The Kolar Schist Belt: A possible Archaean suture zone

The Kolar Schist Belt represents a N-S trending discontinuity in the structures, lithologies, and emplacement and metamorphic ages of late Archean gneisses. The suggestion of a much older basement on the west side of the belt is not seen on the east. Within the schist belt amphibolites from each side have distinctly different chemical characteristics, suggesting different sources at similar mantle depths. These amphibolites were probably not part of a single volcanic sequence, but may have formed about the same time in two completely different settings. Could the amphibolites with depleted light REE patterns represent Archean ocean floor volcanics which are derived from a mantle source with a long term depletion of the light REE? Why are the amphibolites giving an age which may be older than the exposed gneisses immediately on either side of the belt? These results suggest that it is necessary to seriously consider whether the Kolar Schist Belt may be a suture between two late Archean continental terranes

Activating extrinsic and intrinsic toughening mechanisms in polycrystalline ceramics and their composites via microstructural engineering

Ferroelastic toughening is one of a limited number of intrinsic toughening mechanisms available for ceramics, yet rarely is it effectively implemented due to a limited understanding of the activation mechanisms within a polycrystalline framework. In the earliest descriptions of ferroelastic toughening, terms such as the transformation strain, coercive stress and process zone parameters were included following other (extrinsic) crack tip shielding models. In the years that have followed, constitutive models have become more sophisticated, incorporating crystal orientation, rate behaviors, and several other factors. However, further development of these models has been limited by the paucity of experimental observations linking ferroelastic switching with critical, yet common, microstructural variations (i.e. grain size, nearest neighbor orientations, secondary/grain boundary phases, etc.). Here we present a multi-scale experimental approach to explore the role of stress concentration, stress transfer and localized constraint in ferroelastic domain nucleation, motion and subsequent toughening. In situ TEM nanopillar and ex situ micropillar compression on single crystal specimens extracted from a polycrystalline ceramic have been used to correlate crystallographic orientation with coercive stresses for domain nucleation and motion. The comparison between these two length scales also highlights the importance of boundary conditions on the nucleation of ferroelastic domains and sheds light on the grain size dependence of domain nucleation probability extracted from Vicker’s indentation of polycrystalline ceramics of the same composition. This insight will ultimately be coupled with grain orientation, elastic anisotropy data and quantified stress distributions during deformation to establish the early foundations of a microstructural design framework for ferroelastically toughened ceramics. Please click Additional Files below to see the full abstract

The subchalcogenides Ir₂In₈Q (Q = S, Se, Te): Dirac semimetal candidates with re-entrant structural modulation

Subchalcogenides are uncommon compounds where the metal atoms are in unusually low formal oxidation states. They bridge the gap between intermetallics and semiconductors, and can have unexpected structures and properties because of the exotic nature of their chemical bonding, as they contain both metal-metal and metal-main group (e.g. halide, chalcogenide) interactions. Finding new members of this class of materials presents synthetic challenges, as attempts to make them often result in phase separation into binary compounds. We overcome this difficulty by utilizing indium as a metal flux to synthesize large (mm scale) single crystals of novel subchalcogenide materials. Herein, we report two new compounds Ir2In8Q (Q = Se, Te) and compare their structural and electrical properties to the previously reported Ir2In8S analogue. Ir2In8Se and Ir2In8Te crystallize in the P42/mnm space group and are isostructural to Ir2In8S but also have commensurately modulated (with q-vectors q = 1/6a* + 1/6b* and q= 1/10a* + 1/10b* for Ir2In8Se and Ir2In8Te, respectively) low temperature phase transitions, where the chalcogenide anions in the channels experience a distortion in the form of In-Q bond alternation along the ab plane. Both compounds display re-entrant structural behavior, where the supercells appear on cooling but revert to the original subcell below 100 K, suggesting competing structural and electronic interactions dictate the overall structure. Notably, these materials are topological semimetal candidates with symmetry-protected Dirac crossings near the Fermi level, and exhibit high electron mobilities (~1500 cm2 V-1 s-1 at 1.8 K) and moderate carrier concentrations (~1020 cm-3) from charge transport measurements. This work highlights metal flux as a powerful synthetic route to high quality single crystals of novel intermetallic subchalcogenides

