24378 research outputs found
Sort by
Insights into laser induced magnetization dynamics using picosecond ultrasonics with x ray diffraction
No full textUnusually High Occupation of Co 3d State in Magnetic Weyl Semimetal Co3Sn2S2
Full text linkThe physical properties of magnetic topological materials are strongly influenced by their nontrivial band topology coupled with the magnetic structure. Co3Sn2S2 is a ferromagnetic kagome Weyl semimetal displaying giant intrinsic anomalous Hall effect which can be further tuned via elemental doping, such as Ni substitution for Co. Despite significant interest, the exact valency of Co and the magnetic order of the Ni dopants remained unclear. Here, we report a study of Ni doped Co3Sn2S2 single crystals using synchrotron based X ray magnetic circular dichroism XMCD , X ray photoelectron emission microscopy XPEEM , and hard soft X ray photoemission spectroscopy XPS techniques. We confirm the presence of spin dominated magnetism from Co in the host material, and also the establishment of ferromagnetic order from the Ni dopant. The oxygen free photoemission spectrum of the Co 2p core levels in the crystal well resembles that of a metallic Co film, indicating a Co0 valency. Surprisingly, we find the electron filling in the Co 3d state can reach 8.7 9.0 electrons in these single crystals. Our results highlight the importance of element specific X ray spectroscopy in understanding the electronic and magnetic properties that are fundamental to a heavily studied Weyl semimetal, which could aid in developing future spintronic applications based on magnetic topological material
Neural Networks for Quantifying Laboratory Confocal Micro X ray Fluorescence Measurements
Full text linkThe quantification of confocal micro X ray fluorescence spectroscopy CMXRF data obtained with polychromatic excitation in a laboratory setup is challenging. Complex dependencies, an elaborate setup calibration and nontrivial data evaluation makes it a time consuming and intricate task. In this work we introduce the first application of a neural network for the quantification of homogeneous bulk samples, which significantly simplifies the evaluation and effectively eliminates the need for human input. The training of the neural network is performed on simulated data. For this, a simulation routine for CMXRF data of homogeneous bulk samples is introduced. The neural network is trained to simultaneously quantify the elemental concentrations of 53 elements, the density of the sample and the surface position directly from depth profiling measurements. As a result, the CMXRF evaluation is substantially simplified and the potential of the used neural network for feature extraction and prediction is demonstrate
Design of HOM Damped Multi Cell SRF Cavities for CW Operation in High Current Storage Rings
Full text linkPresently, SRF systems for high current storage rings are generally limited to low frequency, moderate voltage, single cell cavities. For a new class of cavities to be used in longitudinal beam phase space manipulation, high voltage third harmonic multi cell cavities are required, resulting in very challenging impedance considerations and higher order mode powers of the order of several kW per cavity. Thus, the cavity design requires far more attention on the HOM spectrum to be off resonance with circulating beam harmonics. Special techniques are developed to analyze the HOM spectrum and damping beyond the standard frequency range laying typically at a few GHz as required by the VSR Demo project. Within the presented work, a 4 cell 1.5 GHz cavity is designed including end groups with multi waveguide damping for a space saving design capable of handling over 2.5 kW of HOM power per cavity. These cavities are designed for high voltage operation with beam currents of at least 300 mA. Prototype systems are now in production. This paper provides an overview on the advanced techniques for SRF cavity design and their application to tailoring the HOM spectrum and its application for the VSR Demo projec
Operando Measurement of Transition Metal Deposition in a NMC Li Ion Battery Using Laboratory Confocal Micro X ray Fluorescence Spectroscopy
