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P. Kumar

Publications and source records attributed to P. Kumar.

At least 91 records · Page 5Linked to original sources
arXiv2009

Controlled defects in ZnO by low energy Ar irradiation

We report interesting observations in 1.2 MeV Ar8+ ion irradiated ZnO which, to the best of our knowledge, have not been published earlier and will be useful for the scientific community engaged in research on ZnO. Irradiation with the initial fluence 1 X 10^15 ions/cm^2 changes the colour of the sample from white to orange while the highest irradiation fluence makes it dark reddish brown that appears as black. Such changes in colour can be correlated with the oxygen vacancy type defects. No significant change in the grain size of the irradiated samples, as revealed from the x-ray diffraction (XRD) line width broadening, has been observed. Increase of surface roughness due to sputtering is clearly visible in scanning electron micrographs (SEM) with highest fluence of irradiation. Room temperature Photoluminescence (PL) spectrum of the unirradiated sample shows intense ultra-violet (UV) emission (~ 3.27 eV) and less prominent defect level emissions (2-3 eV). The overall emission is largely quenched due to initial irradiation fluence. But with increasing fluence UV emission is enhanced along with prominent defect level emissions. Remarkably, the resistivity of the irradiated sample with highest fluence is reduced by four orders of magnitude compared to that of the unirradiated sample. This indicates increase of donor concentration as well as their mobility due to irradiation. Oxygen vacancies are deep donors in ZnO, but surely they influence the stability of the shallow donors (presumably zinc interstitial related) and vice versa. This is in conformity with recent theoretical calculations.

cond-mat.mtrl-sci↗
arXiv2009

Phase Transitions and Dynamics in Bulk and Interfacial Water

New experiments for water at the surface of proteins at very low temperature display intriguing dynamic behaviors. The extreme conditions of these experiments make it difficult to explore the wide range of thermodynamic state points needed to offer a suitable interpretation. Detailed simulations suffer the same problem, where equilibration times at low temperature become unreasonably long. We show how Monte Carlo simulations and mean field calculations of a tractable model of water help interpret the experimental results. Here we summarize the results for bulk water and investigate the thermodynamic and dynamic properties of supercooled water at an interface.

cond-mat.soft↗
arXiv2009

Multi-Wavelength View of Flare Events on November 20, 2003

We analyze two flare events which occurred in active region NOAA 501 on November 20, 2003. The H-alpha and magnetogram measurements show interaction between two filaments which produced a slowly rising flare event, corresponding to two stages of magnetic reconnection. The relative clockwise rotation between the two sunspot systems caused filament destabilization. The cusp-shaped magnetic field in the main phase of the second flare and its evolution in correlation with ribbon separation provide evidence for the cause of the CME eruption. The propagation and orientation of the CME with respect to the ecliptic plane is illustrated by IPS images.

astro-ph.SR↗
arXiv2009

Major Surge Activity of Super-Active Region NOAA 10484

We observed two surges in H-alpha from the super-active region NOAA 10484. The first surge was associated with an SF/C4.3 class flare. The second one was a major surge associated with a SF/C3.9 flare. This surge was also observed with SOHO/EIT in 195 angstrom and NoRh in 17 GHz, and showed similar evolution in these wavelengths. The major surge had an ejective funnel-shaped spray structure with fast expansion in linear (about 1.2 x 10^5 km) and angular (about 65 deg) size during its maximum phase. The mass motion of the surge was along open magnetic field lines, with average velocity about 100 km/s. The de-twisting motion of the surge reveals relaxation of sheared and twisted magnetic flux. The SOHO/MDI magnetograms reveal that the surges occurred at the site of companion sunspots where positive flux emerged, converged, and canceled against surrounding field of opposite polarity. Our observations support magnetic reconnection models for the surges and jets.

astro-ph.SR↗
arXiv2009

Adiabatic expansion, early x-ray data and the central engine in GRBs

The Swift satellite early x-ray data shows a very steep decay in most of the Gamma-Ray Bursts light curves. This decay is either produced by the rapidly declining continuation of the central engine activity or by some left-over radiation starting right after the central engine shuts off. The latter scenario consists of the emission from an "ember" that cools via adiabatic expansion and, if the jet angle is larger than the inverse of the source Lorentz factor, the large angle emission. In this work, we calculate the temporal and spectral properties of the emission from such a cooling ember, providing a new treatment for the micro-physics of the adiabatic expansion. We use the adiabatic invariance of p_{\perp}^2/B (p_{\perp} is the component of the electrons' momentum normal to the magnetic field, B) to calculate the electrons' Lorentz factor during the adiabatic expansion; the electron momentum becomes more and more aligned with the local magnetic field as the expansion develops. We compare the theoretical expectations of the adiabatic expansion (and the large angle emission) with the current observations of the early x-ray data and find that only about 20% of our sample of 107 bursts is potentially consistent with this model. This leads us to believe that, for most bursts, the central engine does not turn off completely during the steep decay of the x-ray light curve; therefore, this phase is produced by the continued rapidly declining activity of the central engine.

