesis, promote neuronal sprouting and prevent atrophy. The molecular mechanisms underlying the effects of antidepressant treatments including adaptations in the cAMP transduction cascade, CREB and BDNF gene expression, and structural neuronal plasticity are discussed.

Improved 3D triple resonance experiments, HNN and HN(C)N, for HN and 15N sequential correlations in (13C, 15N) labeled proteins: application to unfolded proteins

Abstract: Two triple resonance experiments, HNN and HN(C)N, are presented which correlate HN and 15N resonances sequentially along the polypeptide chain of a doubly (13C, 15N) labeled protein These incorporate several improvements over the previously published sequences for a similar purpose and have several novel features The spectral characteristics enable direct identification of certain triplets of residues, which provide many starting points for the sequential assignment procedure The experiments are sensitive and their utility has been demonstrated with a 22 kDa protein under unfolding conditions where most of the standard triple resonance experiments such as HNCA, CBCANH etc have limited success because of poor amide, Cα and Cβ chemical shift dispersions

THE COSMIC-RAY ANTIPROTON FLUX BETWEEN 3 AND 49 GeV

Abstract: We report on a new measurement of the cosmic ray antiproton spectrum. The data were collected by the balloon-borne experiment CAPRICE98, which was —own on 1998 May 28¨29 from Fort Sumner, New Mexico. The experiment used the NMSU-WiZard/CAPRICE98 balloon-borne magnet spectrometer equipped with a gas Ring Imaging Cherenkov (RICH) detector, a time-of-—ight system, a tracking device consisting of drift chambers and a superconducting magnet, and a silicon-tungsten calorimeter. The RICH detector was the —rst ever —own capable of mass-resolving charge-one particles at energies above 5 GeV. A total of 31 antiprotons with rigidities between 4 and 50 GV at the spectrometer were identi—ed with small backgrounds from other particles. The absolute antiproton energy spectrum was determined in the kinetic energy region at the top of the atmosphere between 3.2 and 49.1 GeV. We found that the observed antiproton spectrum and the antiproton-to-proton ratio are consistent with a pure secondary origin. However, a primary component may not be excluded.

Projected Wave Functions and High Temperature Superconductivity

Abstract: Strong correlations are essential to understand d-wave high temperature superconductivity in doped Mott insulators [1]. The no-double occupancy constraint arising from strong correlations has been treated within two complementary approaches. Within the gauge theory approach [2], which is valid at all temperatures, the constraint necessitates the inclusion of strong gauge fluctuations. Alternatively, the constraint can be implemented exactly at T = 0 using the variational Monte Carlo (VMC) method. Previous variational studies [3–6] have focussed primarily on the energetics of competing states. In this letter, we revisit projected wavefunctions of the form proposed by Anderson in 1987 [1]. We compute physically interesting correlations using VMC and show that projection leads to loss of coherence. We obtain information about low energy excitations from the singular behavior of moments of the occupied spectral function. Remarkably, our results for various observables are in semi-quantitative agreement with experiments on the cuprates. We also make qualitative predictions for the doping (x) dependence of correlation functions in projected states (for x ≪ 1) from general arguments which are largely independent of the detailed form of the wavefunction and the Hamiltonian. We use the Hubbard Hamiltonian H = K + Hint. The kinetic energy K = P k,σ ǫ(k)c †σ ckσ with ǫ(k) =

Estimate of the primordial magnetic field helicity.

Abstract: Electroweak baryogenesis proceeds via changes in the non-Abelian Chern-Simons number. It is argued that these changes generate a primordial magnetic field with left-handed helicity. The helicity density of the primordial magnetic field today is then estimated to be given by approximately 10(2)n(b), where n(b) approximately 10(-6)/cm(3) is the present cosmological baryon number density. With certain assumptions about the inverse cascade we find that the field strength at recombination is approximately 10(-13) G on a comoving coherence scale approximately 0.1 pc.

OSSE and RXTE Observations of GRS 1915+105: Evidence for Nonthermal Comptonization

Abstract: GRS 1915+105 was observed by the Oriented Scintillation Spectroscopy Experiment (OSSE) aboard the Compton Gamma Ray Observatory nine times in 1995-2000, and eight of those observations were simultaneous with those by the Rossi X-Ray Timing Explorer (RXTE). We present an analysis of all of the OSSE data and of two RXTE-OSSE spectra with the lowest and highest X-ray fluxes. The OSSE data show a power-law-like spectrum extending up to 600 keV without any break. We interpret this emission as strong evidence for the presence of nonthermal electrons in the source. The broadband spectra cannot be described by either thermal or bulk-motion Comptonization, whereas they are well described by Comptonization in hybrid thermal/nonthermal plasmas.