Full text linkThe degradation of batteries has very different causes depending on the material and operation modes. However, most of these causes are associated with changes in one or more interfaces, in particular through depositions and their potential chemical changes under operating conditions. Over the last decade operando investigations have therefore become increasingly state of the art, elemental analysis of full cell systems, though, is still missing due to a lack of depth resolved methods. Using laboratory confocal micro X ray fluorescence spectroscopy the analysis of a Li ion battery coin cell during 10600 cycles are presented. It is shown that the confocal setup enables to differentiate between the nickel manganese cobalt oxide NMC cathode with high levels of transition metal concentration and a possible deposition of traces of Mn, Ni, Co in the underlying layers. This allows for spatially resolved insights in operando without changing the layer stack, nor electrode area. This paper is the first to demonstrate the non destructive and quantitative elemental analysis of battery interfaces under operating conditions. This quantitative analysis is the prerequisite for the determination of absolute transport and conversion rates, without which the transition from empirical research to a focused development of batteries will not succee
Halogenated and microhydrated transition metal ions insights from x ray absorption spectroscopy
No full textTransition metals are of great interest to fields like catalysis, electrochemistry, and biochemistry as their electronic structure enables a rich chemistry. Therefore, the refining of spectroscopic methods sensitive to the electronic structure of transition metals is essential to advances in aforementioned fields. In this study gas phase x ray absorption spectroscopy data of diatomic iron and nickel halide cations in the oxidation state of 2 are presented. The sensitivity of the used method enables insights into the changes of the electronic structure of transition metal halogen bonds upon changing of the halogen ligand. Combining the experimental data with charge transfer multiplet and multiplet calculations enables a detailed investigation of gradual and abrupt changes in the valence electronic configuration of the diatomic transition metal halide cations. A shift of the median of the iron L amp; 8323; edge excitation energy towards lower energy with increasing covalency of the iron halogen bond within the oxidation state of 2 is observed. The observed shift amounts to 420 60meV from [FeF] amp; 8314; the most ionic species of the series to the more covalently bonded [FeI] amp; 8314;. In addition, a shift of the nickel L amp; 8323; edge excitation energy median of 2.3 0.2eV is discerned between monatomic Ni amp; 8314; in the ground state and a low lying excited state differing by exact one electron in 3d occupation. Furthermore, an abrupt change in the nickel L amp; 8323; edge spectrum between [NiF] amp; 8314; and [NiCl] amp; 8314; is observed. This abrupt change suggests that the ground state wave function of [NiF] amp; 8314; is best described by a 3d amp; 8312; valence configuration. In contrast, the ground state wave functions of [NiX] amp; 8314; X Cl, Br, I are best described by two electronic configurations with a dominating 3d amp; 8313; amp; 7738; configuration that includes a ligand hole and a 3d amp; 8312; configuration with no ligand hole. Therefore, [NiF] amp; 8314; electronic structure can be described by a classic ligand field while the electronic structures of [NiX] amp; 8314; X Cl, Br, I are best described by an inverted ligand field. Moreover, iron II with its first complete hydration shell was prepared in the gas phase. The x ray absorption spectra of the gas phase [Fe H amp; 8322;O amp; 8326;]2 amp; 8314; at the iron L edge and oxygen K edge is compared to the x ray absorption spectra of iron in aqueous solutions as well as molecular water. In contrast, to liquid phase experiments with a dominating signal of bulk solvent molecules at the oxygen K edge in the gas phase the signal originates solely from the water ligands in the first solvation shell of the metal. This enables detailed insight to the electronic structure of the water metal bond. Hence, observation and assignment of spectral features at the oxygen K edge of [Fe H amp; 8322;O amp; 8326;]2 amp; 8314; originating from the hybridization of the irons 3d valence orbitals with the water ligands valence orbitals is achieved. Supporting DFT calculations enabled the assignment of specific spectral features to transitions resulting from photon excitation
Dynamic control of electron correlations in photodoped charge transfer insulators