astro-ph↗
arXiv2008

What did we learn from gamma-ray burst 080319B ?

The optical and gamma-ray observations of GRB 080319B allow us to determine a fairly complete physical picture for this remarkable burst. The data indicate that the prompt optical and gamma-ray photons were produced at the same location but by different radiation processes: synchrotron and synchrotron self-Compton, respectively. The burst emission was produced at a distance of 10^{16.5} cm from the center of explosion by an ultra-relativistic source moving at Lorentz factor of ~500. A straightforward inference is that about 10 times more energy must have been radiated at tens of GeV than that released at ~1 MeV. Assuming that the GRB outflow was baryonic and that the gamma-ray source was shock-heated plasma, the collimation-corrected kinetic energy of the jet powering GRB 080319B was larger than 10^{52.3} erg. The decay of the early afterglow optical emission (up to 1 ks) is too fast to be attributed to the reverse shock crossing the GRB ejecta but is consistent with the expectations for the "large-angle emission" released during the burst. The pure power-law decay of the optical afterglow flux from 1 ks to 10 day is most naturally identified with the (synchrotron) emission from the shock propagating into a wind-like medium. However, the X-ray afterglow requires a departure from the standard blast-wave model.

astro-ph↗
arXiv2008

Quantum algorithm design using dynamic learning

We present a dynamic learning paradigm for "programming" a general quantum computer. A learning algorithm is used to find the control parameters for a coupled qubit system, such that the system at an initial time evolves to a state in which a given measurement corresponds to the desired operation. This can be thought of as a quantum neural network. We first apply the method to a system of two coupled superconducting quantum interference devices (SQUIDs), and demonstrate learning of both the classical gates XOR and XNOR. Training of the phase produces a gate congruent to the CNOT modulo a phase shift. Striking out for somewhat more interesting territory, we attempt learning of an entanglement witness for a two qubit system. Simulation shows a reasonably successful mapping of the entanglement at the initial time onto the correlation function at the final time for both pure and mixed states. For pure states this mapping requires knowledge of the phase relation between the two parts; however, given that knowledge, this method can be used to measure the entanglement of an otherwise unknown state. The method is easily extended to multiple qubits or to quNits.

quant-ph↗
arXiv2008

Scattered Emission from A Relativistic Outflow and Its Application to Gamma-Ray Bursts

We investigate a scenario of photons scattering by electrons within a relativistic outflow. The outflow is composed of discrete shells with different speeds. One shell emits radiation for a short duration. Some of this radiation is scattered by the shell(s) behind. We calculate in a simple two-shell model the observed scattered flux density as a function of the observed primary flux density, the normalized arrival time delay between the two emission components, the Lorentz factor ratio of the two shells and the scattering shell's optical depth. Thomson scattering in a cold shell and inverse Compton scattering in a hot shell are both considered. The results of our calculations are applied to the Gamma-Ray Bursts and the afterglows. We find that the scattered flux from a cold slower shell is small and likely to be detected only for those bursts with very weak afterglows. A hot scattering shell could give rise to a scattered emission as bright as the X-ray shallow decay component detected in many bursts, on a condition that the isotropically equivalent total energy carried by the hot electrons is large, $\sim 10^{52-56}$ erg. The scattered emission from a faster shell could appear as a late short $γ$-ray/MeV flash or become part of the prompt emission depending on the delay of the ejection of the shell.

astro-ph↗
arXiv2007

On the nature of two superconducting transitions in the specific heat of PrOs$_4$Sb$_{12}$: Effects of crystal grinding

