pawan kumar - Academia.edu (original) (raw)
Papers by pawan kumar
Advanced Powder Technology, 2014
CoFe 2Àx Y x O 4 (x = 0.0 and 0.05) compound has been synthesized by citrate precursor method and... more CoFe 2Àx Y x O 4 (x = 0.0 and 0.05) compound has been synthesized by citrate precursor method and, their structural and magnetic properties have been investigated. X-ray diffraction and Raman spectroscopy have been used to confirm the formation of single phase cubic spinel structure. The X-ray diffraction patterns have been analyzed employing Rietveld refinement technique. The surface morphology and particle size of the samples have been examined using FE-SEM and TEM. Substituting small amount of Y 3+ cation causes significant reduction of the particle size. The magnetic hysteresis curve recorded at room temperature using VSM over a field range of ±2 T shows enhancement in coercivity and reduction in highest magnetization with the Y 3+ substitution. The enhancement of coercivity is attributed to the transition from multidomain to single domain state. The cubic magnetocrystalline anisotropy constants for present samples has been determined by ''law of approach'' to saturation which suggests the smaller value for substituted sample (x = 0.05) than that of unsubstituted sample (x = 0.0). ZFC and FC magnetization measurement over the temperature range 300-900 K shows blocking temperature is far above than room temperature. The decrease of highest magnetization and magnetocrystalline anisotropy constant is ascribed to weakening of superexchange interaction and surface effect.
Materials Science Forum, 2012
Letters in Drug Design & Discovery, 2012
A series of novel 4-arylthiazolylhydrazones (TZHs) derived from 1-indanones were synthesized in g... more A series of novel 4-arylthiazolylhydrazones (TZHs) derived from 1-indanones were synthesized in good yields (66e92%) in a simple procedure using microwave irradiation and then characterized by spectroscopy studies. The compounds were evaluated for their in vitro anti-Trypanosoma cruzi activity against the epimastigote, trypomastigote and amastigote forms of the parasite. Most TZHs displayed excellent activity, and were more potent and selective than the reference drug Benznidazole, used in the current chemotherapy. Analysis of the free sterols from parasite incubated with the compounds showed that inhibition of ergosterol biosynthesis is a possible target for the action of these new TZHs. In particular, TZH 9 emerged as a promising antichagasic compound to be evaluated in animal models.
ChemInform, 2011
A series of eight 2-substituted 3-tolyltropane derivatives were synthesized, and the in vitro and... more A series of eight 2-substituted 3-tolyltropane derivatives were synthesized, and the in vitro and in vivo biological activities as dopamine uptake inhibitors were determined. From the in vitro structure-activity data, it is apparent that a tolyl group in the 2-position, independent of the stereochemical attachment to the tropane ring system, provided compounds (9-12, 14) that exhibit high-affinity binding at the dopamine transporter (DAT). Although a slight stereochemical preference in binding affinity at the DAT was observed for the 2beta-(R)-alcohol 10 over the 2beta-(S)-isomer 11, no significant differences in behavioral effects were observed. Furthermore, despite a relatively low potency of 10 for the inhibition of dopamine uptake compared to its affinity for the DAT, its behavioral profile did not vary significantly from cocaine. These data indicate that a behavioral characterization of compounds is a critical feature of efforts to discover pharmacological treatments for cocaine abuse.
Journal of Applied Physics, 2015
Enhanced surface plasmon polariton propagation length using a buried metal grating How many surfa... more Enhanced surface plasmon polariton propagation length using a buried metal grating How many surface plasmons are locally excited on the ridges of metallic lamellar gratings? Appl. Phys. Lett. 96, 051115 (2010); 10.1063/1.3304021
ABSTRACT Hematite (α-Fe2O3) film is formed on liquid-vapor interface using ammonia (NH3) vapors. ... more ABSTRACT Hematite (α-Fe2O3) film is formed on liquid-vapor interface using ammonia (NH3) vapors. Thus formed film was transferred to a glass substrate. The film was characterized for structural, morphological and optical properties. The film is observed uniform morphologically with about 21 nm grain size inside the film as observed by SEM and XRD characterizations. The film after annealing at 500°C is observed crystalline in nature. The observed direct and indirect band gaps of the annealed film are 2.61eV and 2.07 eV, respectively.
