Magnetic anisotropy in YbAlO3 studied by electron spin resonance

The magnetic anisotropy of the quantum-critical spin-chain system YbAlO$$_3$$is studied in detail in its paramagnetic phase by means of electron spin resonance. We find an uniaxial g factor anisotropy with $$g_\parallel =7.01$$and $$g_\perp = 0.47$$. The existence of two distinct magnetic Yb sites explains the resonance spectra well

Magnetic Excitations in the Multiferroic N\'eel-type Skyrmion Host GaV4_4S8_8

Broadband microwave spectroscopy has been performed on single-crystalline GaV$_4$S$_8$, which exhibits a complex magnetic phase diagram including cycloidal, N\'eel-type skyrmion lattice, as well as field-polarized ferromagnetic phases below 13 K. At zero and small magnetic fields two collective modes are found at 5 and 15 GHz, which are characteristic of the cycloidal state in this easy-axis magnet. In finite fields, entering the skyrmion lattice phase, the spectrum transforms into a multi-mode pattern with absorption peaks near 4, 8, and 15 GHz. The spin excitation spectra in GaV$_4$S$_8$ and their field dependencies are found to be in close relation to those observed in materials with Bloch-type skyrmions. Distinct differences arise from the strong uniaxial magnetic anisotropy of GaV4S8 not present in so-far known skyrmion hosts

Development of a density-based topology optimization of homogenized lattice structures for individualized hip endoprostheses and validation using micro-FE

Prosthetic implants, particularly hip endoprostheses, often lead to stress shielding because of a mismatch in compliance between the bone and the implant material, adversely affecting the implant’s longevity and effectiveness. Therefore, this work aimed to demonstrate a computationally efficient method for density-based topology optimization of homogenized lattice structures in a patient-specific hip endoprosthesis. Thus, the root mean square error (RMSE) of the stress deviations between the physiological femur model and the optimized total hip arthroplasty (THA) model compared to an unoptimized-THA model could be reduced by 81 % and 66 % in Gruen zone (GZ) 6 and 7. However, the method relies on homogenized finite element (FE) models that only use a simplified representation of the microstructural geometry of the bone and implant. The topology-optimized hip endoprosthesis with graded lattice structures was synthesized using algorithmic design and analyzed in a virtual implanted state using micro-finite element (micro-FE) analysis to validate the optimization method. Homogenized FE and micro-FE models were compared based on averaged von Mises stresses in multiple regions of interest. A strong correlation (CCC > 0.97) was observed, indicating that optimizing homogenized lattice structures yields reliable outcomes. The graded implant was additively manufactured to ensure the topology-optimized result’s feasibility

On the Relation of Four-Dimensional N=2,4 -- Supersymmetric String Backgrounds to Integrable Models

In this letter we discuss the relation of four-dimensional, $N=2$ supersymmetric string backgrounds to integrable models. In particular we show that non-K\"ahlerian gravitational backgrounds with one $U(1)$ isometry plus non-trivial antisymmetric tensor and dilaton fields arise as the solutions of the Liouville equation or, for the case of vanishing central charge deficit, as the solutions of the continual Toda equation. When performing an Abelian duality transformation, a particular class of solutions of the continual Toda equation leads to the well-known gravitational Eguchi-Hanson instanton background with self-dual curvature tensor.Comment: 10 pages,latex (references added

Fractal diffusion in high temperature polymer electrolyte fuel cell membranes

© 2018 Author(s). The performance of fuel cells depends largely on the proton diffusion in the proton conducting membrane, the core of a fuel cell. High temperature polymer electrolyte fuel cells are based on a polymer membrane swollen with phosphoric acid as the electrolyte, where proton conduction takes place. We studied the proton diffusion in such membranes with neutron scattering techniques which are especially sensitive to the proton contribution. Time of flight spectroscopy and backscattering spectroscopy have been combined to cover a broad dynamic range. In order to selectively observe the diffusion of protons potentially contributing to the ion conductivity, two samples were prepared, where in one of the samples the phosphoric acid was used with hydrogen replaced by deuterium. The scattering data from the two samples were subtracted in a suitable way after measurement. Thereby subdiffusive behavior of the proton diffusion has been observed and interpreted in terms of a model of fractal diffusion. For this purpose, a scattering function for fractal diffusion has been developed. The fractal diffusion dimension dw and the Hausdorff dimension df have been determined on the length scales covered in the neutron scattering experiments

On-ice Vibroseis: What lie beneath Ekström Ice Shelf, East Antarctica?

