Anisotropic Magnetoresistance in Charge-Ordering Na0.34(H3O)0.15CoO2Na_{0.34}(H_3O)_{0.15}CoO_2:Strong Spin-Charge Coupling and Spin Ordering

Angular-dependent in-plane magnetoresistance (AMR) for single crystal $Na_{0.34}(H_3O)_{0.15}CoO_2$ with charge ordering is studied systematically. The anisotropic magnetoresistance shows a twofold symmetry at high temperature with rotating H in the Co-O plane, while a sixfold symmetry below a certain temperature ($T_\rho$). At $T_\rho$, the symmetry of AMR changes from twofold to fourfold with rotating magnetic field (H) in the plane consisting of the current and c-axis. The variation of AMR symmetry with temperature arises from the subtle changes of the spin structure. These results give a direct evidence for the itinerant electrons directly coupled to the localized spins.Comment: 4 pages, 4 figure

Large enhancement of the thermopower in Nax_xCoO2_2 at high Na doping

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate $\rm Na_xCoO_2$ displays several interesting electronic phases as $x$ is varied including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, $\rm Na_xCoO_2$ displays moderately large thermopower $S$ and conductivity $\sigma$. However, the prospects for thermoelectric cooling applications faded when the figure of merit $Z$ was found to be small at this composition (0.6$<x<$0.7). Here we report that, in the poorly-explored high-doping region $x>$0.75, $S$ undergoes an even steeper enhancement. At the critical doping $x_p\sim$ 0.85, $Z$ (at 80 K) reaches values $\sim$40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.Comment: 6 pages, 7 figure

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterised by a complex pattern of faulting, thin continental fault blocks, and the serpentinisation, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Travel-time tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here we demonstrate the application of two-dimensional (2D) time-domain acoustic full-waveform inversion to deep water seismic data collected at the Deep Galicia Margin, in order to attain a high resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modelled waveforms, checkerboard tests, testing of parameter and inversion strategy, and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinisation. This study supports the hypothesis that normal faulting acts to hydrate the upper mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinisation. Our results confirm the feasibility of applying the full-waveform inversion method to sparse, deep water crustal datasets

Moho depth and crustal thinning in the Marmara Sea region from gravity data inversion

The free‐air gravity in the Marmara Sea reveals that the low density of sedimentary basins is partly compensated in the lower crust. We compiled geophysical upper crust studies to determine the sediment basin geometries in and around the Marmara Sea and corrected the gravity signal from this upper crust geology with the Parker method. Then, assuming long wavelength anomalies in the residual gravity signal is caused by variations in the Moho topography, we inverted the residual to build the Moho topography. The result shows that the Moho is uplifted on an area greater than the Marmara Sea with a maximum crust thinning beneath the basins where the Moho is at about 25 km, 5 km above the reference depth. We then evaluated the Neogene extension by comparing the surface covered by our 3‐D thinned model with the surface covered by an unthinned model with same crustal volume. Comparing this surface with areal extension rate from GPS data, we found a good compatibility indicating that the extension rate averaged over the Sea of Marmara area probably remained close to its present‐day value during major changes of tectonic regime, as the incursion of the North Anatolian Fault system during the Pliocene leads to the establishment of the dominantly strike‐slip present‐day system. We also show that crustal extension is distributed over a wider domain in the lower crust than in the upper crust, and that this may be accounted for by a relatively minor component of lower crustal ductile flow

Phase segregation in NaxCoO2 for large Na contents

We have investigated a set of sodium cobaltates (NaxCoO2) samples with various sodium content (0.67 \le x \le 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of 59Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x=2/3 composition which appears to be the most stable phase in this part of phase diagram.Comment: 5 pages, 4 figure

Precise Control of Band Filling in NaxCoO2

Electronic properties of the sodium cobaltate NaxCoO2 are systematically studied through a precise control of band filling. Resistivity, magnetic susceptibility and specific heat measurements are carried out on a series of high-quality polycrystalline samples prepared at 200 C with Na content in a wide range of 0.35 =< x =< 0.70. It is found that dramatic changes in electronic properties take place at a critical Na concentration x* that lies between 0.58 and 0.59, which separates a Pauli paramagnetic and a Curie-Weiss metals. It is suggested that at x* the Fermi level touches the bottom of the a1g band at the gamma point, leading to a crucial change in the density of states across x* and the emergence of a small electron pocket around the gamma point for x > x*.Comment: 4 pages, 5 figures, submitted to J. Phys. Soc. Jp

Anisotropic Physical Properties of Mafic and Ultramafic Rocks From an Oceanic Core Complex

We analyzed the physical properties of altered mafic and ultramafic rocks drilled at the Atlantis Massif (Mid‐Atlantic Ridge, 30°N; Integrated Ocean Discovery Program Expeditions 304‐305 and 357). Our objective was to find a physical property that allows direct distinction between these lithologies using remote geophysical methods. Our data set includes the density, the porosity, P and S wave velocities, the electrical resistivity, and the permeability of mafic and ultramafic samples under shallow subsurface conditions (confining pressure up to 50 MPa equivalent to ~2‐km depth). In shallow subsurface conditions, mafic and ultramafic samples showed distinct differences in the density, the seismic wave velocities, and the electrical resistivity (mafic samples: 2,840 to 2,860 kg/m3, 5.92 to 6.70 km/s, and 60 to 221 Ω m; ultramafic samples: 2,370 to 2,790 kg/m3, 3.36 and 3.62 km/s, and 8 to 44 Ω m). However, we observed an overlap between physical properties of mafic and ultramafic rocks when we compared our measurements with those acquired from similar environments. The anisotropic homogeneous electrical resistivity inversion shows transverse isotropy symmetry, which is typical of a foliated microstructure. In both the inversion results and the thin sections, the direction of high resistivity axes of ultramafic rock samples is systematically perpendicular to the equivalent axes in mafic rock samples analyzed in this study. Our sample scale study suggests that electrical resistivity anisotropy may allow us to distinguish mafic and ultramafic lithologies via controlled source electromagnetic surveys. When surface conduction is negligible, the electrical resistivity can be used as proxy for permeability