The Astrophysical Journal, 1991
TFW ASTROPHYSICAL JoURNAL, 374:366-368, 1991 June 10 © 1991. The American Astronomical Society. A... more TFW ASTROPHYSICAL JoURNAL, 374:366-368, 1991 June 10 © 1991. The American Astronomical Society. All rights reserved. Printed in USA THERMAL AND MECHANICAL DAMPING OF SOLAR p-MODES PETER GOLDREICH California Institute of Technology, 170-25 Pasadena, CA ...
BiFeO 3 ceramics were prepared by using the sol-gel technique with adding excess amount of bismut... more BiFeO 3 ceramics were prepared by using the sol-gel technique with adding excess amount of bismuth. Different concentrations of excess bismuth were used to study the evolution of crystal structure and physical properties of BiFeO 3 . The phase purity and crystal structure of all samples have been studied by powder X-ray diffraction. The primary BiFeO 3 phase could be indexed to R3C space group and analyzed by Rietveld method to extract different crystallographic parameters. Secondary phases were suppressed with addition of 4% excess bismuth. The leakage current for all the samples for all samples have been studied by ferroelectric hysteresis loop at room temperature in the maximum electric field of ±100 kV/cm. The leakage current for 4% sample is least (1.86 nA) among all samples due to reduction of extra phases.
The Astrophysical Journal, 1994
It is well known that the standard transport equations violate causality when gradients are large... more It is well known that the standard transport equations violate causality when gradients are large or when temporal variations are rapid. We derive a modified set of transport equations that satisfy causality. These equations are obtained from the underlying Boltzmann equation. We use a simple model for particle collisions which enables us to derive moment equations non-perturbatively, i.e. without making the usual assumption that the distribution function deviates only slightly from its equilibrium value. We also retain time derivatives of various moments and choose closure relations so that the final set of equations are causal. We apply the model to two problems: particle diffusion and viscous transport. In both cases we show that signals propagate at a finite speed and therefore that the formalism obeys causality. When spatial gradients or temporal variations are small, our theory for particle diffusion and viscous flows reduces to the usual diffusion and Navier-Stokes equations respectively. However, in the opposite limit of strong gradients the theory produces causal results with finite transport fluxes, whereas the standard theory gives results that are physically unacceptable. We find that when the velocity gradient is large on the scale of a mean free path, the viscous shear stress is suppressed relative to the prediction of the standard diffusion approximation. The shear stress reaches a maximum at a finite value of the shear amplitude and then decreases as the velocity gradient increases. The decrease of the stress in the limit of large shear is qualitatively different from the case of scalar particle diffusion where the diffusive flux asymptotes to a constant value in the limit of large density gradient. In the case of a steady Keplerian accretion disk with hydrodynamic turbulent viscosity, the stress-limit translates to an upper bound on the Shakura-Sunyaev α-parameter, namely α < 0.07. The limit on α is much stronger in narrow boundary layers where the velocity shear is larger than in a Keplerian disk. † Alfred P. Sloan Research Fellow
The Astrophysical Journal, 1990
... Printed in USA WAVE GENERATION BY TURBULENT CONVECTION PETER GOLDREICH California Institute o... more ... Printed in USA WAVE GENERATION BY TURBULENT CONVECTION PETER GOLDREICH California Institute of Technology PAWAN KUMAR High Altitude Observatory, National Center for Atmospheric Research' Received 1990 January 29; accepted 1990 May 9 We consider ...
The Astrophysical Journal, 1999
ABSTRACT We estimate the fraction of the total energy in a gamma-ray burst (GRB) that is radiated... more ABSTRACT We estimate the fraction of the total energy in a gamma-ray burst (GRB) that is radiated in photons during the main burst. Random internal collisions among different shells limit the efficiency for converting bulk kinetic energy to photons. About 1% of the energy of explosion is ...
