18,172 research outputs found
Post-acquisition image based compensation for thickness variation in microscopy section series
Serial section Microscopy is an established method for volumetric anatomy
reconstruction. Section series imaged with Electron Microscopy are currently
vital for the reconstruction of the synaptic connectivity of entire animal
brains such as that of Drosophila melanogaster. The process of removing
ultrathin layers from a solid block containing the specimen, however, is a
fragile procedure and has limited precision with respect to section thickness.
We have developed a method to estimate the relative z-position of each
individual section as a function of signal change across the section series.
First experiments show promising results on both serial section Transmission
Electron Microscopy (ssTEM) data and Focused Ion Beam Scanning Electron
Microscopy (FIB-SEM) series. We made our solution available as Open Source
plugins for the TrakEM2 software and the ImageJ distribution Fiji
Designing a fully compensated half-metallic ferrimagnet
Recent experimental work on Mn2RuxGa demonstrates its potential as a compensated ferrimagnetic
half-metal (CFHM). Here we present a set of high-throughput ab initio density functional
theory calculations and detailed experimental characterisation, that enable us to correctly describe
the nominal Mn2RuxGa thin films, in particular with regard to site-disorder and defects. We then
construct models that accurately capture all the key features of the Mn-Ru-Ga system, including
magnetic compensation and the spin gap at the Fermi level. We find that electronic doping is neccessary,
which is achieved with a Mn/Ga ratio smaller than two. Our study shows how composition
and substrate-induced biaxial strain can be combined to design a ferrimagnetic half-metal with a
compensation point close to room temperature
Nanoscale piezoelectric response across a single antiparallel ferroelectric domain wall
Surprising asymmetry in the local electromechanical response across a single
antiparallel ferroelectric domain wall is reported. Piezoelectric force
microscopy is used to investigate both the in-plane and out-of- plane
electromechanical signals around domain walls in congruent and
near-stoichiometric lithium niobate. The observed asymmetry is shown to have a
strong correlation to crystal stoichiometry, suggesting defect-domain wall
interactions. A defect-dipole model is proposed. Finite element method is used
to simulate the electromechanical processes at the wall and reconstruct the
images. For the near-stoichiometric composition, good agreement is found in
both form and magnitude. Some discrepancy remains between the experimental and
modeling widths of the imaged effects across a wall. This is analyzed from the
perspective of possible electrostatic contributions to the imaging process, as
well as local changes in the material properties in the vicinity of the wall
Effect of ferromagnetic film thickness on magnetoresistance of thin-film superconductor-ferromagnet hybrids
We study the influence of the thickness Df of the plain ferromagnetic (F)
film on the electrical resistance of the flux-coupled hybrids, consisting of
superconducting (S) Al film and multilayer [Co/Pt] F film with out-of-plain
magnetization. The behavior of such hybrids at high and low temperatures is
found to be different: the nucleation of superconductivity at high temperatures
is governed mainly by the typical lateral dimensions of the magnetic domains,
while low temperature properties are determined by topology of the magnetic
template. We show that an increase in the Df value leads to a broadening of the
field- and temperature intervals where non-monotonous dependence of the
superconducting critical temperature Tc on the applied magnetic field H is
observed (for demagnetized F films). Further increase in the Df value results
in a global suppression of superconductivity. Thus, we determined an optimal
thickness, when the non-monotonous dependence Tc(H) can be observed in rather
broad T and H range, what can be interesting for further studies of the
localized superconductivity in planar Al-based S/F hybrids and for development
of the devices which can exploit the localized superconductivity.Comment: 10 pages, 9 figure
Adaptive optics wavefront compensation for solid immersion microscopy in backside imaging
Thesis (Ph.D.)--Boston UniversityThis dissertation concerns advances in high-resolution optical microscopy needed to detect faults in next generation semiconductor chips. In this application, images are made through the chips' back side to avoid opaque interconnect metal layers on the frontside. Near infrared wavelengths are required, since the silicon is relatively transparent at these wavelengths. A significant challenge in this technique is to resolve features as small as 200nm using wavelengths exceeding 1OOOnm. The highest imaging resolution achievable with refractive optics at infrared wavelengths is demonstrated in this dissertation using an aplanatic solid immersion lens (SIL). This is the only method that has been found to be of sufficient resolution to image the next generation of integrated circuits. While the use of an aplanatic solid immersion lens theoretically allows numerical aperture far in excess of conventional microscopy (NASIL ~ 3.5), it also makes the system performance particularly sensitive to aberrations, especially when the samples have thicknesses that are more than a few micrometers thicker or thinner than designed thickness, or when the refractive index of the SIL is slightly different than that of the sample.
In the work described here, practical design considerations of the SILs are examined. A SIL-based confocal scanning microscope system is designed and constructed. The aberrations of the system due to thickness uncertainty and material mismatch are simulated using both analytical model and ray-tracing software, and are measured in the SIL experimental apparatus. The dominant aberration for samples with thickness mismatch is found to be spherical aberration. Wavefront errors are compensated by a microelectromechanical systems deformable mirror (MEMS DM) in the optical system's pupil. The controller is implemented either with closed-loop real time sensor feedback or with predictive open-loop estimation of optical aberrations. Different DM control algorithms and aberration compensation techniques are studied and compared. The experimental results agree well with simulation and it has been demonstrated through models and experiments in this work that the stringent sample thickness tolerances previously needed for high numerical aperture SIL microcopy can be relaxed considerably through aberration compensation. Near-diffraction-limited imaging performance has been achieved in most cases that correspond to practical implementation of the technique
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