Specific heat, dc- and ac-magnetic susceptibility are reported for a large single crystal of PrOs$_4$Sb$_{12}$ and, after grinding, its powder. The room temperature effective paramagnetic moment of the crystal was consistent with the Pr$^{3+}$ ionic configuration and full occupancy of the Pr-sublattice. The crystal showed two distinct anomalies in the specific heat and an overall discontinuity in $C/T$ of more than 1000 mJ/K$^2$mol. The upper transition (at $T_{c1}$) was rounded, in an agreement with previous reports. The anomaly at $T_{c2}$ was very sharp, consistent with a good quality of the crystal. We observed a shoulder in $χ$' and two peaks in $χ$'' below $T_{c1}$. However, there were no signatures in $χ$' of the lower temperature transition. PrOs$_4$Sb$_{12}$ is extremely sensitive to grinding, which suppresses the upper superconducting transition in both the specific heat and magnetic susceptibility. $ΔC/T_{c}$ was reduced to 140 mJ/K$^2$ mol in the powdered sample. Existing data on ground, polished, and sliced crystals suggests the existence of a length scale of order 100 $μ$, characterizing the higher temperature superconducting phase.

cond-mat.supr-con↗
arXiv2007

A Spectacular Radio Flare from XRF 050416a at 40 days and Implications for the Nature of X-ray Flashes

We present detailed optical, near-infrared, and radio observations of the X-ray flash 050416a obtained with Palomar and Siding Springs Observatories as well as HST and the VLA, placing this event among the best-studied X-ray flashes to date. In addition, we present an optical spectrum from Keck LRIS from which we measure the redshift of the burst, z=0.6528. At this redshift the isotropic-equivalent prompt energy release was about 10^51 erg, and using a standard afterglow synchrotron model we find that the blastwave kinetic energy is a factor of 10 larger, E_K,iso ~ 10^52 erg. The lack of an observed jet break to t ~ 20 days indicates that the opening angle is larger than 7 deg and the total beaming-corrected relativistic energy is larger than 10^50 erg. We further show that the burst produced a strong radio flare at t ~ 40 days accompanied by an observed flattening in the X-ray band which we attribute to an abrupt circumburst density jump or an episode of energy injection (either from a refreshed shock or off-axis ejecta). Late-time observations with HST show evidence for an associated supernova with peak optical luminosity roughly comparable to that of SN 1998bw. Next, we show that the host galaxy of XRF 050416a is actively forming stars at a rate of at least 2 M_solar per year with a luminosity of L_B ~ 0.5L* and metallicity of Z ~ 0.2-0.8 Z_solar. Finally, we discuss the nature of XRF 050416a in the context of short-hard gamma-ray bursts and under the framework of off-axis and dirty fireball models for X-ray flashes.

astro-ph↗
arXiv2007

The nature of the outflow in gamma-ray bursts

The Swift satellite has enabled us to follow the evolution of gamma-ray burst (GRB) fireballs from the prompt gamma-ray emission to the afterglow phase. The early x-ray and optical data obtained by telescopes aboard the Swift satellite show that the source for prompt gamma-ray emission, the emission that heralds these bursts, is short lived and that its source is distinct from that of the ensuing, long-lived afterglow. Using these data, we determine the distance of the gamma-ray source from the center of the explosion. We find this distance to be 1e15-1e16 cm for most bursts and we show that this is within a factor of ten of the radius of the shock-heated circumstellar medium (CSM) producing the x-ray photons. Furthermore, using the early gamma-ray, x-ray and optical data, we show that the prompt gamma-ray emission cannot be produced in internal shocks, nor can it be produced in the external shock; in a more general sense gamma-ray generation mechanisms based on shock physics have problems explaining the GRB data for the ten Swift bursts analyzed in this work. A magnetic field dominated outflow model for GRBs has some attractive features, although the evidence in its favor is inconclusive. Finally, the x-ray and optical data allow us to provide an upper limit on the density of the CSM of about 10 protons per cubic cm at a distance of about 5e16 cm from the center of explosion.

astro-ph↗
arXiv2006

A unified picture for the gamma-ray and prompt optical emissions of GRB 990123

The prompt optical emission of GRB 990123 was uncorrelated to the gamma-ray light-curve and exhibited temporal properties similar to those of the steeply-decaying, early X-ray emission observed by Swift at the end of many bursts. These facts suggest that the optical counterpart of GRB 990123 was the large-angle emission released during (the second pulse of) the burst. If the optical and gamma-ray emissions of GRB 990123 have, indeed, the same origin then their properties require that (i) the optical counterpart was synchrotron emission and the gamma-rays arose from inverse-Compton scatterings ("synchrotron self-Compton model"), (ii) the peak-energy of the optical-synchrotron component was at ~20 eV, and (iii) the burst emission was produced by a relativistic outflow moving at Lorentz factor > 450 and at a radius > 10^{15} cm, which is comparable to the outflow deceleration radius. Because the spectrum of GRB 990123 was optically thin above 2 keV, the magnetic field behind the shock must have decayed on a length-scale of <1% of the thickness of the shocked gas, which corresponds to 10^6-10^7 plasma skin-depths. Consistency of the optical counterpart decay rate and its spectral slope (or that of the burst, if they represent different spectral components) with the expectations for the large-angle burst emission represents the most direct test of the unifying picture proposed here for GRB 990123.

