757 research outputs found
Infrared spectroscopy of solid CO-CO2 mixtures and layers
The spectra of pure, mixed and layered CO and CO2 ices have been studied
systematically under laboratory conditions using infrared spectroscopy. This
work provides improved resolution spectra (0.5 cm-1) of the CO2 bending and
asymmetric stretching mode, as well as the CO stretching mode, extending the
existing Leiden database of laboratory spectra to match the spectral resolution
reached by modern telescopes and to support the interpretation of the most
recent data from Spitzer. It is shown that mixed and layered CO and CO2 ices
exhibit very different spectral characteristics, which depend critically on
thermal annealing and can be used to distinguish between mixed, layered and
thermally annealed CO-CO2 ices. CO only affects the CO2 bending mode spectra in
mixed ices below 50K under the current experimental conditions, where it
exhibits a single asymmetric band profile in intimate mixtures. In all other
ice morphologies the CO2 bending mode shows a double peaked profile, similar to
that observed for pure solid CO2. Conversely, CO2 induces a blue-shift in the
peak-position of the CO stretching vibration, to a maximum of 2142 cm-1 in
mixed ices, and 2140-2146 cm-1 in layered ices. As such, the CO2 bending mode
puts clear constraints on the ice morphology below 50K, whereas beyond this
temperature the CO2 stretching vibration can distinguish between initially
mixed and layered ices. This is illustrated for the low-mass YSO HH46, where
the laboratory spectra are used to analyse the observed CO and CO2 band
profiles and try to constrain the formation scenarios of CO2.Comment: Accepted in A&
Seismic anisotropy in the Sumatra subduction zone
An important tool for understanding deformation occurring within a subduction zone is the measurement of seismic anisotropy through observations of shear wave splitting (SWS). In Sumatra, two temporary seismic networks were deployed between December 2007 and February 2009, covering the fore arc between the fore-arc islands to the back arc. We use SKS and local SWS measurements to determine the type, amount, and location of anisotropy. Local SWS measurements from the fore-arc islands exhibit trench-parallel fast directions which can be attributed to shape preferred orientation of cracks/fractures in the overriding sediments. In the Sumatran Fault region, the predominant fast direction is fault/trench parallel, while in the back-arc region it is trench perpendicular. The trench-perpendicular measurements exhibit a positive correlation between delay time and raypath length in the mantle wedge, while the fault-parallel measurements are similar to the fault-parallel fast directions observed for two crustal events at the Sumatran Fault. This suggests that there are two layers of anisotropy: one due to entrained flow within the mantle wedge and a second layer within the overriding crust due to the shear strain caused by the Sumatran Fault. SKS splitting results show a NNW-SSE fast direction with delay times of 0.8–3.0 s. The fast directions are approximately parallel to the absolute plate motion of the subducting Indo-Australian Plate. The small delay times exhibited by the local SWS (0.05–0.45 s), in combination with the large SKS delay times, suggest that the anisotropy generating the teleseismic SWS is dominated by entrained flow in the asthenosphere below the slab
Prospects for Improving the Intrinsic and Extrinsic Properties of Magnesium Diboride Superconducting Strands
The magnetic and transport properties of magnesium diboride films represent
performance goals yet to be attained by powder-processed bulk samples and
conductors. Such performance limits are still out of the reach of even the best
magnesium diboride magnet wire. In discussing the present status and prospects
for improving the performance of powder-based wire we focus attention on (1)
the intrinsic (intragrain) superconducting properties of magnesium diboride,
Hc2 and flux pinning, (2) factors that control the efficiency with which
current is transported from grain-to-grain in the conductor, an extrinsic
(intergrain) property. With regard to Item-(1), the role of dopants in Hc2
enhancement is discussed and examples presented. On the other hand their roles
in increasing Jc, both via Hc2 enhancement as well as direct
fluxoid/pining-center interaction, are discussed and a comprehensive survey of
Hc2 dopants and flux-pinning additives is presented. Current transport through
the powder-processed wire (an extrinsic property) is partially blocked by the
inherent granularity of the material itself and the chemical or other
properties of the intergrain surfaces. These and other such results indicate
that in many cases less than 15% of the conductor's cross sectional area is
able to carry transport current. It is pointed out that densification in
association with the elimination of grain-boundary blocking phases would yield
five-to ten-fold increases in Jc in relevant regimes, enabling the performance
of magnesium diboride in selected applications to compete with that of Nb-Sn
Field-Dependent Tilt and Birefringence of Electroclinic Liquid Crystals: Theory and Experiment
An unresolved issue in the theory of liquid crystals is the molecular basis
of the electroclinic effect in the smectic-A phase. Recent x-ray scattering
experiments suggest that, in a class of siloxane-containing liquid crystals, an
electric field changes a state of disordered molecular tilt in random
directions into a state of ordered tilt in one direction. To investigate this