The magnetic susceptibility, specific heat and dielectric constant of hexagonal YMnO3, LuMnO3 and ScMnO3

Abstract: We report the magnetic susceptibility, specific heat and dielectric constant for high-purity polycrystalline samples of three hexagonal manganites: YMnO3, LuMnO3 and ScMnO3. These materials can exhibit a ferroelectric transition at very high temperatures (TFE>700 K). At lower temperatures there is magnetic ordering of the frustrated Mn3+ spins (S = 2) on a triangular Mn lattice (YMnO3: TN = 71 K; LuMnO3: TN = 90 K and ScMnO3: TN = 130 K). The transition is characterized by a sharp kink in the magnetic susceptibility at TN below which it continues to increase due to the frustration on the triangular lattice. The specific heat shows one clear continuous phase transition at TN, which is independent of external magnetic field up to 9 T with an entropy content as expected for Mn3+ ions. The temperature-dependent dielectric constant displays a distinct anomaly at TN.

Asymmetric high-energy ion emission from argon clusters in intense laser fields.

Abstract: Clusters of $2\ifmmode\times\else\texttimes\fi{}{10}^{3}$ to $4\ifmmode\times\else\texttimes\fi{}{10}^{4}$ Ar atoms are Coulomb exploded in intense (up to $8\ifmmode\times\else\texttimes\fi{}{10}^{15}\mathrm{W}{\mathrm{cm}}^{\ensuremath{-}2}$) laser fields. The dependence of multiply charged argon ion energies on the polarization state of light is probed. A directional asymmetry in the ion-explosion energies is observed for the highest charge states. The ion-energy distribution consists of a low-energy isotropic component, and a high-energy anisotropic one. The results are discussed in terms of an asymmetric Coulomb-explosion scenario.

Einstein-Hermitian connections on polystable principal bundles over a compact Kähler manifold

Abstract: Given a compact Kahler manifold M and a connected reductive algebraic group G over [inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="01i" /], a prinlcipal G -bundle over M admits an Einstein-Hermitian connection if and only if the principal bundle is polystable. If M is a projective manifold, a Higgs G -bundle over M admits an Einstein-Hermitian connection if and only if the Higgs bundle is polystable.

Radial scalelengths of the galactic thin and thick disc with 2MASS data

Abstract: This paper presents a global analysis of the 2MASS (Two Micron All Sky Survey) data as observed in seven fields at different galactic latitudes in our Galaxy. The data allow the preliminary determination of the scale parameters, which lead to strong constraints on the radial and vertical structure of the galactic thin and thick disc. The interpretation of star counts and colour distributions of stars in the near-infrared with the synthetic stellar population model gives strong evidence that the galactic thin disc density scalelength (h R ) is rather short (2.8 ± 0.3 kpc). The galactic thick disc population is revisited in the light of new data. We find the thick disc to have a local density of 3.5 ± 2.0 per cent of the thin disc, exponential scaleheight (h z ) of 860±200 pc and exponential scalelength (h R ) of 3.7 ± 0.8 0.5 kpc.

Structure is lost incrementally during the unfolding of barstar

Abstract: Coincidental equilibrium unfolding transitions observed by multiple structural probes are taken to justify the modeling of protein unfolding as a two-state, N ⇌ U, cooperative process. However, for many of the large number of proteins that undergo apparently two-state equilibrium unfolding reactions, folding intermediates are detected in kinetic experiments. The small protein barstar is one such protein. Here the two-state model for equilibrium unfolding has been critically evaluated in barstar by estimating the intramolecular distance distribution by time-resolved fluorescence resonance energy transfer (TR-FRET) methods, in which fluorescence decay kinetics are analyzed by the maximum entropy method (MEM). Using a mutant form of barstar containing only Trp 53 as the fluorescence donor and a thionitrobenzoic acid moiety attached to Cys 82 as the fluorescence acceptor, the distance between the donor and acceptor has been shown to increase incrementally with increasing denaturant concentration. Although other probes, such as circular dichroism and fluorescence intensity, suggest that the labeled protein undergoes two-state equilibrium unfolding, the TR-FRET probe clearly indicates multistate equilibrium unfolding. Native protein expands progressively through a continuum of native-like forms that achieve the dimensions of a molten globule, whose heterogeneity increases with increasing denaturant concentration and which appears to be separated from the unfolded ensemble by a free energy barrier.

