The effective crystal field in multiorbital correlated materials can be either enhanced or reduced by electronic correlations with crucial consequences for the topology of the Fermi surface and, hence, on the physical properties of these systems. In this respect, recent local density approximation plus dynamical mean-field theory studies of Ni-based heterostructure have shown contradicting results, depending on whether the less correlated $p$ orbitals are included or not. We investigate the origin of this problem and identify the key parameters controlling the Fermi surface properties of these systems. Without the $p$ orbitals, the model is quarter-filled, while the $d$ manifold moves rapidly towards half-filling when the $p$ orbitals are included. This implies that the local Hund's exchange, while rather unimportant for the former case, can play a predominant role in controlling the orbital polarization for the extended basis set by favoring the formation of a larger local magnetic moment.
In this work, we report the integration of an atomic force microscope (AFM) into a helium ion microscope (HIM). The HIM is a powerful instrument, capable of sub-nanometer resolution imaging and machining of nanoscale structures, while the AFM is a well-established versatile tool for multiparametric nanoscale characterization. Combining the two techniques opens the way for unprecedented, in situ, correlative analysis at the nanoscale. Nanomachining and analysis can be performed without contamination of the sample and environmental changes between processing steps. The practicality of the resulting tool lies in the complementarity of the two techniques. The AFM offers not only true 3D topography maps, something the HIM can only provide in an indirect way but also allows for nanomechanical property mapping, as well as for electrical and magnetic characterization of the sample after focused ion beam materials modification with the HIM. The experimental setup is described and evaluated through a series of correlative experiments, demonstrating the feasibility of the integration.
We present here two alternative schemes designed to correct the high-frequency truncation errors in the numerical treatment of the Bethe-Salpeter equations. The schemes are applicable to all Bethe-Salpeter calculations with a local two-particle irreducible vertex, which is relevant, e.g., for the dynamical mean-field theory (DMFT) and its diagrammatic extensions. In particular, within a purely diagrammatic framework, we could extend existing algorithms for treating the static case in the particle-hole sector to more general procedures applicable to all bosonic frequencies and all channels. After illustrating the derivation and the theoretical interrelation of the two proposed schemes, these have been applied to the Bethe-Salpeter equations for the auxiliary Anderson impurity models of selected DMFT calculations, where results can be compared against a numerically ``exact'' solution. The successful performance of the proposed schemes suggests that their implementation can significantly improve the accuracy of DMFT calculations at the two-particle level, in particular for more realistic multiorbital calculations where the large number of degrees of freedom substantially restricts the actual frequency range for numerical calculations, as well as---on a broader perspective---of the diagrammatic extensions of DMFT.
The demand for clean and green energy has raised the consumption of hydrogen continuously during the last years. Hydrogen is most economically produced in large scale systems by methane steam reforming followed by pressure swing adsorption (PSA). However, with a rising demand for small-scale production of hydrogen, and as down-scaling to smaller PSA-systems ( < 500 Nm3/h H2) is not economic, a substantial demand for hydrogen generation using palladium membranes has emerged.Porous tubes made of an oxide dispersion strengthened powder metallurgy Fe-Cr alloy (trade name ITM) constitute the backbone for the thin solid Pd films. The tubes provide mechanical and chemical long-term stability in atmospheres with hydrogen- and carbon-species at operation temperatures up to 600°C. A porous ceramic diffusion barrier layer (DBL) is deposited between the ITM-backbone and the Pd thin-film to avoid Pd diffusion into the Fe-Cr substrate and thereby ensure long-term integrity of the system. The Pd thin-film with a thickness < 10 μm is applied onto the DBL by a proprietary coating technology.This paper describes the production route of a tube/diffusion-barrier-layer/Pd-membrane system, its structure and permeation properties.
