235 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
Adapting the control of the magnetic bearings of a highly flexible and gyroscopic rotor to the excitations by the motor
A test rig was built to perform fatigue tests on thick-walled cylinders made of fibre reinforced plastic (FRP). During the fatigue test, the rotational speed of an FRP cylinder is periodically varied until it fails. The FRP cylinder is connected to a drive spindle that accelerates and decelerates it using a permanent magnet synchronous machine (PMSM). To avoid excessive wear, the rotor is supported by active magnetic bearings (AMB). After the fatigue test was finished with the first cylinder, a new cylinder was attached to the test stand. With this new specimen, previously uncritical radial vibrations became more severe. For high accelerations, these vibrations led to instability of the rotor. However, high accelerations are desirable to perform the fatigue tests in the shortest possible time. Hence, the AMB control should be made insensitive to these vibrations. Since the vibrations depend on the acceleration of the rotor, it is reasonable to assume that they are induced by the PMSM. To reduce the vibrations, these excitations from the PMSM are included in the model-based controller parametrization process for the radial AMB, in which the parameters are adjusted via optimization. With the adjusted control, the amplitude of the vibration was significantly reduced and higher accelerations were possible. The described parameter tuning process can easily be adapted to different AMB systems with disturbances and changes in the system
In-situ electron-beam lithography of deterministic single-quantum-dot mesa-structures using low-temperature cathodoluminescence spectroscopy
We report on the deterministic fabrication of sub-um mesa structures
containing single quantum dots by in-situ electron-beam lithography. The
fabrication method is based on a two-step lithography process using a
low-temperature cathodoluminescence (CL) spectroscopy setup. In the first step
the position and spectral features of single InGaAs quantum dots (QDs) are
detected by CL. Then circular sub-um mesa-structures are exactly defined by
high-resolution electron-beam lithography and subsequent etching in the second
step. CL spectroscopy and micro-photoluminscence spectroscopy demonstrate the
high optical quality of the single-QD mesa-structures with emission linewidths
below 15 ueV and g(2)(0) = 0.04. Our lithography method allows for an alignment
precision better than 100 nm which paves the way for a fully-deterministic
device technology using in-situ CL lithography.Comment: 4 pages, 4 figure
Electronic structure of NiSSe across the phase transition
We report very highly resolved photoemission spectra of NiS(1-x)Se(x) across
the so-called metal-insulator transition as a function of temperature as well
as composition. The present results convincingly demonstrate that the low
temperature, antiferromagnetic phase is metallic, with a reduced density of
states at E. This decrease is possibly due to the opening of gaps along
specific directions in the Brillouin zone caused by the antiferromagnetic
ordering.Comment: Revtex, 4 pages, 3 postscript 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
Ultrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet
We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L3 resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state
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
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