120 research outputs found
Lifetimes of antiferromagnetic magnons in two and three dimensions: experiment, theory, and numerics
A high-resolution neutron spectroscopic technique is used to measure
momentum-resolved magnon lifetimes in the prototypical two- and
three-dimensional antiferromagnets Rb2MnF4 and MnF2, over the full Brillouin
zone and a wide range of temperatures. We rederived theories of the lifetime
resulting from magnon-magnon scattering, thereby broadening their applicability
beyond asymptotically small regions of wavevector and temperature.
Corresponding computations, combined with a small contribution reflecting
collisions with domain boundaries, yield excellent quantitative agreement with
the data.Comment: 5 pages, 4 figure
Large enhancement of the thermopower in NaCoO 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
displays several interesting electronic phases as is varied
including water-induced superconductivity and an insulating state that is
destroyed by field. Initial measurements showed that, in the as-grown
composition, displays moderately large thermopower and
conductivity . However, the prospects for thermoelectric cooling
applications faded when the figure of merit was found to be small at this
composition (0.60.7). Here we report that, in the poorly-explored
high-doping region 0.75, undergoes an even steeper enhancement. At the
critical doping 0.85, (at 80 K) reaches values 40 times
larger than in the as-grown crystals. We discuss prospects for low-temperature
thermoelectric applications.Comment: 6 pages, 7 figure
Synthesis, Characterization, and Magnetic Properties of gamma-NaxCoO2 (0.70 < x <0.84)
Powder NaCoO () samples were synthesized and
characterized carefully by X-ray diffraction analysis, inductive-coupled plasma
atomic emission spectroscopy, and redox titration. It was proved that
-NaCoO is formed only in the narrow range of . Nevertheless, the magnetic properties depend strongly on . We
found, for the first time, two characteristic features in the magnetic
susceptibility of NaCoO, a sharp peak at K and an
anomaly at K, as well as the transition at K and the broad
maximum at K which had already been reported. A type of weak
ferromagnetic transition seems to occur at . The transition at ,
which is believed to be caused by spin density wave formation, was observed
clearly for with constant and independent of .
On the other hand, ferromagnetic moment varies systematically depending on .
These facts suggest the occurrence of a phase separation at the microscopic
level, such as the separation into Na-rich and Na-poor domains due to the
segregation of Na ions. The magnetic phase diagram and transition mechanism
proposed previously should be reconsidered.Comment: 4 pages (2 figures included) and 2 extra figures (gif), to be
published in J. Phys. Soc. Jpn. 73 (8) with possible minor revision
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
Bulk antiferromagnetism in single crystals
Susceptibility, specific heat, and muon spin rotation measurements on
high-quality single crystals of have revealed bulk
antiferromagnetism with N\'{e}el temperature K and an
ordered moment perpendicular to the layers. The magnetic order
encompasses nearly 100% of the crystal volume. The susceptibility exhibits a
broad peak around 30 K, characteristic of two-dimensional antiferromagnetic
fluctuations. The in-plane resistivity is metallic at high temperatures and
exhibits a minimum at .Comment: published versio
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
A critical assessment of the pairing symmetry in NaxCoO2.yH2O
We examine each of the symmetry-allowed pairing states of NaxCoO2.yH2O and
compare their properties to what is experimentally and theoretically
established about the compound. In this way, we can eliminate the vast majority
of states that are technically allowed and narrow the field to two, both of
f-wave type states. We discuss the expected features of these states and
suggest experiments that can distinguish between them. We also discuss
odd-frequency gap pairing and how it relates to available experimental
evidence
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
Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: similarities with the Tohoku forearc
The 300 km long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments coordinated within the ¿Thales was right¿ proposal to the European Union action (Laigle et al., Tectonophys., in rev.) The top of the subducting plate can be followed under the wide accretionary wedge by a dense grid of dip- and strike-lines of multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop thrust onto the slab. Two OBS refraction seismic profiles constrained a 26 km large crustal thickness from the volcanic arc throughout the forearc domain (Kopp et al., EPSL, 2011). These new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone, in the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes from the dense temporary array of 80 OBS and land seismometers appears with kinks which increase the dip from 10-20° under the forearc domain, to 60° on the segment from 70 km depth down to under the volcanic arc. There, at 140 km depth just north of Martinique the 2007 M 7.4 earthquake, largest for half a century, was accompanied by an increased seismic activity over the whole depth range, which provides a new focused image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku ( NE Japan) and since in some other subduction zones. Other types of seismic activity uniquely observed in Tohoku, are resolved now here, such as ¿supraslab¿ earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and ¿deep flat-thrust¿ earthquakes at 45 km depth on the interplate fault under the Caribbean plate forearc mantle. None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is serpentinized by the fluids provided from the dehydrating slab beneath, and which is commonly considered as limiting the downward extent of the interplate seismic coupling. If the upper plate here comprised lithospheric segments related to the earlier formation of the Caribbean oceanic plateau by the material advection from a mantle plume, it could then be underlain by a correspondingly modified, heterogeneous mantle, which may impose regions of stick-slip behaviour on the interplate under the mantle corner among stable-gliding areas. The Tohoku 2011 M9 earthquake was unexpected not only in its slip reaching to the trench, but also in its slip reaching far under the mantle corner against the serpentinization decoupling paradigm, and its structural setting may be revisited for resolving corresponding structural heterogeneityPeer Reviewe
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