Structure of charge density waves in La1.875 Ba0.125 CuO4

Although charge density wave (CDW) correlations exist in several families of cuprate superconductors, they exhibit substantial variation in CDW wave vector and correlation length, indicating a key role for CDW-lattice interactions. We investigated this interaction in La1.875Ba0.125CuO4 using single-crystal x-ray diffraction to collect a large number of CDW peak intensities and determined the Cu and La/Ba atomic distortions induced by the formation of CDW order. Within the CuO2 planes, the distortions involve a periodic modulation of the Cu-Cu spacing along the direction of the ordering wave vector. The charge ordering within the copper-oxygen layer induces an out-of-plane breathing modulation of the surrounding lanthanum layers, which leads to a related distortion on the adjacent copper-oxygen layer. Our result implies that the CDW-related structural distortions do not remain confined to a single layer but rather propagate an appreciable distance through the crystal. This leads to overlapping structural modulations, in which CuO2 planes exhibit distortions arising from the orthogonal CDWs in adjacent layers as well as distortions from the CDW within the layer itself. We attribute this striking effect to the weak c-axis charge screening in cuprates and suggest this effect could help couple the CDWs between adjacent planes in the crystal

De Novo Design of Bioactive Protein-Resembling Nanospheres via Dendrimer-Templated Peptide Amphiphile Assembly

Self-assembling peptide amphiphiles (PAs) have been extensively used in the development of novel biomaterials. Because of their propensity to form cylindrical micelles, their use is limited in applications where small spherical micelles are desired. Here we present a platform method for controlling the self-assembly of biofunctional PAs into spherical 50 nm particles using dendrimers as shape-directing scaffolds. This templating approach results in biocompatible, stable protein-like assemblies displaying peptides with native secondary structure and biofunctionality

Lattice dynamical analogies and differences between SrTiO3 and EuTiO3 revealed by phonon-dispersion relations and double-well potentials

A comparative analysis of the structural phase transitions of EuTiO3 and SrTiO3 (at TS = 282 and 105 K, respectively) is made on the basis of phonon-dispersion and density functional calculations. The phase transition of EuTiO3 is predicted to arise from the softening of a transverse acoustic zone-boundary mode caused by the rotations of the TiO6 octahedra, as also found for the phase transition of SrTiO3. While the temperature dependence of the soft mode is similar in both compounds, their elastic properties differ drastically due to a large difference in the double-well potentials associated with the soft zone boundary-acoustic mode.Comment: 16 pages, 6 figure

Structure of Charge Density Waves in La1.875_{1.875}Ba0.125_{0.125}CuO4_4

Although charge-density wave (CDW) correlations exist in several families of cuprate supercon-ductors, they exhibit substantial variation in CDW wavevector and correlation length, indicating a key role for CDW-lattice interactions. We investigated this interaction in La$_{1.875}$Ba$_{0.125}$CuO$_4$ using single crystal x-ray diffraction to collect a large number of CDW peak intensities, and determined the Cu and La/Ba atomic distortions induced by the formation of CDW order. Within the CuO$_2$ planes, the distortions involve a periodic modulation of the Cu-Cu spacing along the direction of the ordering wave vector. The charge ordering within the copper-oxygen layer induces an out-of-plane breathing modulation of the surrounding lanthanum layers, which leads to a related distortion on the adjacent copper-oxygen layer. Our result implies that the CDW-related structural distortions do not remain confined to a single layer but rather propagate an appreciable distance through the crystal. This leads to overlapping structural modulations, in which CuO$_2$ planes exhibit distortions arising from the orthogonal CDWs in adjacent layers as well as distortions from the CDW within the layer itself. We attribute this striking effect to the weak c-axis charge screening in cuprates and suggest this effect could help couple the CDW between adjacent planes in the crystal.Comment: 9 pages; Accepted in Phys. Rev.

The SPAIR method: Isolating incident and reflected directional wave spectra in multidirectional wave basins

Wave tank tests aiming to reproduce realistic or site specific conditions will commonly involve using directionally spread, short-crested sea states. The measurement of these directional characteristics is required for the purposes of calibrating and validating the modelled sea state. Commonly used methods of directional spectrumreconstruction, based on directional spreading functions, have an inherent level of uncertainty associated with them. In this paper we aim to reduce the uncertainty in directional spectrum validation by introducing the SPAIR (Single-summation PTPD Approach with In-line Reflections) method, in combination with a directional wave gauge array. A variety of wave conditions were generated in the FloWave Ocean Energy Research Facility, Edinburgh, UK, to obtain a range of sea state and reflection scenarios. The presented approach is found to provide improved estimates of directional spectra over standardmethods, reducing the mean apparent directional deviation down to below 6% over the range of sea states. Additionally, the method isolates incident and reflected spectra in both the frequency and time domain, and can separate these wave systems over 360°. The accuracy of themethod is shown to be only slightly sensitive to the level of in-line reflectionpresent,but at present cannot dealwithoblique reflections. The SPAIRmethod, as presented or with slightmodification, will allow complex directional sea states to be validated more effectively, enabling multidirectional wave basins to simulate realistic wave scenarios with increased confidence