The Astrophysical Journal, 1996
We calculate the damping of quadrupole f-and low order g-modes (primary modes) by nonlinear coupl... more We calculate the damping of quadrupole f-and low order g-modes (primary modes) by nonlinear coupling to other modes of the star. Primary modes destabilize high degree g-modes of half their frequency (daughter modes) by 3-mode coupling in radiative zones. For sunlike stars, the growth time ≡ η −1 ≈ 4E
The Astrophysical Journal, 1989
... Printed in USA NONLINEAR INTERACTIONS AMONG SOLAR ACOUSTIC MODES PAWANKUMAR High Altitude Obs... more ... Printed in USA NONLINEAR INTERACTIONS AMONG SOLAR ACOUSTIC MODES PAWANKUMAR High Altitude Observatory, National Center for Atmospheric Research&amp;amp;#x27; PETER GOLDREICH California Institute of Technology Received 1988 August 15; accepted 1988 ...
The Astrophysical Journal, 1994
... Printed in USA PROPERTIES OF ACOUSTIC SOURCES IN THE SUN PAWAN KUMAR Department of Physics, M... more ... Printed in USA PROPERTIES OF ACOUSTIC SOURCES IN THE SUN PAWAN KUMAR Department of Physics, Massachusetts Institute ofTechnology, Cambridge, MA 02139 Received 1993 October 18; accepted 1993 December17 The power spectrum of solar acoustic ...
The Astrophysical Journal, 1988
... Printed in USA THE INTERACTION OF ACOUSTIC RADIATION WITH TURBULENCE PETER GOLDREICH AND PAWA... more ... Printed in USA THE INTERACTION OF ACOUSTIC RADIATION WITH TURBULENCE PETER GOLDREICH AND PAWAN KUMAR California Institute of Technology Received 1987 June 22; accepted 1987 August24 We derive expressions for the spectral emissivity and ...
The Astrophysical Journal, 1993
We show that low-order g-modes with large enough amplitudes to affect significantly the solar neu... more We show that low-order g-modes with large enough amplitudes to affect significantly the solar neutrino fluxes would produce surface velocities that are 10 4 times larger than the observed upper limits and hence are ruled out by existing data. We also demonstrate that any large-amplitude, short-period oscillations that grow on a Kelvin-Helmholtz time scale will require, to affect solar neutrino fluxes, a large amount of energy (for g-modes, 10 9 times the energy in the observed p− mode oscillations) and a tiny amount of dissipation (for g-modes, 10 −8 the dissipation rate of the p-modes).
The Astrophysical Journal, 1994
... Printed in USA EXCITATION OF SOLAR p-MODES PETER GOLDREICH AND NORMAN MURRAY California Insti... more ... Printed in USA EXCITATION OF SOLAR p-MODES PETER GOLDREICH AND NORMAN MURRAY California Institute ofTechnology, Pasadena, CA 91125 PAWAN KUMAR Massachusetts Institute ofTechnology, Cambridge, MA 02139 Received 1993 March 31 ; accepted 1993 ...
The Astrophysical Journal, 2000
We calculate the afterglow emission for Gamma-Ray Bursts (GRBs) going off in an extremely low den... more We calculate the afterglow emission for Gamma-Ray Bursts (GRBs) going off in an extremely low density medium, referred to as naked bursts. Our results also apply to the case where the external medium density falls off sharply at some distance from the burst. The observed afterglow flux in this case originates at high latitudes, i.e. where the angle between the fluid velocity and the observer line of sight is greater than Γ −1 . The observed peak frequency of the spectrum for naked bursts decreases with observer time as t −1 , and the flux at the peak of the spectrum falls off as t −2 . The 2-10 keV X-ray flux from a naked burst of average fluence should be observable by the SWIFT satellite for time duration of about 10 3 longer than the burst variability timescale. The high latitude emission contributes to the early X-ray afterglow flux for any GRB, not just naked bursts, and can be separated from the shocked inter-stellar medium (ISM) emission by their different spectral and temporal properties. Measurements of the high latitude emission could be used to map the angular structure of GRB producing shells.
![Research paper thumbnail of Optimized synthesis of [3-15N]-labeled uridine phosphoramidites](https://attachments.academia-assets.com/47292623/thumbnails/1.jpg)
](Tetrahedron Letters, 2009
A short and high-yielding synthetic route to [3-15 N]-labeled uridine phosphoramidite 1 (26% over... more A short and high-yielding synthetic route to [3-15 N]-labeled uridine phosphoramidite 1 (26% overall yield from uridine) has been developed. This will enable automated synthesis of isotopically labeled RNA strands and facilitate their use in structural studies.