Between 2010-2018 an extensive grid of seismic reflection data were collected across the grounding line and on the Ekström ice-shelf, using an on-ice vibroseis source and snowstreamer. Here they are used to investigate current ice dynamics and reconstruct the glaciological history of this region. These data show the ice-shelf thickness ranges from 170 m, near the ice-shelf front, to ~600 m near the grounding line. Relic crevasses are seen at the ice base, in the Western part of the ice shelf, which can be tracked back to a current crevasse field at the grounding line. There is also evidence of an ice-shelf basal channel, with a corresponding surface depression. Beneath the sea floor the outcrop and sub-ice extent of the volcanic Explora Wedge (generated through Jurassic rifting and seafloor spreading) is clearly imaged. The wedge is overlain by a sequence of truncated, dipping marine-sediment layers. The sediment layers were likely truncated by former ice advance and subsequent retreat; which has also left evidence in the form topographic over-deepening and glacial debris deposits at the sea floor. The debris deposits range from elongated bedforms in a topographic trough (indicating probable former ice-stream flow) to layered sediment wedges at the current ice-shelf front (indicating the likely former extent of grounded ice). The vibroseis method is fast and effective allowing for a high volume of data collection. For example, in the 2016/17 season ~280 km of multi-fold seismic reflection data were collected over a 25-day period. Future integration of these results with numerical models will provide a better understanding of past and present interactions between the ice sheet and the solid Earth in Dronning Maud Land, which will in turn improve understanding of future contributions of this region to sea-level rise

Sediment features at the grounding zone and beneath Ekström Ice Shelf, East Antarctica, imaged using on-ice vibroseis

An extensive grid of seismic reflection data collected on Ekström Ice Shelf, East Antarctica, between 2010 – 2017 are used to constrain palaeo-ice flow and retreat in the region. Sets of elongated bedforms are seen in a topographic trough, indicating a probable former ice-stream under the western part of the ice shelf. A smoother bed and truncated marine sediments under the eastern part of the ice shelf suggest there was slower ice flow in this area. Sediment wedges at the current ice-shelf front show the grounded ice extent likely reached this region; further investigation of these features will allow the style of deglaciation to be better determined. It is common to map geomorphological sediment features in front of ice shelves and beneath modern-day ice streams using geophysical methods, but there are much less data documenting landforms beneath ice shelves. The data presented here were collected as part of pre-site survey for an envisaged sub-ice shelf geological drilling campaign. These data cover a large portion of the Ekström ice shelf and also extend into the grounded ice areas. Data were collected using an on-ice vibroseis source combined with a snowstreamer. This method of collection is fast and effective allowing for a high volume of data collection. For example, in the 2016/17 season ~280 km of multi-fold seismic reflection data were collected over a 25-day period. In contrast to AUVs with capabilities to measure bathymetry in full spatial coverage, seismic reflection data also allows the vertical structure of sea floor strata and properties to be investigated. The acoustic properties of the sub-surface allow information about the material characteristics, and potentially age and origin of the sediment features to be determined. Moreover, the thickness of the glacial debris, deposited on pre-existing marine sediments, is a first-order indication of where to retrieve short sediment cores. Future integration of these geophysical results with numerical models of ice and sediment dynamics will provide a better understanding of past and present interactions between ice and the solid Earth in East Antarctica; leading to an improved understanding of future contributions of this region to sea-level rise

What lies beneath: A detailed bathymetry of the sea-floor below Ekström Ice Shelf, East Antarctica

An extensive grid of seismic reflection data collected on Ekstro ̈m Ice Shelf, East Antarctica, between 2010–2018, using an on-ice vibroseis source and snowstreamer, are used to make a detailed bathymetry map of the sea floorand ice-shelf cavity. The maps shows a deep sea-floor trough, likely a paleao-ice stream, under the western side ofthe ice shelf. The trough contains a number of points of higher topography, indicating probable former grounding line positions. At the shelf front a sill running across the width of the shelf has implications for ocean circulation and thus ice-ocean interaction and ice shelf melt. This new bathymetry is markedly different from previous models, which show a generally flat and shallow sea floor in the region. This is presumably the case for many of the smaller ice-shelves in Dronning Maud Land, which highlights the need for better bathymetry measurements in these key threshold regions

Relevance of field observations as boundary conditions for understanding ice-sheet-ocean interactions

The direct contact of warm ocean water with the front and base of ice shelves is the main driver for accelerated mass loss of the Antarctic ice sheet. We present a compilation of observations from various projects and methodological approaches applied over the last decade along the Dronning Maud Land coast and highlight their importance for understanding the ice-ocean interactions. With a focus on the Ekström ice shelf, these include spatially continuous seismic observations in combination with airborne gravity inversion to yield sub-shelf bathymetry and geomorphological evidence of past ice-flow activity; ice-dynamic numerical modelling to investigate the role of seafloor/subglacial substrate characteristics to enhance or reduce ice-sheet extent and advance/retreat rates; sub-shelf CTD measurements to determine ocean properties driving basal melting; satellitebased remote sensing to determine ice-shelf height changes and spatially-distributed basal melting; and point measurements of basal melt with surface-based phase-sensitive radar to determine ocean-driven melt and validate remote-sensing products. As the Dronning Maud Land coast plays a critical role in preconditioning the water mass of the coastal current before it enters the Filcher ice-shelf cavity, we argue that a coordinated inter- and transdisciplinary observational network is required to facilitate monitoring a potential ice-sheet mass loss in this part of Antarctica