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Abstract: We have studied the magnetic and electronic properties of (Ga,Mn)As ferromagnetic semiconductor layers containing low concentrations of room temperature magnetic MnAs precipitates, using x-ray magnetic circular dichroism in x-ray absorption spectroscopy. Measurements below and above the Curie temperature of the (Ga,Mn)As show localized multiplet structures and broad metallic-like line shapes, respectively. By applying magnetic fields below the coercivity of the (Ga,Mn)As layer, the magnetization reversal behavior of the ferromagnetic semiconductor and the MnAs precipitates can be resolved and are found to be independent of each other.

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Abstract: [1] Paleoproterozioc (ca. 1.9 Ga) stromatolites from the Biwabik Iron-Formation (Minnesota, United States) contain fossil forms which in the literature have been attributed to ancient microbes including iron-oxidising bacteria and cyanobacteria. To search for valence state fossils, we measured Fe3+/?Fe (?Fe = Fe2+ + Fe3+) transverse to the laminae of a Biwabik, probably biogenic stromatolite and compared the results with a scan across the enclosing, likely abiotic sedimentary rock (or interstromatolite fill). To obtain Fe3+/?Fe and information about the site symmetry and crystal field strength, we used L3 (2p3/2 ? 3d) soft X-ray absorption spectroscopy and interpreted the measurements using our calculated spectra. We found that Fe3+/?Fe is approximately constant at 0.3 within the stromatolite, increases steeply to around 0.6 in the region of the stromatolite-fill boundary, and then reduces significantly in the interstromatolite fill. The crystal field strength for octahedral Fe3+ increased and became more irregular as the scan moved from the stromatolite into the fill. Our results tentatively suggest that a combination of ancient biological and later diagenetic processes can produce steep gradients in Fe3+/?Fe and alterations in the crystal field strength across a probable abiotic-biotic boundary. To describe the L3 absorption spectra in the stromatolite-fill region, it was necessary to include a significant Fe3+ tetrahedral component, which suggests the presence of magnetite. At other positions, fits using only octahedral Fe2+ and Fe3+ theoretical curves satisfactorily reproduce the spectra. The search for a possible new type of spatial biosignature in the valence state record for ancient stromatolites might help differentiate between the biogenic stromatolites and abiogenic stromatolite-like structures and could even be relevant to the search for fossil evidence of life in extraterrestrial rocks.

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Abstract: Different mechanisms of formation of purely resonant reflections hhl (h = 2n + 1) during resonant diffraction of X-ray synchrotron radiation in zinc oxide, whose intensity very strongly changes with an increase in temperature, are considered. Such reflections, forbidden in the dipole-dipole scattering, can be excited owing to the dipole-quadrupole scattering and the anisotropy of X-ray susceptibility induced by thermal motion of atoms and point defects. Different contributions are theoretically considered and it is shown that they can be separated owing to their different temperature dependences.

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Abstract: By using absorption and fluorescence spectroscopy, we compared the effects of heat and light treatments on the phycobilisome (PBS) antenna of Synechocystis sp. PCC 6803 cells. Fluorescence emission spectra obtained upon exciting predominantly PBS, recorded at 25 °C and 77 K, revealed characteristic changes upon heat treatment of the cells. A 5-min incubation at 50 °C, which completely inactivated the activity of photosystem II, led to a small but statistically significant decrease in the F680/F655 fluorescence intensity ratio. In contrast, heat treatment at 60 °C resulted in a much larger decrease in the same ratio and was accompanied by a blue-shift of the main PBS emission band at around 655 nm (F655), indicating an energetic decoupling of PBS from chlorophylls and reorganizations in its internal structure. (Upon exciting PBS, F680 originates from photosystem II and from the terminal emitter of PBS.). Very similar changes were obtained upon exposing the cells to high light (600–7500 μmol photons m−2 s−1) for different time periods (10 min to 3 h). In cells with heat-inactivated photosystem II, the variations caused by light treatment could clearly be assigned to a similar energetic decoupling of the PBS from the membrane and internal reorganizations as induced at around 60 °C. These data can be explained within the frameworks of thermo-optic mechanism [Cseh et al. 2000, Biochemistry 39, 15250]: in high light the heat packages originating from dissipation might lead to elementary structural changes in the close vicinity of dissipation in heat-sensitive structural elements, e.g. around the site where PBS is anchored to the membrane. This, in turn, brings about a diminishment in the energy supply from PBS to the photosystems and reorganization in the molecular architecture of PBS.