issue, we measure the optical tilt and birefringence of these liquid crystals
as functions of field and temperature, and we develop a theory for the
distribution of molecular orientations under a field. Comparison of theory and
experiment confirms that these materials have a disordered distribution of
molecular tilt directions that is aligned by an electric field, giving a large
electroclinic effect. It also shows that the net dipole moment of a correlated
volume of molecules, a key parameter in the theory, scales as a power law near
the smectic-A--smectic-C transition.Comment: 18 pages, including 9 postscript figures, uses REVTeX 3.0 and
epsf.st
The structure of the Sumatran Fault revealed by local seismicity
[1] The combination of the Sunda megathrust and the (strike-slip) Sumatran Fault (SF) represents a type example of slip-partitioning. However, superimposed on the SF are geometrical irregularities that disrupt the local strain field. The largest such feature is in central Sumatra where the SF splits into two fault strands up to 35 km apart. A dense local network was installed along a 350 km section around this bifurcation, registering 1016 crustal events between April 2008 and February 2009. 528 of these events, with magnitudes between 1.1 and 6.0, were located using the double-difference relative location method. These relative hypocentre locations reveal several new features about the crustal structure of the SF. Northwest and southeast of the bifurcation, where the SF has only one fault strand, seismicity is strongly focused below the surface trace, indicating a vertical fault that is seismogenic to ∼15 km depth. By contrast intense seismicity is observed within the bifurcation, displaying streaks in plan and cross-section that indicate a complex system of faults bisecting the bifurcation. In combination with analysis of topography and focal mechanisms, we propose that the bifurcation is a strike-slip duplex system with complex faulting between the two main fault branches
Close Packing of Atoms, Geometric Frustration and the Formation of Heterogeneous States in Crystals
To describe structural peculiarities in inhomogeneous media caused by the
tendency to the close packing of atoms a formalism based on the using of the
Riemann geometry methods (which were successfully applied lately to the
description of structures of quasicrystals and glasses) is developed. Basing on
this formalism we find in particular the criterion of stability of precipitates
of the Frank-Kasper phases in metallic systems. The nature of the ''rhenium
effect'' in W-Re alloys is discussed.Comment: 14 pages, RevTex, 2 PostScript figure
Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in-situ processed MgB2 strands
Monocore powder-in-tube MgB2 strands were cold-drawn and heat-treated at 600C
and 700C for times of up to 71 hours and structure-property relationships
examined. Drawing-induced elongation of the Mg particles led, after HT, to a
textured macrostructure consisting of elongated polycrystalline MgB2 fibers
separated by elongated pores. The superconducting Tc, Jc and Fp were correlated
with the macrostructure and grain size. Grain size increased with HT time at
both 600C and 700C. Jc and hence Fp decreased monotonically but not linearly
with grain size. Overall, it was observed that at 700C, the MgB2 reaction was
more or less complete after as little as 30 min; at 600C, full reaction
completion did not occur until 71 h. into the HT. Transport, Jct(B) was
measured in a perpendicular applied field, and the magnetic critical current
densities, Jcm\bot(B) and Jcm{\phi}(B), were measured in perpendicular and
parallel (axial) applied fields, respectively. Particularly noticeable was the
premature dropoff of Jcm\bot(B) at fields well below the irreversibility field
of Jct(B). This effect is attributed to the fibrous macrostructure and its
accompanying anisotropic connectivity. Magnetic measurements with the field
directed along the strand axis yielded a critical density, Jcm\bot(B), for
current flowing transversely to the strand axis that was less than and dropped
off more rapidly than Jct(B). In the conventional magnetic measurement, the
loop currents that support the magnetization are restricted by the lower of
Jct(B) and Jcm{\phi} (B). In the present case the latter, leading to the
premature dropoff of the measured Jcm(B) compared to Jct(B) with increasing
field. This result is supported by Kramer plots of the Jcm{\phi} (B) and Jct(B)
data which lead to an irreversibility field for transverse current that is very
much less than the usual transport-measured longitudinal one, Birr,t.Comment: 41 pages, 14 figure
Anisotropic Connectivity and its Influence on Critical Current Densities, Irreversibility Fields, and Flux Creep in In-Situ-Processed MgB2 Strands
The anisotropy of the critical current density (Jc) and its influence on
measurement of irreversibility field (Birr) has been investigated for high
quality, in-situ MgB2 strands. Comparison of transport and magnetization
measurements has revealed the onset of a regime where large differences exist
between transport and magnetically measured values of the critical current
density and Birr. These effects, initially unexpected due to the lack of
crystalline texture in these in-situ processed strands, appear to be due to a
fibrous microstructure, connected with the details of the wire fabrication and
MgB2 formation reactions. Scanning electron micrographs of in-situ-processed
MgB2 monocore strands have revealed a fibrous microstructure. Grains (~100 nm)
are randomly oriented, and there is no apparent local texture of the grains.