Advanced Powder Technology, 2014
CoFe 2Àx Y x O 4 (x = 0.0 and 0.05) compound has been synthesized by citrate precursor method and... more CoFe 2Àx Y x O 4 (x = 0.0 and 0.05) compound has been synthesized by citrate precursor method and, their structural and magnetic properties have been investigated. X-ray diffraction and Raman spectroscopy have been used to confirm the formation of single phase cubic spinel structure. The X-ray diffraction patterns have been analyzed employing Rietveld refinement technique. The surface morphology and particle size of the samples have been examined using FE-SEM and TEM. Substituting small amount of Y 3+ cation causes significant reduction of the particle size. The magnetic hysteresis curve recorded at room temperature using VSM over a field range of ±2 T shows enhancement in coercivity and reduction in highest magnetization with the Y 3+ substitution. The enhancement of coercivity is attributed to the transition from multidomain to single domain state. The cubic magnetocrystalline anisotropy constants for present samples has been determined by ''law of approach'' to saturation which suggests the smaller value for substituted sample (x = 0.05) than that of unsubstituted sample (x = 0.0). ZFC and FC magnetization measurement over the temperature range 300-900 K shows blocking temperature is far above than room temperature. The decrease of highest magnetization and magnetocrystalline anisotropy constant is ascribed to weakening of superexchange interaction and surface effect.
Materials Science Forum, 2012
Letters in Drug Design & Discovery, 2012
A series of novel 4-arylthiazolylhydrazones (TZHs) derived from 1-indanones were synthesized in g... more A series of novel 4-arylthiazolylhydrazones (TZHs) derived from 1-indanones were synthesized in good yields (66e92%) in a simple procedure using microwave irradiation and then characterized by spectroscopy studies. The compounds were evaluated for their in vitro anti-Trypanosoma cruzi activity against the epimastigote, trypomastigote and amastigote forms of the parasite. Most TZHs displayed excellent activity, and were more potent and selective than the reference drug Benznidazole, used in the current chemotherapy. Analysis of the free sterols from parasite incubated with the compounds showed that inhibition of ergosterol biosynthesis is a possible target for the action of these new TZHs. In particular, TZH 9 emerged as a promising antichagasic compound to be evaluated in animal models.
ChemInform, 2011
A series of eight 2-substituted 3-tolyltropane derivatives were synthesized, and the in vitro and... more A series of eight 2-substituted 3-tolyltropane derivatives were synthesized, and the in vitro and in vivo biological activities as dopamine uptake inhibitors were determined. From the in vitro structure-activity data, it is apparent that a tolyl group in the 2-position, independent of the stereochemical attachment to the tropane ring system, provided compounds (9-12, 14) that exhibit high-affinity binding at the dopamine transporter (DAT). Although a slight stereochemical preference in binding affinity at the DAT was observed for the 2beta-(R)-alcohol 10 over the 2beta-(S)-isomer 11, no significant differences in behavioral effects were observed. Furthermore, despite a relatively low potency of 10 for the inhibition of dopamine uptake compared to its affinity for the DAT, its behavioral profile did not vary significantly from cocaine. These data indicate that a behavioral characterization of compounds is a critical feature of efforts to discover pharmacological treatments for cocaine abuse.
Journal of Applied Physics, 2015
Enhanced surface plasmon polariton propagation length using a buried metal grating How many surfa... more Enhanced surface plasmon polariton propagation length using a buried metal grating How many surface plasmons are locally excited on the ridges of metallic lamellar gratings? Appl. Phys. Lett. 96, 051115 (2010); 10.1063/1.3304021
ABSTRACT Hematite (α-Fe2O3) film is formed on liquid-vapor interface using ammonia (NH3) vapors. ... more ABSTRACT Hematite (α-Fe2O3) film is formed on liquid-vapor interface using ammonia (NH3) vapors. Thus formed film was transferred to a glass substrate. The film was characterized for structural, morphological and optical properties. The film is observed uniform morphologically with about 21 nm grain size inside the film as observed by SEM and XRD characterizations. The film after annealing at 500°C is observed crystalline in nature. The observed direct and indirect band gaps of the annealed film are 2.61eV and 2.07 eV, respectively.
