arXiv subjects

Christian Fella

Publications and source records attributed to Christian Fella.

4 recordsLinked to original sources

arXiv2021

A Novel Nano Tomography Setup for Material Science and Engineering Applications

In a comprehensive study on several samples we demonstrate for our laboratory-based computed tomography system resolutions down to 150nm. The achieved resolution is validated by imaging com-mon test structures in 2D and Fourier Shell Correlation of 3D volumes. As representative application examples from nowadays material research, we show metallization processes in multilayer integrated circuits, ageing in lithium battery electrodes, and volumetric of metallic sub-micrometer fillers of com-posites. Thus, our laboratory system provides the unique possibility to image non-destructively struc-tures in the range of hundred nanometers, even for high density materials.

physics.ins-det↗

arXiv2020

Lenseless X-ray Nano-Tomography down to 150nm Resolution: On the Quantification of Modulation Transfer and Focal Spot of the Lab-based ntCT System

The ntCT nano tomography system is a geometrically magnifying X-ray microscopy system integrating the recent Excillum NanoTube nano-focus X-ray source and a CdTe photon counting detector from Dectris. The system's modulation transfer function (MTF) and corresponding point spread function (PSF) are characterized by analyzing the contrast visibility of periodic structures of a star pattern featuring line width from 150nm to 1.5$μ$m. The results, which can be attributed to the characteristics of the source spot, are crosschecked by scanning the source's electron focus over an edge of the structured transmission target in order to obtain an independent measurement of its point spread function. For frequencies above 1000 linepairs/mm, the MTF is found to correspond to a Gaussian PSF of 250nm full width at half maximum (FWHM). The lower frequency range down to 340 linepairs/mm shows an additional Gaussian contribution of 1$μ$m FWHM. The resulting resolution ranges at 3200 linepairs/mm, which is consistent with the visual detectability of the smallest 150nm structures within the imaged star pattern.

physics.ins-det↗

arXiv2019

Comparing Image Quality in Phase Contrast sub$μ$ X-Ray Tomography -- A Round-Robin Study

How to evaluate and compare image quality from different sub-micrometer (sub$μ$) CT scans? A simple test phantom made of polymer microbeads is used for recording projection images as well as 13 CT scans in a number of commercial and non-commercial scanners. From the resulting CT images, signal and noise power spectra are modeled for estimating volume signal-to-noise ratios (3D SNR spectra). Using the same CT images, a time- and shape-independent transfer function (MTF) is computed for each scan, including phase contrast effects and image blur ($\mathrm{MTF_{blur}}$). The SNR spectra and MTF of the CT scans are compared to 2D SNR spectra of the projection images. In contrary to 2D SNR, volume SNR can be normalized with respect to the object's power spectrum, yielding detection effectiveness (DE) a new measure which reveals how technical differences as well as operator-choices strongly influence scan quality for a given measurement time. Using DE, both source-based and detector-based sub$μ$ CT scanners can be studied and their scan quality can be compared. Future application of this work requires a particular scan acquisition scheme which will allow for measuring 3D signal-to-noise ratios, making the model fit for 3D noise power spectra obsolete.

physics.ins-det↗

arXiv2013

Microwave spectroscopy on heavy-fermion systems: probing the dynamics of charges and magnetic moments

Investigating solids with light gives direct access to charge dynamics, electronic and magnetic excitations. For heavy fermions, one has to adjust the frequency of the probing light to the small characteristic energy scales, leading to spectroscopy with microwaves. We review general concepts of the frequency-dependent conductivity of heavy fermions, including the slow Drude relaxation and the transition to a superconducting state, which we also demonstrate with experimental data taken on UPd2Al3. We discuss the optical response of a Fermi liquid and how it might be observed in heavy fermions. Microwave studies with focus on quantum criticality in heavy fermions concern the charge response, but also the magnetic moments can be addressed via electron spin resonance (ESR). We discuss the case of YbRh2Si2, the open questions concerning ESR of heavy fermions, and how these might be addressed in the future. This includes an overview of the presently available experimental techniques for microwave studies on heavy fermions, with a focus on broadband studies using the Corbino approach and on planar superconducting resonators.

cond-mat.str-el↗