227 research outputs found
Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering
We report the direct observation of slow fluctuations of helical
antiferromagnetic domains in an ultra-thin holmium film using coherent resonant
magnetic x-ray scattering. We observe a gradual increase of the fluctuations in
the speckle pattern with increasing temperature, while at the same time a
static contribution to the speckle pattern remains. This finding indicates that
domain-wall fluctuations occur over a large range of time scales. We ascribe
this non-ergodic behavior to the strong dependence of the fluctuation rate on
the local thickness of the film.Comment: to appear in Phys. Rev. Let
Revealing natural relationships among arbuscular mycorrhizal fungi: culture line BEG47 represents Diversispora epigaea, not Glomus versiforme
Background: Understanding the mechanisms underlying biological phenomena, such as evolutionarily conservative trait inheritance, is predicated on knowledge of the natural relationships among organisms. However, despite their enormous ecological significance, many of the ubiquitous soil inhabiting and plant symbiotic arbuscular mycorrhizal fungi (AMF, phylum Glomeromycota) are incorrectly classified.
Methodology/Principal Findings:
Here, we focused on a frequently used model AMF registered as culture BEG47. This fungus is a descendent of the ex-type culture-lineage of Glomus epigaeum, which in 1983 was synonymised with Glomus versiforme. It has since then been used as ‘G. versiforme BEG47’. We show by morphological comparisons, based on type material, collected 1860–61, of G. versiforme and on type material and living ex-type cultures of G. epigaeum, that these two AMF species cannot be conspecific, and by molecular phylogenetics that BEG47 is a member of the genus Diversispora.
Conclusions: This study highlights that experimental works published during the last >25 years on an AMF named ‘G. versiforme’ or ‘BEG47’ refer to D. epigaea, a species that is actually evolutionarily separated by hundreds of millions of years from all members of the genera in the Glomerales and thus from most other commonly used AMF ‘laboratory strains’. Detailed redescriptions substantiate the renaming of G. epigaeum (BEG47) as D. epigaea, positioning it systematically in the order Diversisporales, thus enabling an evolutionary understanding of genetical, physiological, and ecological traits, relative to those of other AMF. Diversispora epigaea is widely cultured as a laboratory strain of AMF, whereas G. versiforme appears not to have been cultured nor found in the field since its original description
Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction
Strong resonant enhancements of the charge-order and spin-order
superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when
x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The
pronounced photon-energy and polarization dependences of these diffraction
intensities allow for a critical determination of the local symmetry of the
ordered spin and charge carriers. We found that not only the antiferromagnetic
order but also the charge-order superstructure resides within the NiO2 layers;
the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with
the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the
cuprates.Comment: 4 pages, 3 figure
Aligning the design of intermediary organisations with the ecosystem
Intermediary organisations such as technology transfer organisations (TTOs) are an important mechanism of open ecosystem governance, as they support how ecosystem participants search for knowledge. While scholars have identified TTO activities to support knowledge search, little is known about how these activities relate to the struc-tural dimensions of TTOs or ecosystem-level factors. We propose that ecosystem search scope and problem complexity are key ecosystem- level factors that influence how TTOs support knowledge search. We further argue that coupling, specialisation, centralisation, and forma-lisation are the key structural dimensions of TTOs. We combine these arguments to develop TTO designs that detail the interplay of the structural dimensions and activities of a TTO given varying ecosys-tem-level factors. Our paper contributes to research on the open governance of ecosystems, ecosystem structures, and the ecosystem structure–intermediary organisations relation
Direct observation of t2g orbital ordering in magnetite
Using soft-x-ray diffraction at the site-specific resonances in the Fe L23
edge, we find clear evidence for orbital and charge ordering in magnetite below
the Verwey transition. The spectra show directly that the (001/2) diffraction
peak (in cubic notation) is caused by t2g orbital ordering at octahedral Fe2+
sites and the (001) by a spatial modulation of the t2g occupation.Comment: to appear in Phys. Rev. Let
Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation
We have succeeded in growing epitaxial and highly stoichiometric films of EuO
on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation
process during the molecular beam epitaxy assisted growth enables the
consistent achievement of stoichiometry. We have also succeeded in growing the
films in a layer-by-layer fashion by fine tuning the Eu vs. oxygen deposition
rates. The initial stages of growth involve the limited supply of oxygen from
the YSZ substrate, but the EuO stoichiometry can still be well maintained. The
films grown were sufficiently smooth so that the capping with a thin layer of
aluminum was leak tight and enabled ex situ experiments free from trivalent Eu
species. The findings were used to obtain recipes for better epitaxial growth
of EuO on MgO (001).Comment: 10 pages, 15 figure
Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)
We have succeeded in preparing high-quality Gd-doped single-crystalline EuO
films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic
variation in the Gd and oxygen deposition rates, we have been able to observe
sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia
(001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We
used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm
the absence of Eu3+ contaminants and to determine the actual Gd concentration.
The distillation process ensures the absence of oxygen vacancies in the films.
From magnetization measurements we found the Curie temperature to increase
smoothly as a function of doping from 70 K up to a maximum of 125 K. A
threshold behavior was not observed for concentrations as low as 0.2%.Comment: 8 pages, 9 figure
Determination of the orbital moment and crystal field splitting in LaTiO
Utilizing a sum-rule in a spin-resolved photoelectron spectroscopic
experiment with circularly polarized light, we show that the orbital moment in
LaTiO is strongly reduced both below and above the N\'{e}el temperature.
Using Ti x-ray absorption spectroscopy as a local probe, we found
that the crystal field splitting in the subshell is about 0.12-0.30
eV. This large splitting does not facilitate the formation of an orbital
liquid
Analysis of charge and orbital order in Fe_{3}O_{4} by Fe L_{2,3} resonant x-ray diffraction
To elucidate charge and orbital order below the Verwey transition temperature
TV∼125 K, a thin layer of magnetite partially detwined by growth on the
stepped MgO(001) substrate has been studied by means of soft x-ray diffraction
at the Fe L2,3 resonance. The azimuth angle, incident photon polarization, and
energy dependence of the (0012)c and (001)c reflection intensities have been
measured, and analyzed using a configuration-interaction FeO6 cluster model.
The azimuth dependence of the (0012)c reflection intensities directly
represents the space-group symmetry of the orbital order in the initial state
rather than indirectly through the intermediate-state level shifts caused by
the order-induced lattice distortions. From the analysis of the (0012)c
reflection intensities, the orbital order in the t2g orbitals of B sites below
TV is proved to have a large monoclinic deformation with the value of
Re[Fxy]/Re[Fyz]∼2. This finding contradicts the majority of theories on the
Verwey transition so far proposed. We show that the experimentally observed
resonance spectra cannot be explained by orbital and charge orders obtained
with recent LDA+U and GGA+U band structure calculations but by a complex-
number orbital order with excellent agreement
- …