The Astrophysical Journal, 1991
TFW ASTROPHYSICAL JoURNAL, 374:366-368, 1991 June 10 © 1991. The American Astronomical Society. A... more TFW ASTROPHYSICAL JoURNAL, 374:366-368, 1991 June 10 © 1991. The American Astronomical Society. All rights reserved. Printed in USA THERMAL AND MECHANICAL DAMPING OF SOLAR p-MODES PETER GOLDREICH California Institute of Technology, 170-25 Pasadena, CA ...
BiFeO 3 ceramics were prepared by using the sol-gel technique with adding excess amount of bismut... more BiFeO 3 ceramics were prepared by using the sol-gel technique with adding excess amount of bismuth. Different concentrations of excess bismuth were used to study the evolution of crystal structure and physical properties of BiFeO 3 . The phase purity and crystal structure of all samples have been studied by powder X-ray diffraction. The primary BiFeO 3 phase could be indexed to R3C space group and analyzed by Rietveld method to extract different crystallographic parameters. Secondary phases were suppressed with addition of 4% excess bismuth. The leakage current for all the samples for all samples have been studied by ferroelectric hysteresis loop at room temperature in the maximum electric field of ±100 kV/cm. The leakage current for 4% sample is least (1.86 nA) among all samples due to reduction of extra phases.
The Astrophysical Journal, 1994
It is well known that the standard transport equations violate causality when gradients are large... more It is well known that the standard transport equations violate causality when gradients are large or when temporal variations are rapid. We derive a modified set of transport equations that satisfy causality. These equations are obtained from the underlying Boltzmann equation. We use a simple model for particle collisions which enables us to derive moment equations non-perturbatively, i.e. without making the usual assumption that the distribution function deviates only slightly from its equilibrium value. We also retain time derivatives of various moments and choose closure relations so that the final set of equations are causal. We apply the model to two problems: particle diffusion and viscous transport. In both cases we show that signals propagate at a finite speed and therefore that the formalism obeys causality. When spatial gradients or temporal variations are small, our theory for particle diffusion and viscous flows reduces to the usual diffusion and Navier-Stokes equations respectively. However, in the opposite limit of strong gradients the theory produces causal results with finite transport fluxes, whereas the standard theory gives results that are physically unacceptable. We find that when the velocity gradient is large on the scale of a mean free path, the viscous shear stress is suppressed relative to the prediction of the standard diffusion approximation. The shear stress reaches a maximum at a finite value of the shear amplitude and then decreases as the velocity gradient increases. The decrease of the stress in the limit of large shear is qualitatively different from the case of scalar particle diffusion where the diffusive flux asymptotes to a constant value in the limit of large density gradient. In the case of a steady Keplerian accretion disk with hydrodynamic turbulent viscosity, the stress-limit translates to an upper bound on the Shakura-Sunyaev α-parameter, namely α < 0.07. The limit on α is much stronger in narrow boundary layers where the velocity shear is larger than in a Keplerian disk. † Alfred P. Sloan Research Fellow
The Astrophysical Journal, 1990
... Printed in USA WAVE GENERATION BY TURBULENT CONVECTION PETER GOLDREICH California Institute o... more ... Printed in USA WAVE GENERATION BY TURBULENT CONVECTION PETER GOLDREICH California Institute of Technology PAWAN KUMAR High Altitude Observatory, National Center for Atmospheric Research' Received 1990 January 29; accepted 1990 May 9 We consider ...
The Astrophysical Journal, 1999
ABSTRACT We estimate the fraction of the total energy in a gamma-ray burst (GRB) that is radiated... more ABSTRACT We estimate the fraction of the total energy in a gamma-ray burst (GRB) that is radiated in photons during the main burst. Random internal collisions among different shells limit the efficiency for converting bulk kinetic energy to photons. About 1% of the energy of explosion is ...