However, this randomly oriented polycrystalline material has a fibrous texture
at a larger length scale, with stringers of MgB2 (~ 60 {\mu}m long and ~5
{\mu}m in diameter) partially separated by elongated pores -- the spaces
previously occupied by stringers of elemental Mg. This leads to an
interpretation of the differences observed in transport and magnetically
determined critical currents, in particular a large deviation between the two
at higher fields, in terms of different transverse and longitudinal
connectivities within the strand. The different values of connectivity also
lead to different resistive transition widths, and thus irreversibility field
values, as measured by transport and magnetic techniques. Finally, these
considerations are seen to influence estimated pinning potentials for the
strands.Comment: 43 Pages, 11 Figures, accepted by Supercon. Sci. Tec
An ultracold molecular beam for testing fundamental physics
We use two-dimensional transverse laser cooling to produce an ultracold beam
of YbF molecules. Through experiments and numerical simulations, we study how
the cooling is influenced by the polarization configuration, laser intensity,
laser detuning and applied magnetic field. The ultracold part of the beam
contains more than molecules per shot and has a temperature
below 200 K, and the cooling yields a 300-fold increase in the brightness
of the beam. The method can improve the precision of experiments that use
molecules to test fundamental physics. In particular, the beam is suitable for
measuring the electron electric dipole moment with a statistical precision
better than e cm.Comment: 25 pages, 14 figures. Trajectory simulations added and results
compared to experiment; other minor revision
The clumpy structure of the chemically active L1157 outflow
We present high spatial resolution maps, obtained with the Plateau de Bure
Interferometer, of the blue lobe of the L1157 outflow. We observed four lines
at 3 mm, namely CH3OH (2_K-1_K), HC3N (11-10), HCN (1-0) and OCS (7-6).
Moreover, the bright B1 clump has also been observed at better spatial
resolution in CS (2-1), CH3OH (2_1-1_1)A-, and 34SO (3_2-2_1). These high
spatial resolution observations show a very rich structure in all the tracers,
revealing a clumpy structure of the gas superimposed to an extended emission.
In fact, the three clumps detected by previous IRAM-30m single dish
observations have been resolved into several sub-clumps and new clumps have
been detected in the outflow. The clumps are associated with the two cavities
created by two shock episodes driven by the precessing jet. In particular, the
clumps nearest the protostar are located at the walls of the younger cavity
with a clear arch-shape form while the farthest clumps have slightly different
observational characteristics indicating that they are associated to the older
shock episode. The emission of the observed species peaks in different part of
the lobe: the east clumps are brighter in HC3N (11-10), HCN (1-0) and CS (2-1)
while the west clumps are brighter in CH3OH(2_K-1_K), OCS (7-6) and 34SO
(3_2-2_1). This peak displacement in the line emission suggests a variation of
the physical conditions and/or the chemical composition along the lobe of the
outflow at small scale, likely related to the shock activity and the precession
of the outflow. In particular, we observe the decoupling of the silicon
monoxide and methanol emission, common shock tracers, in the B1 clump located
at the apex of the bow shock produced by the second shock episode.Comment: 11 pages, 8 figures, accepted for publication in the MNRA
- …