965 research outputs found
1995, Spatial and temporal variability of late Neogene equatorial Pacific carbonate
High-resolution, continuous records of GRAPE wet bulk density (a carbonate proxy) from Ocean Drilling Program Leg 138 provide one the opportunity for a detailed study of eastern equatorial Pacific Ocean carbonate sedimentation during the last 6 m.y. The transect of sites drilled spans both latitude and longitude in the eastern equatorial Pacific from 90° to 110°W and from 5°S to 10°N. Two modes of variability are resolved through the use of Empirical Orthogonal Function (EOF) analysis. In the presence of large tectonic and climatic boundary condition changes over the last 6 m.y., the dominant mode of spatial variability in carbonate sedimentation is remarkably constant. The first mode accounts for over 50% of the variance in the data, and is consistent with forcing by equatorial divergence. This mode characterizes both carbonate concentration and carbonate mass accumulation rate time series. Variability in the first mode is highly coherent with insolation, indicating a strong linear relationship between equatorial Pacific car bonate sedimentation and Milankovitch variability. Frequency domain analysis indicates that the coupling to equatorial divergence in carbonate sedimentation is strongest in the precession band (19-23 k.y.) and weakest though present at lower frequencies. The second mode of variability has a consistent spatial pattern of east-west asymmetry over the past 4 m.y. only; prior to 4 Ma, a different mode of spatial variability may have been present, possibly suggesting influence by closure of the Isthmus of Panama or other tectonic changes. The second mode of variability may indicate influence by CaCO3 dissolution. The second mode of variability is not highly coherent with insolation. Comparison of the modes of carbonate variability to a 4 m.y. record of benthic δ 1 8 indicates that although overall correlation between carbonate and δ 1 8 is low, both modes of variability in carbonate sedimentation are coherent with δ 1 8 changes at some frequencies. The first mode of carbonate variability is coherent with Sites 846/849 δ 1 8 at the dominant insolation periods, and the second mode is coherent at 100 k.y. during the last 2 m.y. The coherence between carbonate sedimentation and δ 1 8 in both EOF modes suggests that multiple uncorrelated modes of variability operated within the climate system during the late Neogene
Downhole Logging as a Paeoceanographic Tool on Ocean Drilling Program Leg 138: Interface Between High-Resolution Stratigraphy and Regional Syntheses
On Ocean Drilling Program (ODP) Leg 138, standard shipboard procedures were modified to allow for the real-time monitoring of several laboratory core-scanning systems that provide centimeter-scale measurements of saturated bulk density, magnetic susceptibility and digital color reflectance. These continuous, high-resolution data sets were used to ensure the proper offset of multiple holes and to splice together complete sedimentary sections. Typically, the spliced, continuous sediment sections were found to be about 10% longer than the section drilled, as measured by the length of the drill string. While the source of this elongation is not yet fully understood, it must be compensated for in order to property determine sediment fluxes and mass accumulation rates. Downhole logging, in conjunction with inverse correlation techniques provided a means to determine where the distortion occurred and to correct back to true in situ depths.