Upper limits on sparticle masses from g -- 2 and the possibility for discovery of supersymmetry at colliders and in dark matter searches.

Abstract: We analyze the implications of the new physics effect seen in the $g\ensuremath{-}2$ Brookhaven measurement and show that if the effect arises from supersymmetry, then the sign of the Higgs mixing parameter $\ensuremath{\mu}$ is determined to be positive in the standard sign convention. Further, analyses within the minimal supergravity model show that the Brookhaven result leads to upper limits on the universal gaugino and scalar masses of ${m}_{1/2}\ensuremath{\le}800\mathrm{GeV}$ and ${m}_{0}\ensuremath{\le}1.5\mathrm{TeV}$ for $\mathrm{tan}\ensuremath{\beta}\ensuremath{\le}55$. Our analysis strongly suggests that supersymmetry via production of sparticles must be found at the CERN Large Hadron Collider. Further, $\mathrm{sgn}(\ensuremath{\mu})$ positive is favorable for the discovery of supersymmetric cold dark matter.

Synthesis of nanocrystalline material by sputtering and laser ablation at low temperatures

Abstract: Physical vapor deposition techniques such as sputtering and laser ablation – which are very commonly used in thin film technology – appear to hold much promise for the synthesis of nanocrystalline thin films as well as loosely aggregated nanoparticles. We present a systematic study of the process parameters that facilitate the growth of nanocrystalline metals and oxides. The systems studied include TiO2, ZnO, γ-Al2O3, Cu2O, Ag and Cu. The mean particle size and crystallographic orientation are influenced mainly by the sputtering power, the substrate temperature and the nature, pressure and flow rate of the sputtering gas. In general, nanocrystalline thin films were formed at or close to 300 K, while loosely adhering nanoparticles were deposited at lower temperatures.

Coronal Mass Ejection of 2000 July 14 Flare Event: Imaging from Near-Sun to Earth Environment

Abstract: We report the results of our investigation of interplanetary effects caused by the large solar flare (X5.7/3B) that occurred on 2000 July 14. In association with this event a bright, fast, halo coronal mass ejection (CME) was observed. The analysis of multiwavelength, high-cadence images obtained from the Nancay Radioheliograph shows the on-disk signatures of the initiation of the CME at low-coronal heights, ≤2 R☉. The formation of the CME inferred from the radio data indicates a nearly developed halo at the low corona. The white-light images and CME follow-up measurements in the interplanetary medium also show, in agreement with the radio data, the propagation of the fully developed halo CME. The inference on the consequences of the CME in the inner heliosphere is from the interplanetary scintillation (IPS) observations obtained with the Ooty Radio Telescope and multiantenna system at the Solar-Terrestrial Environment Laboratory. Scintillation measurements at Ooty on a grid of a large number of radio sources provided an opportunity to image the disturbance associated with the CME at different distances from the Sun before its arrival at the near-Earth space. The scintillation data in particular also played a crucial role in understanding the radial evolution of the speed of the CME in the inner heliosphere. The "speed-distance" plot indicates a two-level deceleration: (1) a low decline in speed at distances within or about 100 solar radii and (2) a rapid decrease at larger distances from the Sun. The linear increase in the size of the CME with radial distance is also briefly discussed. The expansion of the CME, formation of the halo in the low corona, and its speed history in the interplanetary medium suggest a driving energy, which is likely supplied by the twisted magnetic flux rope system associated with the CME.

Giant gravitons, Bogomol'nyi-Prasad-Sommerfield bounds, and noncommutativity

Abstract: It has been recently suggested that gravitons moving in AdS{sub m}xS{sup n} spacetimes along the S{sup n} blow up into spherical (n-2)-branes whose radius increases with increasing angular momentum. This leads to an upper bound on the angular momentum, thus ''explaining'' the stringy exclusion principle. We show that this bound is present only for states which saturate a BPS-like condition involving the energy E and angular momentum J,E{>=}J/R, where R is the radius of S{sup n}. Restriction of motion to such states lead to a noncommutativity of the coordinates on S{sup n}. As an example of motions which do not obey the exclusion principle bound, we show that there are finite action instanton configurations interpolating between two possible BPS states. We suggest that this is consistent with the proposal that there is an effective description in terms of supergravity defined on noncommutative spaces and noncommutativity arises here because of imposing supersymmetry.