Ptychography provides a sophisticated means of retrieving the complex object function via coherent diffractive imaging. It has become successfully established in the x‐ray and visible light communities as a means of lensless imaging and for its super‐resolution capability. Super‐resolution was also the original use of the method in electron microscopy [1]. However the technique did not become popular in the high resolution electron microscopy community due to the difficulty of acquiring and processing the four dimensional datasets required. Recent advances in detector technology however have resulted in a resurgence of interest in the method. As aberration correction now provides atomic resolution in hardware without the need for super‐resolution techniques, interest in ptychography in scanning transmission electron microscopy (STEM) has shifted towards achieving efficient phase contrast imaging. STEM provides sensitivity to atomic number via Z‐contrast annular dark field (ADF) imaging. The approximately quadratic variation of the intensity in ADF images with atomic number provides relatively facile compositional interpretability as compared to phase contrast imaging. However a relatively small proportion of the beam current is scattered out to the high angles sampled by ADF detectors, particularly for thin samples composed of light elements. Most of the transmitted electrons are contained within the bright field (BF) disk. Ptychography has recently been shown to be more efficient than other phase contrast imaging methods used in STEM, including conventional BF, annular bright field (ABF), and differential phase contrast (DPC) [2,3]. It has also proven superior to these modes at revealing the positions of light elements hidden by the scattering of heavy elements in the ADF signal [4]. Furthermore, ptychographic phase imaging requires no aberrations to achieve contrast, meaning the electron probe can be tuned to maximum capability of the aberration corrector. Here we investigate the sensitivity of STEM ptychography for two different applications. The first makes use of the sensitivity of phase contrast imaging to electromagnetic fields to detect charge transfer. Such charge transfer sensitivity was demonstrated in conventional TEM by Meyer et. al. by making use of lens aberrations to reveal contrast changes in N‐doped graphene and hexagonal boron nitride (hBN) that only matched with simulations based on potentials including the effects of charge transfer produced by density functional theory (DFT) and not the neutral atom potentials. We will present the results of testing charge transfer sensitivity in STEM with ptychography and various low dimensional materials. Figure 1 compares the projected potentials of (hBN) simulated with and without charge transfer. Figure 2 shows an example of simultaneously acquired ADF and ptychographic phase images of a region of single layer hBN surrounded by a double layer taken with the microscope fully tuned with the aberration corrector. As residual aberrations can affect phase images, we will also investigate the use of post acquisition aberration quantification and correction applied to ptychographic datasets of samples with the relatively subtle contrast effects of charge transfer. The second application of the sensitivity of STEM ptychography is its use for beam sensitive samples. We will assess the dose effectiveness of the method through simulations of varies samples, including biological samples frozen in amorphous ice, and compare to conventional TEM imaging. Consideration will be made of the pixelated detector technologies currently available, as the sensitivity and speed of the detector directly influence the dose effectiveness of the ptychographic phase images.
The atomic structure of disordered materials is one of the remaining challenges in materials science due to the difficulties related to imaging non-repeating arrangements of atomic positions.In fact, since over 80 years, the popular concept of the atomic structure of amorphous materials has been for a large part based on the drawings of a random network by Zachariasen [1].
Abstract We demonstrate the use of combined simultaneous atomic force microscopy (AFM) and laterally resolved Raman spectroscopy to study the strain distribution around highly localised deformations in suspended two-dimensional materials. Using the AFM tip as a nanoindentation probe, we induce localised strain in suspended few-layer graphene, which we adopt as a two-dimensional membrane model system. Concurrently, we visualise the strain distribution under and around the AFM tip in situ using hyperspectral Raman mapping via the strain-dependent frequency shifts of the few-layer graphene’s G and 2D Raman bands. Thereby we show how the contact of the nm-sized scanning probe tip results in a two-dimensional strain field with μm dimensions in the suspended membrane. Our combined AFM/Raman approach thus adds to the critically required instrumental toolbox towards nanoscale strain engineering of two-dimensional materials.
Micro power converters for energy recovery are increasingly important for a number of future applications. The Austrian Institute of Technology (AIT) is presently developing an innovative μ-scale turbine expander for work recovery in transcritical CO2 heat pumps. The main drawback of a lower COP (coefficient of performance) of transcritical CO2 heat pumps compared to conventional heat pump systems can be compensated by utilizing the pressure difference between the high pressure and low pressure part of the pump for work recovery. Work recovery can be realized by substituting the expansion valve between the high and low pressure side by a Pelton turbine with specific two phase flow turbine blades. In order to increase the power output, the generator was integrated into the turbine to reduce the friction losses and hence increase the overall efficiency. An important aspect is that the generator is directly connected with the high pressure part of the turbine. One part of the project is the optimization of the turbine geometry via simulation tools. The paper will give an overview about our microturbine development as well as a comparison of the power output of each turbine generation. Furthermore the present paper discusses a concept that utilizes our microturbine together with a micro combustion module that enables a micro power generator with very high power-to-weight ratios based on green fuels.