Downhole logging also provides a means, through the generation of synthetic seismograms, of precisely relating the paleoceanographic events found in the core record to the high-resolution seismic record. Once correlated to the seismic record, the spatial and temporal extent of paleoceanographic events can be traced well beyond the borehole. Most seismic events in the equatorial Pacific are related to rapid changes in carbonate content that, in turn, are related to both productivity events (often expressed as monospecific laminated diatom oozes) and times of enhanced dissolution. While many of these events may have oceanwide extent, others, like the absence of carbonate in the late-Miocene to Recent in the Guatemala Basin have been shown to be regional and confined to only the deeper portions of the Guatemala Basin. As we identify and trace specific paleoceanographic events in the seismic record, we can begin to explore the response of the ocean through gradients of latitude, productivity, and depth
Downhole logging as a paeoceanographic tool on ocean drilling program leg 138: Interface between high-resolution stratigraphy and regional syntheses
On Ocean Drilling Program (ODP) Leg 138, standard shipboard procedures were modified to allow for the real-time monitoring of several laboratory core-scanning systems that provide centimeter-scale measurements of saturated bulk density, magnetic susceptibility and digital color reflectance. These continuous, high-resolution data sets were used to ensure the proper offset of multiple holes and to splice together complete sedimentary sections. Typically, the spliced, continuousediment sections were found to be about 10% longer than the section drilled, as measured by the length of the drill string. While the source of this elongation is not yet fully understood, it must be compensated for in order to property determine sediment fluxes and mass accumulation rates. Downhole logging, in conjunction with inverse correlation techniques provided a means to determine where the distortion occurred and to correct back to true in sire depths. Downhole logging also provides a means, through the generation of synthetic seismograms, of precisely relating the paleoceanographic events found in the core record to the high-resolution seismic record. Once correlated to the seismic record, the spatial and temporal extent of paleoceanographic events can be traced well beyond the borehole. Most seismic events in the equatorial Pacific are related to rapid changes in carbonate contenthat, in turn, are related to both productivity events (often expressed as monospecific laminated diatom oozes) and times of enhanced dissolution. While many of these events may have oceanwide extent, others, like the absence of carbonate in the late-Miocene to Recent in the Guatemala Basin have been shown to be regional and confined to only the deeper portions of the Guatemala Basin. As we identify and trace specific paleoceanographic events in the seismic record, we can begin to explore the response of the ocean through gradients of latitude, productivity, and depth
New iron-based Heusler compounds Fe2YZ: Comparison with theoretical predictions of the crystal structure and magnetic properties
The present work reports on the new soft ferromagnetic Heusler phases
Fe2NiGe, Fe2CuGa, and Fe2CuAl, which in previous theoretical studies have been
predicted to exist in a tetragonal regular Heusler structure. Together with the
known phases Fe2CoGe and Fe2NiGa these materials have been synthesized and
characterized by powder XRD, 57 Fe M\"ossbauer spectroscopy, SQUID and EDX
measurements. In particular M\"ossbauer spectroscopy was used to monitor the
degree of local atomic order/disorder and to estimate magnetic moments at the
Fe sites from the hyperfine fields. It is shown that in contrast to the
previous predictions all the materials except Fe2NiGa basically adopt the
inverse cubic Heusler (X-) structure with differing degrees of disorder. The
disorder is more enhanced in case of Fe2NiGa, which was predicted as an inverse
Heusler phase. The experimental data are compared with results from ab-inito
electronic structure calculations on LDA level incorporating the effects of
atomic disorder by using the coherent potential approximation (CPA). A good
agreement between calculated and experimental magnetic moments is found for the
cubic inverse Heusler phases. Model calculations on various atomic
configurations demonstrate that antisite disorder tends to enhance the
stability of the X-structure. Given the fundamental scientific and
technological importance of tetragonal Heusler phases the present results call
for further investigations to unravel the factors stabilizing tetragonal
Heusler materials
Ferromagnetism and superconductivity in P-doped CeFeAsO
We report on superconductivity in CeFeAs1-xPxO and the possible coexistence
with Ce- ferromagnetism (FM) in a small homogeneity range around x = 30% with
ordering temperatures of T_SC = T_C = 4K. The antiferromagnetic (AFM) ordering
temperature of Fe at this critical concentration is suppressed to T^N_Fe ~ 40K
and does not shift to lower temperatures with further increase of the P
concentration. Therefore, a quantum-critical-point scenario with T^N_Fe -> 0K
which is widely discussed for the iron based superconductors can be excluded
for this alloy series. Surprisingly, thermal expansion and X-ray powder
diffraction indicate the absence of an orthorhombic distortion despite clear
evidence for short range AFM Fe-ordering from muon-spin-rotation measurements.