Instantons and Chiral Anomaly in Fuzzy Physics

Abstract: In continuum physics, there are important topological aspects like instantons, θ-terms and the axial anomaly. Conventional lattice discretizations often have difficulties in treating one or the other of these aspects. In this paper, we develop discrete quantum field theories on fuzzy manifolds using noncommutative geometry. Basing ourselves on previous treatments of instantons and chiral fermions (without fermion doubling) on fuzzy spaces and especially fuzzy spheres, we present discrete representations of θ-terms and topological susceptibility for gauge theories and derive axial anomaly on the fuzzy sphere. Our gauge field action for four dimensions is bounded by a constant times the modulus of the instanton number as in the continuum.

Supergravity description of non-Bogomol'nyi-Prasad-Sommerfield branes

Abstract: We construct supergravity solutions that correspond to N $\mathrm{D}p$-branes coinciding with $\overline{N}$ $\overline{\mathrm{D}p}$-branes. We study the physical properties of the solutions and analyze the supergravity description of tachyon condensation. We construct an interpolation between the brane-antibrane solution and the Schwarzschild solution and discuss its possible application to the study of non-supersymmetric black holes.

Extreme-value statistics of hierarchically correlated variables deviation from Gumbel statistics and anomalous persistence.

Abstract: We study analytically the distribution of the minimum of a set of hierarchically correlated random variables E1, E2,ellipsis, E(N) where E(i) represents the energy of the ith path of a directed polymer on a Cayley tree. If the variables were uncorrelated, the minimum energy would have an asymptotic Gumbel distribution. We show that due to the hierarchical correlations, the forward tail of the distribution of the minimum energy becomes highly nonuniversal, depends explicitly on the distribution of the bond energies epsilon, and is generically different from the superexponential forward tail of the Gumbel distribution. The consequence of these results to the persistence of hierarchically correlated random variables is discussed and the persistence is also shown to be generically anomalous.

Two coexisting vortex phases in the peak effect regime in a superconductor

Abstract: The critical current in the vortex phase of a type-II superconductor such as NbSe2 displays a striking anomaly in the vicinity of the superconductor-to-normal-metal transition. Instead of going to zero smoothly, it rebounds to a sharp and pronounced maximum, just before vanishing at the transition. This counter-intuitive phenomenon, known as the peak effect1,2,3, has remained an unsolved problem for 40 years. Here we use a scanning a.c. Hall microscope to visualize the real-space distribution of the critical current in NbSe2. We show that in the peak-effect regime two distinct vortex-matter phases with intrinsically different pinning strengths coexist on a macroscopic scale. The composition of the two-phase mixture and the transformation of one phase into another are responsible for the history effects4,5,6 and anomalous voltage response4,5 commonly seen when external parameters such as temperature, magnetic field or transport current are varied. We argue that the observed phase coexistence is, in fact, the hallmark of a disorder-driven non-thermal phase transition.

X-Ray Probes of Cosmic Star Formation History

Abstract: We discuss the imprints left by a cosmological evolution of the star formation rate (SFR) on the evolution of X-ray luminosities Lx of normal galaxies, using the scheme earlier proposed by us, wherein the evolution of LX of a galaxy is driven by the evolution of its X-ray binary population. As indicated in our earlier work, the profile of Lx with redshift can both serve as a diagnostic probe of the SFR profile and constrain evolutionary models for X-ray binaries. We report here the first calculation of the expected evolution of X-ray luminosities of galaxies, updating our work by using a suite of more recently developed SFR profiles that span the currently plausible range. The first Chandra deep imaging results on Lx evolution are beginning to probe the SFR profile of bright spiral galaxies; the early results are consistent with predictions based on current SFR models. Using these new SFR profiles, the resolution of the "birthrate problem" of low-mass X-ray binaries and recycled, millisecond pulsars in terms of an evolving global SFR is more complete. We discuss the possible impact of the variations in the SFR profile of individual galaxies and galaxy types.