Mode Mixing and Rotational Splittings: I. Near-Degeneracy Effects Revisited

Rotation is typically assumed to induce strictly symmetric rotational splitting into the rotational multiplets of pure p- and g-modes. However, for evolved stars exhibiting mixed modes, avoided crossings between different multiplet components are known to yield asymmetric rotational splitting, particularly for near-degenerate mixed-mode pairs, where notional pure p-modes are fortuitiously in resonance with pure g-modes. These near-degeneracy effects have been described in subgiants, but their consequences for the characterisation of internal rotation in red giants has not previously been investigated in detail, in part owing to theoretical intractability. We employ new developments in the analytic theory of mixed-mode coupling to study these near-resonance phenomena. In the vicinity of the most p-dominated mixed modes, the near-degenerate intrinsic asymmetry from pure rotational splitting increases dramatically over the course of stellar evolution, and depends strongly on the mode mixing fraction $\zeta$. We also find that a linear treatment of rotation remains viable for describing the underlying p- and g-modes, even when it does not for the resulting mixed modes undergoing these avoided crossings. We explore observational consequences for potential measurements of asymmetric mixed-mode splitting, which has been proposed as a magnetic-field diagnostic. Finally, we propose improved measurement techniques for rotational characterisation, exploiting the linearity of rotational effects on the underlying p/g modes, while still accounting for these mixed-mode coupling effects.Comment: 21 pages, 13 figures. Accepted to Ap

XPS and EELS characterization of Mn2SiO4, MnSiO3 and MnAl2O4

NSERCX-ray Photoelectron Spectroscopy (XPS) and Electron Energy Loss Spectroscopy (EELS) are strong candi- date techniques for characterizing steel surfaces and substrate-coating interfaces when investigating the selective oxidation and reactive wetting of advanced high strength steels (AHSS) during the continuous galvanizing process. However, unambiguous identification of ternary oxides such as Mn2SiO4, MnSiO3, and MnAl2O4 by XPS or EELS, which can play a significant role in substrate reactive wetting, is difficult due to the lack of fully characterized standards in the literature. To resolve this issue, samples of Mn2 SiO4 , MnSiO3 and MnAl2O4 were synthesized and characterized by XPS and EELS. The unique features of the XPS and EELS spectra for the Mn2 SiO4 , MnSiO3 and MnAl2 O4 standards were successfully derived, thereby allowing investigators to fully differentiate and identify these oxides at the surface and subsurface of Mn, Si and Al alloyed AHSS using these techniques

Operando analysis of a solid oxide fuel cell by environmental transmission electron microscopy

Correlating the microstructure of an energy conversion device to its performance is often a complex exercise, notably in solid oxide fuel cell (SOFC) research. SOFCs combine multiple materials and interfaces that evolve in time due to high operating temperatures and reactive atmospheres. We demonstrate here that operando environmental transmission electron microscopy can simplify the identification of structure-property links in such systems. By contacting a cathode-electrolyte-anode cell to a heating and biasing microelectromechanical system in a single-chamber configuration, a direct correlation is found between the environmental conditions (O2 and H2 partial pressures, temperature), the cell voltage, and the microstructural evolution of the fuel cell, down to the atomic scale. The results shed new insights into the impact of the anode oxidation state and its morphology on the cell electrical properties.Comment: 18 pages, 5 figure

New U-Pb ages for syn-orogenic magmatism in the SW sector of the Ossa Morena Zone (Portugal)

The Ossa-Morena Zone (OMZ) is a major geotectonic unit within the Iberian Massif (which constitutes an important segment of the European Variscan Belt) and one of its distinguishing features is the presence of a noteworthy compositional diversity of plutonic rocks. In the SW sector of the OMZ, the tonalitic Hospitais intrusion (located to the W of Montemor-o-Novo) is considered a typical example of syn-orogenic magmatism, taking into account that both the long axis of the plutonic body and its mesoscopic foliation are oriented parallel to the Variscan WNW-ESE orientation. Another relevant feature of the Hospitais intrusion is the occurrence of mafic microgranular enclaves within the main tonalite. In previous works (Moita et al., 2005; Moita, 2007), it was proposed that: (1) the Hospitais intrusion is part of a calc-alkaline suite, represented by a large number of intrusions in this sector of the OMZ, ranging from gabbros to granites; (2) the enclaves are co-genetic to the host tonalite in the Hospitais pluton. In this study, zircon populations from one sample of the main tonalite (MM-17) and one sample of the associated enclave (MM-17E) were analysed by ID-TIMS for U-Pb geochronology. In each sample, three fractions of nice glassy, euhedral, long prismatic and inclusion free crystals were analysed. The results from the three fractions of MM-17 yielded a 206Pb/238U age of 337.0 ± 2.0 Ma. Similarly, for the enclave MM-17E a 206Pb/238U zircon age of 336.5 ± 0.47 Ma was obtained. These identical ages, within error, are in agreement with a common parental magma for the tonalite and mafic granular enclaves. Similar U-Pb ages have been reported in previous works for plutonic and metamorphic events in this region (e.g.: Pereira et al., 2009; Antunes et al., 2011). Furthermore, also in the SW sector of the OMZ, palaeontological studies (Pereira et al., 2006; Machado & Hladil, 2010) carried out in Carboniferous sedimentary basins containing intercalated calc-alkaline volcanics (Santos et al., 1987; Chichorro, 2006) have shown that they are mainly of Visean age. Therefore, magmatism displaying features typical of continental arc setting seems to have been active in this part of the OMZ during the Lower Carboniferous times

