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| 2. |
- Villforth, C., et al.
(författare)
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Variability and stability in blazar jets on time-scales of years : optical polarization monitoring of OJ 287 in 2005-2009
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 402:3, s. 2087-2111
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Tidskriftsartikel (refereegranskat)abstract
- OJ 287 is a BL Lac object at redshift z = 0.306 that has shown double-peaked bursts at regular intervals of similar to 12 yr during the last similar to 40 yr. We analyse optical photopolarimetric monitoring data from 2005 to 2009, during which the latest double-peaked outburst occurred. The aim of this study is twofold: firstly, we aim to analyse variability patterns and statistical properties of the optical polarization light curve. We find a strong preferred position angle in optical polarization. The preferred position angle can be explained by separating the jet emission into two components: an optical polarization core and chaotic jet emission. The optical polarization core is stable on time-scales of years and can be explained as emission from an underlying quiescent jet component. The chaotic jet emission sometimes exhibits a circular movement in the Stokes plane. We find six such events, all on the time-scales of 10-20 d. We interpret these events as a shock front moving forwards and backwards in the jet, swiping through a helical magnetic field. Secondly, we use our data to assess different binary black hole models proposed to explain the regularly appearing double-peaked bursts in OJ 287. We compose a list of requirements a model has to fulfil to explain the mysterious behaviour observed in OJ 287. The list includes not only characteristics of the light curve but also other properties of OJ 287, such as the black hole mass and restrictions on accretion flow properties. We rate all existing models using this list and conclude that none of the models is able to explain all observations. We discuss possible new explanations and propose a new approach to understanding OJ 287. We suggest that both the double-peaked bursts and the evolution of the optical polarization position angle could be explained as a sign of resonant accretion of magnetic field lines, a 'magnetic breathing' of the disc.
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| 3. |
- Plucinski, L, et al.
(författare)
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Resonant shake-up satellites in photoemission at the Ga 3p photothreshold in GaN
- 2005
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Ingår i: Solid State Communications. - : Elsevier BV. - 1879-2766 .- 0038-1098. ; 136:4, s. 191-195
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Tidskriftsartikel (refereegranskat)abstract
- Photoemission spectra recorded near the Ga 3p photothreshold from GaN have been found to contain satellites of the main Ga 3d emission line. The intensity of these satellites resonate at this threshold, and are associated with a 3d(8) state. The correlation energies and binding energies for the satellite multiplet have been measured for the satellite and related Auger transitions. The satellite multiplet contains additional constant binding energy features not observed in previous studies of other Ga compounds. The present results are compared with those for Gal? and GaAs.
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| 4. |
- Godlewski, M, et al.
(författare)
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Compensation mechanisms in magnesium doped GaN
- 2004
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Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:2, s. 216-220
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Tidskriftsartikel (refereegranskat)abstract
- Compensation processes in magnesium doped GaN epilayers and bulk samples are studied. We demonstrate enhancement of potential fluctuations in Mg doped samples, from Kelvin probe atomic force microscopy measurements. Large- and small-scale light emission fluctuations are also demonstrated. Micro-photoluminescence (PL) study indicates an unusual anti-correlation between the intensities of excitonic and defect-related emission processes in p-type doped structures and also the presence of the so-called hot-PL. Hot-PL observed in compensated p-type samples, we relate to the presence of strong potential fluctuations. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 5. |
- Godlewski, M., et al.
(författare)
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Mechanism of radiative recombination in acceptor-doped bulk GaN crystals
- 1999
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Ingår i: 20th International Conference on Defects in Semiconductors ICDS-20,1999. - Physica B, Vol. 273-274 : Elsevier. ; , s. 39-
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Konferensbidrag (refereegranskat)abstract
- Optical and electrical properties of acceptor-doped bulk GaN crystals are discussed. Though introducing Zn and Ca to bulk GaN does not significantly change electron concentration, it results in the appearance of a blue photoluminescence band accompanying the relatively strong yellow band usually present. Highly resistive GaN : Mg crystals are obtained when high amount of Mg is introduced to the Ga melt during high-pressure synthesis. Change of electrical properties of Mg-doped bulk crystals is accompanied by the appearance of a strong blue emission of GaN similar to that in Ca- and Zn-doped crystals. Optically detected magnetic resonance investigations indicate a multi-band character of this blue emission and suggest possible mechanism of compensation in acceptor-doped bulk GaN.
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| 6. |
- Godlewski, M., et al.
(författare)
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Photoluminescence mechanisms in undoped and in Mg doped bulk GaN
- 1999
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Ingår i: 24th International Conference on the Physics of Semiconductors,1998. - Proc. of the 24th International Conference on the Physics of Semiconductors, ed. by D. Gershoni : World Scientific, Singapore. ; , s. IX B17-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 7. |
- Kowalik, I. A., et al.
(författare)
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MnAs dots grown on GaN( 000(1)over-bar)-(1x1) surface
- 2007
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 75:23, s. 11-
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Tidskriftsartikel (refereegranskat)abstract
- MnAs has been grown by means of MBE on the GaN(000 (1) over bar)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1x10(11) cm(-2) and 2.5x10(11) cm(-2), respectively (as observed by atomic force microscopy), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d-related contribution to the total density of states distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both systems behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intradot Curie temperatures substantially different. The intradot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure, and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.
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| 8. |
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| 9. |
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| 10. |
- Kowalski, BJ, et al.
(författare)
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MnAs overlayer on GaN(000(1)under-bar)-(1 x 1) its growth, morphology and electronic structure
- 2004
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Ingår i: Acta Physica Polonica. Series A: General Physics, Physics of Condensed Matter, Optics and Quantum Electronics, Atomic and Molecular Physics, Applied Physics. - 0587-4246. ; 105:6, s. 645-650
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Tidskriftsartikel (refereegranskat)abstract
- MnAs layer has been grown by means of MBE on the GaN(000 (1) under bar)-(1 x 1) surface. Spontaneous formation of MnAs grains with a diameter of 30-60 nm (as observed by atomic force microscopy) occurred for the layer thickness bigger than 7 ML. Ferromagnetic properties of the layer with Curie temperature higher than 330 K were detected by SQUID measurements. Electronic structure of the system was investigated in situ by resonant photoemission spectroscopy for MnAs layer thickness of 1, 2, and 8 ML. Density of the valence band states of MnAs and its changes due to the increase in the layer thickness were revealed.
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| 11. |
- Kowalski, BJ, et al.
(författare)
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Surface states on GaN(000(1)over-bar)(1x1) - an angle-resolved photoemission study
- 2002
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Ingår i: Surface Science. - 0039-6028. ; 507, s. 186-191
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of angle-resolved photoemission studies on the (0 0 0 (1) over bar) (N-polar) surface of bulk gallium nitride crystals. The bulk and surface band Structure was investigated along Gamma-A and Gamma-K-M directions on the surface with (1 x 1) symmetry. The sets of spectra were obtained in normal and off-normal emission for the surface prepared by Ar+ ion sputtering and annealing and for the same surface enriched with Ga under molecular beam epitaxy conditions. The bulk and surface related features were identified by analysis of the acquired experimental data and by comparison with the results of band structure Calculations.
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