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Sökning: WFRF:(Gupta V)

  • Resultat 811-820 av 1000
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811.
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812.
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813.
  • Guo, J. H., et al. (författare)
  • X-ray spectroscopic study of the charge state and local ordering of room-temperature ferromagnetic Mn-doped ZnO
  • 2007
  • Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:17
  • Tidskriftsartikel (refereegranskat)abstract
    • The charge state and local ordering of Mn doped into a pulsed laser deposited single-phase thin film of ZnO are investigated by using x-ray absorption spectroscopy at the O K-edge, Mn K-edge and L-edge, and x-ray emission spectroscopy at the O K-edge and Mn L-edge. This film is ferromagnetic at room temperature. EXAFS measurement shows that Mn2+ replaces the Zn site in tetrahedral symmetry, and there is no evidence for either metallic Mn or MnO in the film. Upon Mn doping, the top of O 2p valence band extends into the bandgap, indicating additional charge carriers being created.
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814.
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815.
  • Gupta, Alok C., et al. (författare)
  • Long-term Multiband Near-infrared Variability of the Blazar OJ 287 during 2007-2021
  • 2022
  • Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 260:2, s. 39-
  • Tidskriftsartikel (refereegranskat)abstract
    • We present the most extensive and well-sampled long-term multiband near-infrared (NIR) temporal and spectral variability study of OJ 287, considered to be the best candidate binary supermassive black hole blazar. These observations were made between 2007 December and 2021 November. The source underwent similar to 2-2.5 mag variations in the J, H, and Ks NIR bands. Over these long-term timescales there were no systematic trends in either flux or spectral evolution with time or with the source's flux states. However, on shorter timescales, there are significant variations in flux and spectra indicative of strong changes during different activity states. The NIR spectral energy distributions show diverse facets at each flux state, from the lowest to the highest. The spectra are, in general, consistent with a power-law spectral profile (within 10%) and many of them indicate minor changes (observationally insignificant) in the shift of the peak. The NIR spectra generally steepen during bright phases. We briefly discuss these behaviors in the context of blazar emission scenarios/mechanisms, OJ 287's well-known traditional behavior, and implications for models of the source central engine invoked for its long-term optical semiperiodic variations.
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816.
  • Gupta, Alok C., et al. (författare)
  • Multiband optical variability of the blazar OJ 287 during its outbursts in 2015-2016
  • 2017
  • Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 465:4, s. 4423-4433
  • Tidskriftsartikel (refereegranskat)abstract
    • We present recent optical photometric observations of the blazar OJ 287 taken during 2015 September-2016 May. Our intense observations of the blazar started in 2015 November and continued until 2016 May and included detection of the large optical outburst in 2015 December that was predicted using the binary black hole model for OJ 287. For our observing campaign, we used a total of nine ground-based optical telescopes of which one is in Japan, one is in India, three are in Bulgaria, one is in Serbia, one is in Georgia, and two are in the USA. These observations were carried out in 102 nights with a total of similar to 1000 image frames in BVRI bands, though the majority were in the R band. We detected a second comparably strong flare in 2016 March. In addition, we investigated multiband flux variations, colour variations, and spectral changes in the blazar on diverse time-scales as they are useful in understanding the emission mechanisms. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux, colour, and spectral variability.
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817.
  • Gupta, A., et al. (författare)
  • High-temperature ferromagnetism in Cu-doped GaP by SQUID magnetometry and ferromagnetic resonance measurements
  • 2006
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:22
  • Tidskriftsartikel (refereegranskat)abstract
    • The search for ferromagnetism above room temperature in semiconductors doped with paramagnetic ions has intensified in recent years because of the potential of combining magnetic information storage and electronic switching in one spintronic device. Here we report an observation of ferromagnetism well above room temperature in gallium phosphide doped with Cu2+ detected by ferromagnetic resonance and SQUID magnetometry. Other important features of the results in this p-type Cu-doped GaP are the high Curie temperature above 700 K significantly higher than previous observations, the relatively simple low-temperature bulk sintering process used to synthesize the material, which will significantly reduce the cost of large-scale production, and the use of copper as the dopant rather than manganese, which precludes ferromagnetic clusters or magnetic alloy impurities as the origin of the ferromagnetism. Ab initio calculations also show the existence of ferromagnetism in Cu-doped GaP. When the spin-orbit coupling is included, the total moment is enhanced and we get a total magnetic moment of 0.31 mu(B) with a local moment on Cu 0.082 and on P 0.204 mu(B).
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818.
  • Gupta, Amita, et al. (författare)
  • Room temperature ferromagnetism in transition metal (V, Cr, Ti) doped In(2)O(3)
  • 2007
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:9, s. 09N513-
  • Tidskriftsartikel (refereegranskat)abstract
    • Indium oxide is chosen as the host material for doping Ti, V, and Cr transition metal ions. Theoretical calculations based on density functional theory within a local spin density approximation show that V-V separation of 5.6 A is more stable with a strong ferromagnetic coupling. Our calculations clearly predict that substitution of vanadium for indium should yield ferromagnetism in In(2)O(3). Experimentally, (In(0.95)TM(0.05))O(3) (TM=Ti,V,Cr) were prepared using sol-gel as well as solid state reaction methods. Superconducting quantum interference device magnetization measurements as a function of field and temperature clearly showed that the V and Cr doped samples are ferromagnetic with Curie temperature well above room temperature. Thin films deposited by pulsed laser ablation using these materials on sapphire substrates exhibit a preferred 222 orientation normal to the plane of the film. The magnetic moment for (In(0.95)V(0.05))O(3) film deposited in 0.1 mbar oxygen pressure was estimated to be 1.7 mu(B)/V and is comparable to the theoretical value of 2 mu(B)/V.
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819.
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820.
  • Gupta, H., et al. (författare)
  • Detection of OH+ and H2O+ towards Orion KL
  • 2010
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521, s. L47-
  • Tidskriftsartikel (refereegranskat)abstract
    • We report observations of the reactive molecular ions OH+, H2O+, and H3O+ towards Orion KL with Herschel/HIFI. All three N = 1-0 fine-structure transitions of OH+ at 909, 971, and 1033 GHz and both fine-structure components of the doublet ortho-H2O+ 111-000 transition at 1115 and 1139 GHz were detected; an upper limit was obtained for H3O+. OH+ and H2O+ are observed purely in absorption, showing a narrow component at the source velocity of 9 km s-1, and a broad blueshifted absorption similar to that reported recently for HF and para-H218O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH+ and H2O+ for the 9 km s-1 component of 9 ± 3 × 1012 cm-2 and 7 ± 2 × 1012 cm-2, and those in the outflow of 1.9 ± 0.7 × 1013 cm-2 and 1.0 ± 0.3 × 1013 cm-2. Upper limits of 2.4 × 1012 cm-2 and 8.7 × 1012 cm-2 were derived for the column densities of ortho and para-H3O+ from transitions near 985 and 1657 GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate.
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