Full text linkThe electronic properties of correlated insulators are governed by the strength of Coulomb interactions, enabling the control of electronic conductivity with external stimuli. This work highlights that the strength of electronic correlations in nickel oxide NiO , a prototypical charge transfer insulator, can be coherently reduced by tuning the intensity of an optical pulse excitation. This weakening of correlations persists for hundreds of picoseconds and exhibits a recovery time independent of photodoping density across two orders of magnitude. A broadening of the charge transfer gap is also observed, consistent with dynamical screening. The high degree of control achieved over both the energy and temporal dynamics of electronic correlations offers a promising avenue to a full optical control of correlated systems and the Mott transitio
Periodic Inverted Micropyramids for Optically Optimized Fully Textured Solution Processed Perovskite Solar Cells
Full text linkOptical performance of perovskite based solar cells can be enhanced by utilizing fully textured interfaces. However, solution processing of perovskite films on textured surfaces is a nonstraightforward and challenging process, particularly if optically most efficient micrometer sized textures are used. In this work, we present fully textured solution processed perovskite solar cells on periodic inverted micropyramids. The textures have a period of 4 amp; 956;m with varying pyramid depths and are fabricated by wet chemical etching of silicon with subsequent replication on glass substrates using nanoimprint lithography. Inverted pyramids are shown to enable low reflectance similar to random micropyramids on silicon. Additionally, they are able to confine perovskite precursor solution within its structure during spin coating, resulting in a conformal, fully textured perovskite film. We demonstrate that the resulting fully textured single junction perovskite solar cells feature a reduced reflection loss of up to 1.2 amp; 8201;mA cm2 in short circuit current density. Moreover, we observe that the amount of lead iodide in the perovskite precursor solution crucially impacts growth and nonradiative recombination losses of the fully textured perovskite solar cells on inverted micropyramids. Finally, we prove the versatility of our approach by also demonstrating conformal coating with slot die coating, which is a scalable process considered for industrial applicatio
Co2 3 and Fe2 3 charge transition levels in La,Sr CoO3 delta and La,Sr FeO3 delta
No full textX ray photoelectron spectroscopy XPS is used to determine the energy levels associated with the Co2 3 and Fe2 3 charge transition levels in La, Sr CoO3 delta and La, Sr FeO3 delta, respectively. The charge transition levels are revealed by varying the Fermi level by means of oxidizing and reducing treatments. The Co2 3 transition occurs at 0.15 0.05 eV above the valence band maximum of La, Sr CoO3 delta and the Fe2 3 transition at 1.25 0.05 eV above the valence band maximum of La, Sr FeO3 delta, respectively. In combination with the energy band alignment determined from XPS analysis in the course of interface formation, it is derived that the energetic difference of the charge transition levels coincides with that in Co doped BiFeO3, suggesting transferability of the energy levels in perovskite type materials. The difference in charge transition levels directly relates to the easier reduction of Co compared to Fe. The result emphasizes that the oxidation states of Fe and Co in mixed compounds must be treated independently for a quantitative description of defect properties. They also provide a natural direct explanation for the higher electronic conductivity of La, Sr CoO3 delta compared to that of La, Sr FeO3 delta, and for the acceptor behavior of Co in La, Sr FeO3 delt
Operando investigation of the two phase flow behavior of a zero gap alkaline electrolysis cell using neutron radiography
Full text linkThe two phase flow behavior inside a zero gap alkaline electrolysis cell is investigated using operando neutron radiography. The cell was operated with a highly concentrated potassium hydroxide solution. The two phase flow is evaluated at different electrolyte volume flows, current densities, and temperatures. The amount of gas inside the parallel flow channels is identified and the gas bubble velocity over the channel s length and time is evaluated depending on the different operating conditions. The gas bubble motion requires a high degree of temporal resolution. At the Institut Laue Langevin, a high frame rate of 50 fps was achieved using the NeXT Neutron and X Ray Tomograph neutron imaging instrument, which is fed by the world s most powerful neutron source. This study demonstrates the importance and limitations of high temporal and spatial resolution in neutron radiography for the investigation of two phase flow in electrochemical flow cell