Furthermore, we discovered the formation of a sharp electron spin resonance
signal unambiguously connected with the emergence of FM ordering.Comment: 5 pages, 4 figures, published in Phys. Rev. B (Rapid Communication,
Editors suggestion
AC/DC Susceptibility of the Heavy-Fermion Superconductor CePt3Si under Pressure
We have investigated the pressure dependence of ac and dc susceptibilities of
the heavy-fermion superconductor CePt3Si (Tc= 0.75 K) that coexists with
antiferromagnetism (TN = 2.2 K). As hydrostatic pressure is increased, Tc first
decreases rapidly, then rather slowly near the critical pressure Pc = 0.6 GPa
and shows a stronger decrease again at higher pressures, where Pc is the
pressure at which TN becomes zero. A transition width and a difference in the
two transition temperatures defined in the form of structures in the
out-of-phase component of ac susceptibilities also become small near Pc,
indicating that a double transition observed in CePt3Si is caused by some
inhomogeneous property in the sample that leads to a spatial variation of local
pressure. A sudden increase in the Meissner fraction above Pc suggests the
influence of antiferromagnetism on superconductivity.Comment: 4 pages with 5 figures. This paper will be published in J. Phys. Soc.
Jp
Fluctuation-Driven Quantum Phase Transitions in Clean Itinerant Ferromagnets
The quantum phase transition in clean itinerant ferromagnets is analyzed. It
is shown that soft particle-hole modes invalidate Hertz's mean-field theory for
. A renormalized mean-field theory predicts a fluctuation-induced
first order transition for , whose stability is analyzed by
renormalization group techniques. Depending on microscopic parameter values,
the first order transition can be stable, or be pre-empted by a
fluctuation-induced second order transition. The critical behavior at the
latter is determined. The results are in agreement with recent experiments.Comment: 4 pp., REVTeX, no figs; final version as publishe
The quantum critical point in CeRhIn_5: a resistivity study
The pressure--temperature phase diagram of CeRhIn_5 has been studied under
high magnetic field by resistivity measurements. Clear signatures of a quantum
critical point has been found at a critical pressure of p_c = 2.5 GPa. The
field induced magnetic state in the superconducting state is stable up to the
highest field. At p_c the antiferromagnetic ground-state under high magnetic
field collapses very rapidly. Clear signatures of p_c are the strong
enhancement of the resistivity in the normal state and of the inelastic
scattering term. No clear T2 temperature dependence could be found for
pressures above T_c. From the analysis of the upper critical field within a
strong coupling model we present the pressure dependence of the coupling
parameter lambda and the gyromagnetic ratio g. No signatures of a spatially
modulated order parameter could be evidenced. A detailed comparison with the
magnetic field--temperature phase diagram of CeCoIn_5 is given. The comparison
between CeRhIn_5 and CeCoIn_5 points out the importance to take into account
the field dependence of the effective mass in the calculation of the
superconducting upper critical field H_c2. It suggests also that when the
magnetic critical field H_(0) becomes lower than H_c2 (0)$, the persistence of
a superconducting pseudo-gap may stick the antiferromagnetism to H_c2 (0).Comment: 15 pages, 20 figures, to be published in J. Phys. Soc. Jp
The GREGOR Fabry-P\'erot Interferometer
The GREGOR Fabry-P\'erot Interferometer (GFPI) is one of three first-light
instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio
del Teide, Tenerife, Spain. The GFPI uses two tunable etalons in collimated
mounting. Thanks to its large-format, high-cadence CCD detectors with
sophisticated computer hard- and software it is capable of scanning spectral
lines with a cadence that is sufficient to capture the dynamic evolution of the
solar atmosphere. The field-of-view (FOV) of 50" x 38" is well suited for quiet
Sun and sunspot observations. However, in the vector spectropolarimetric mode
the FOV reduces to 25" x 38". The spectral coverage in the spectroscopic mode
extends from 530-860 nm with a theoretical spectral resolution R of about
250,000, whereas in the vector spectropolarimetric mode the wavelength range is
at present limited to 580-660 nm. The combination of fast narrow-band imaging
and post-factum image restoration has the potential for discovery science
concerning the dynamic Sun and its magnetic field at spatial scales down to
about 50 km on the solar surface.Comment: 14 pages, 17 figures, 4 tables; pre-print of AN 333, p.880-893, 2012
(AN special issue to GREGOR
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