Atomic-Scale Study of Metal–Oxide Interfaces and Magnetoelastic Coupling in Self-Assembled Epitaxial Vertically Aligned Magnetic Nanocomposites

Vertically aligned nanocomposites (VANs) of metal/oxide type have recently emerged as a novel class of heterostructures with great scientific and technological potential in the fields of nanomagnetism, multiferroism, and catalysis. One of the salient features of these hybrid materials is their huge vertical metal/oxide interface, which plays a key role in determining the final magnetic and/or transport properties of the composite structure. However, in contrast to their well‐studied planar counterparts, detailed information on the structural features of vertical interfaces encountered in VANs is scarce. In this work, high resolution scanning transmission electron microscopy (STEM) and electron energy‐loss spectroscopy (EELS) are used to provide an element selective atomic‐scale analysis of the interface in a composite consisting of ultrathin, self‐assembled Ni nanowires, vertically epitaxied in a SrTiO3/SrTiO3(001) matrix. Spectroscopic EELS measurements evidence rather sharp interfaces (6–7 Å) with the creation of metallic NiTi bonds and the absence of nickel oxide formation is confirmed by X‐ray absorption spectroscopy measurements. The presence of these well‐defined phase boundaries, combined with a large lattice mismatch between the oxide and metallic species, gives rise to pronounced magnetoelastic effects. Self‐assembled columnar Ni:SrTiO3 composites thus appear as ideal model systems to explore vertical strain engineering in metal/oxide nanostructures

Asteroseismic Signatures of Core Magnetism and Rotation in Hundreds of Low-Luminosity Red Giants

Red Giant stars host solar-like oscillations which have mixed character, being sensitive to conditions both in the outer convection zone and deep within the interior. The properties of these modes are sensitive to both core rotation and magnetic fields. While asteroseismic studies of the former have been done on a large scale, studies of the latter are currently limited to tens of stars. We aim to produce the first large catalogue of both magnetic and rotational perturbations. We jointly constrain these parameters by devising an automated method for fitting the power spectra directly. We successfully apply the method to 302 low-luminosity red giants. We find a clear bimodality in core rotation rate. The primary peak is at $\delta \nu_{\mathrm{rot}}$ = 0.32 $\mu$Hz, and the secondary at $\delta \nu_{\mathrm{rot}}$ = 0.47 $\mu$Hz. Combining our results with literature values, we find that the percentage of stars rotating much more rapidly than the population average increases with evolutionary state. We measure magnetic splittings of 2$\sigma$ significance in 23 stars. While the most extreme magnetic splitting values appear in stars with masses > 1.1M$_{\odot}$, implying they formerly hosted a convective core, a small but statistically significant magnetic splitting is measured at lower masses. Asymmetry between the frequencies of a rotationally split multiplet has previously been used to diagnose the presence of a magnetic perturbation. We find that of the stars with a significant detection of magnetic perturbation, 43\% do not show strong asymmetry. We find no strong evidence of correlation between the rotation and magnetic parameters

Imaging the Spatial Distribution of Electronic States in Graphene Using Electron Energy-Loss Spectroscopy: Prospect of Orbital Mapping

The spatial distributions of antibonding π∗ and σ∗ states in epitaxial graphene multilayers are mapped using electron energy-loss spectroscopy in a scanning transmission electron microscope. Inelastic channeling simulations validate the interpretation of the spatially resolved signals in terms of electronic orbitals, and demonstrate the crucial effect of the material thickness on the experimental capability to resolve the distribution of unoccupied states. This work illustrates the current potential of core-level electron energy-loss spectroscopy towards the direct visualization of electronic orbitals in a wide range of materials, of huge interest to better understand chemical bonding among many other properties at interfaces and defects in solids