astro-ph↗
arXiv2006

Glass transition in biomolecules and the liquid-liquid critical point of water

Using molecular dynamics simulations, we investigate the relation between the dynamic transitions of biomolecules (lysozyme and DNA) and the dynamic and thermodynamic properties of hydration water. We find that the dynamic transition of the macromolecules, sometimes called a ``protein glass transition'', occurs at the temperature of dynamic crossover in the diffusivity of hydration water, and also coincides with the maxima of the isobaric specific heat $C_P$ and the temperature derivative of the orientational order parameter. We relate these findings to the hypothesis of a liquid-liquid critical point in water. Our simulations are consistent with the possibility that the protein glass transition results from crossing the Widom line, which is defined as the locus of correlation length maxima emanating from the hypothesized second critical point of water.

cond-mat.soft↗
arXiv2006

Evolution of superconductivity in Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$: Upper critical field measurements

Measurements of the upper critical field $H_{c2}$ near $T_c$ of Pr$_{1- x}$La$_{x}$Os$_{4}$Sb$_{12}$ were performed by specific heat. A positive curvature in $H_{c2}$ versus $T$ was observed in samples and concentrations exhibiting two superconducting transitions. These results argue against this curvature being due to two-band superconductivity. The critical field slope - $dH_{c2}/dT$ suggests the existence of a crossover concentration $x_{cr} \approx 0.25$, below which there is a rapid suppression of effective electron mass with La-alloying. This crossover concentration was previously detected in the measurement of the discontinuity of $C/T$ at $T_c$.

cond-mat.supr-con↗
arXiv2006

No Universality for Electron's Power-Law Index (p) in Gamma-Ray Bursts and Other Relativistic Sources

The Gamma-Ray Burst (GRB) prompt emission is believed to be from highly relativistic electrons accelerated in relativistic shocks. From the GRB high-energy power-law spectral indices $β$ observed by the Burst and Transient Source Experiment (BATSE) Large Area Detectors (LAD), we determine the spectral index, $p$, of the electrons' energy distribution. Both the theoretical calculations and numerical simulations of the particle acceleration in relativistic shocks show that $p$ has a universal value $\approx 2.2-2.3$. We show that the observed distribution of $p$ during GRBs is not consistent with a $δ$-function distribution or an universal $p$ value, with the width of the distribution $\ge$ 0.54. The distributions of $p$ during X-ray afterglows are also investigated and found to be inconsistent with a $δ$-function distribution. The $p$-distributions in blazars and pulsar wind nebulae are also broad, inconsistent with a $δ$-function distribution.

astro-ph↗
arXiv2006

Light element synthesis in baryon isocurvature models

The prejudice against baryon isocurvature models is primarily because of their inconsistency with early universe light element nucleosynthesis results. We propose that incipient low metallicity (Pop II) star forming regions can be expected to have environments conducive to Deuterium production by spallation, up to levels observed in the universe.

astro-ph↗
arXiv2006

A unified picture for gamma-ray burst prompt and X-ray afterglow emissions

Data from the Swift satellite has enabled us for the first time to provide a complete picture of the gamma-ray burst emission mechanism and its relationship with the early afterglow emissions. We show that gamma-ray photons for two bursts, 050126 & 050219A, for which we have carried out detailed analysis were produced as a result of the synchrotron self-Compton process in the material ejected in the explosion when it was heated to a mildly relativistic temperature at a distance from the center of explosion of order the deceleration radius. Both of these bursts exhibit rapidly declining early X-ray afterglow lightcurves; this emission is from the same source that produced the gamma-ray burst. The technique we exploit to determine this is very general and makes no assumption about any particular model for gamma-ray generation except that the basic radiation mechanism is some combination of synchrotron and inverse-Compton processes in a relativistic outflow. For GRB 050219A we can rule out the possibility that energy from the explosion is carried outward by magnetic fields, and that the dissipation of this field produced the $γ$-ray burst.

astro-ph↗