Global and local hard X‐ray tomography of a centimeter‐size tumor vessel tree

The visualization of the vascular network in tumors down to the smallest vessels requires high spatial resolution and reasonable contrast. Stained corrosion casts of the microvasculature network guarantee superior X‐ray absorption contrast and highest reproduction fidelity. Tomography of a centimeter‐size tumor, however, is unfeasible at the spatial resolution needed to reveal the smallest vessels. Therefore, local tomography has been performed to visualize the smallest capillaries within the region of interest. These three‐dimensional data show the detailed morphology, but the reconstructed absorption coefficients obtained in local tomography differ substantially from the absorption coefficients retrieved from the less detailed global tomography data. This paper deals with the adaptation of local tomography data using the global data and considers two‐parameter histogram matching of the radiographs, sinogram extension, and multi‐parameter cupping correction. It is demonstrated that two‐parameter histogram matching of the radiographs already provides reasonable agreement. The change of the lens in front of the detector's camera, however, significantly affects the obtained local X‐ray absorption coefficients in the tomograms predominantly owing to the dissimilar point‐spread functions of the two configurations used, and much less to the fact that one of the data sets was acquired in a local geometry.     

Experimental test of the fluctuation theorem for a driven two-level system with time-dependent rates

A single defect center in diamond periodically excited by a laser is shown to provide a simple realization for a system obeying a fluctuation theorem for nonthermal noise. The distribution of these fluctuations is distinctly non-Gaussian, which has also been verified by numerical calculation. For driving protocols symmetric under time reversal a more restricted form of the theorem holds, which is also known from entropy fluctuations caused by thermal noise.     

The Effect of Substrate Material and Postannealing on the Photoluminescence and Piezo Properties of DC-Sputtered ZnO

Abstact Zinc oxide (ZnO) thin films were deposited on various substrates by DC sputtering deposition. Thermal annealing was performed at up to 1,200°C in N₂ for 30 min. The effect was investigated using x-ray diffraction (XRD), photoluminescence (PL) spectra, scanning electron microscopy (SEM), and piezoresponse force microscopy (PFM). The influence on PL response depends both on substrate material and annealing temperature. The PFM images reveal that the ZnO films have inversion domains. While annealing improves the piezoresponse, the inversion domains still persist. The cross-sectional analysis of the inversion domains shows domain boundary widths of approximately 1.5 nm. (Received ...; accepted ...)     

Electromagnetic finite elements based on a four-potential variational principle

We derive electromagnetic finite elements based on a variational principle that uses the electromagnetic four-potential as primary variable. This choice is used to construct elements suitable for downstream coupling with mechanical and thermal finite elements for the analysis of electromagnetic/mechanical systems that involve superconductors. The main advantages of the four-potential as a basis for finite element formulation are: the number of degrees of freedom per node remains modest as the problem dimensionality increases, jump discontinuities on interfaces are naturally accomodated, and statics as well as dynamics may be treated without any a priori approximations. The new elements are tested on an axisymmetric problem under steady-state forcing conditions. The results are in excellent agreement with analytical solutions.