The Astrophysical Journal, 1996
We calculate the damping of quadrupole f-and low order g-modes (primary modes) by nonlinear coupl... more We calculate the damping of quadrupole f-and low order g-modes (primary modes) by nonlinear coupling to other modes of the star. Primary modes destabilize high degree g-modes of half their frequency (daughter modes) by 3-mode coupling in radiative zones. For sunlike stars, the growth time ≡ η −1 ≈ 4E
The Astrophysical Journal, 1989
... Printed in USA NONLINEAR INTERACTIONS AMONG SOLAR ACOUSTIC MODES PAWANKUMAR High Altitude Obs... more ... Printed in USA NONLINEAR INTERACTIONS AMONG SOLAR ACOUSTIC MODES PAWANKUMAR High Altitude Observatory, National Center for Atmospheric Research&amp;amp;#x27; PETER GOLDREICH California Institute of Technology Received 1988 August 15; accepted 1988 ...
The Astrophysical Journal, 1994
... Printed in USA PROPERTIES OF ACOUSTIC SOURCES IN THE SUN PAWAN KUMAR Department of Physics, M... more ... Printed in USA PROPERTIES OF ACOUSTIC SOURCES IN THE SUN PAWAN KUMAR Department of Physics, Massachusetts Institute ofTechnology, Cambridge, MA 02139 Received 1993 October 18; accepted 1993 December17 The power spectrum of solar acoustic ...
The Astrophysical Journal, 1988
... Printed in USA THE INTERACTION OF ACOUSTIC RADIATION WITH TURBULENCE PETER GOLDREICH AND PAWA... more ... Printed in USA THE INTERACTION OF ACOUSTIC RADIATION WITH TURBULENCE PETER GOLDREICH AND PAWAN KUMAR California Institute of Technology Received 1987 June 22; accepted 1987 August24 We derive expressions for the spectral emissivity and ...
The Astrophysical Journal, 1993
We show that low-order g-modes with large enough amplitudes to affect significantly the solar neu... more We show that low-order g-modes with large enough amplitudes to affect significantly the solar neutrino fluxes would produce surface velocities that are 10 4 times larger than the observed upper limits and hence are ruled out by existing data. We also demonstrate that any large-amplitude, short-period oscillations that grow on a Kelvin-Helmholtz time scale will require, to affect solar neutrino fluxes, a large amount of energy (for g-modes, 10 9 times the energy in the observed p− mode oscillations) and a tiny amount of dissipation (for g-modes, 10 −8 the dissipation rate of the p-modes).
The Astrophysical Journal, 1994
... Printed in USA EXCITATION OF SOLAR p-MODES PETER GOLDREICH AND NORMAN MURRAY California Insti... more ... Printed in USA EXCITATION OF SOLAR p-MODES PETER GOLDREICH AND NORMAN MURRAY California Institute ofTechnology, Pasadena, CA 91125 PAWAN KUMAR Massachusetts Institute ofTechnology, Cambridge, MA 02139 Received 1993 March 31 ; accepted 1993 ...
The Astrophysical Journal, 2000
We calculate the afterglow emission for Gamma-Ray Bursts (GRBs) going off in an extremely low den... more We calculate the afterglow emission for Gamma-Ray Bursts (GRBs) going off in an extremely low density medium, referred to as naked bursts. Our results also apply to the case where the external medium density falls off sharply at some distance from the burst. The observed afterglow flux in this case originates at high latitudes, i.e. where the angle between the fluid velocity and the observer line of sight is greater than Γ −1 . The observed peak frequency of the spectrum for naked bursts decreases with observer time as t −1 , and the flux at the peak of the spectrum falls off as t −2 . The 2-10 keV X-ray flux from a naked burst of average fluence should be observable by the SWIFT satellite for time duration of about 10 3 longer than the burst variability timescale. The high latitude emission contributes to the early X-ray afterglow flux for any GRB, not just naked bursts, and can be separated from the shocked inter-stellar medium (ISM) emission by their different spectral and temporal properties. Measurements of the high latitude emission could be used to map the angular structure of GRB producing shells.
Tetrahedron Letters, 2009
A short and high-yielding synthetic route to [3-15 N]-labeled uridine phosphoramidite 1 (26% over... more A short and high-yielding synthetic route to [3-15 N]-labeled uridine phosphoramidite 1 (26% overall yield from uridine) has been developed. This will enable automated synthesis of isotopically labeled RNA strands and facilitate their use in structural studies.