86 research outputs found
Exciton spectroscopy of hexagonal boron nitride using non-resonant x-ray Raman scattering
We report non-resonant x-ray Raman scattering (XRS) measurements from
hexagonal boron nitride for transferred momentum from 2 to 9
along directions both in and out of the basal plane. A
symmetry-based argument, together with real-space full multiple scattering
calculations of the projected density of states in the spherical harmonics
basis, reveals that a strong pre-edge feature is a dominantly -type
Frenkel exciton with no other \textit{s}-, \textit{p}-, or \textit{d}-
components. This conclusion is supported by a second, independent calculation
of the \textbf{q}-dependent XRS cross-section based on the Bethe-Salpeter
equation
Hard x ray spectroscopy and imaging by a reflection zone plate in the presence of astigmatism
The feasibility of an off axis x ray reflection zone plate to perform wavelength dispersive spectroscopy, on axis point focusing, and two dimensional imaging is demonstrated by means of one and the same diffractive optical element DOE at a synchrotron radiation facility. The resolving power varies between 30 and 400 in the range of 7.6 keV to 9.0 keV, with its maximum at the design energy of 8.3 keV. This result is verified using an adjustable entrance slit, by which horizontal H and vertical V focusing to 0.85 amp; 956;m H and 1.29 amp; 956;m V is obtained near the sagittal focal plane of the astigmatic configuration. An angular and axial scan proves an accessible field of view of at least 0.6 arcmin 0.8 arcmin and a focal depth of plus minus 0.86 mm. Supported by the grating efficiency of around 17.5 and a very short pulse elongation, future precision x ray fluorescence and absorption studies of transition metals at their K edge on an ultrashort timescale could benefit from our finding
Takagi-Taupin Description of X-ray Dynamical Diffraction from Diffractive Optics with Large Numerical Aperture
We present a formalism of x-ray dynamical diffraction from volume diffractive
optics with large numerical aperture and high aspect ratio, in an analogy to
the Takagi-Taupin equations for strained single crystals. We derive a set of
basic equations for dynamical diffraction from volume diffractive optics, which
enable us to study the focusing property of these optics with various grating
profiles. We study volume diffractive optics that satisfy the Bragg condition
to various degrees, namely flat, tilted and wedged geometries, and derive the
curved geometries required for ultimate focusing. We show that the curved
geometries satisfy the Bragg condition everywhere and phase requirement for
point focusing, and effectively focus hard x-rays to a scale close to the
wavelength.Comment: 18 pages, 12 figure
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Laterally graded multilayer double-monochromator.
The authors describe a tunable multilayer monochromator with an adjustable bandpass to be used for reflectivity and grazing incidence diffraction studies on surfaces at energies near 10 keV. Multilayers have a bandpass typically 100 times larger than the Si(111) reflection, and by using multilayers an experimenter can significantly increase data collection rates over those available with a Si monochromator. The transmission through 1 and 2 laterally graded multilayer (LGML) reflections was recorded versus photon energy. The identical LGMLs were comprised of 60 bilayers of W and C on 100 x 25 x 3 mm float glass with a bilayer spacing varying from 35 to 60 {angstrom}. The average gradient was 0.27 {angstrom}/mm along the long dimension. The rms deviation of the data for the bilayer spacing from a linear fit was 0.36 {angstrom}. Data were obtained for a nondispersive ({+-}) double-multilayer arrangement. The relative bandpass width (FWHM) when the two multilayers exposed the same bilayer spacing was measured to be 2.2% with a transmission of 78.7 {+-} 1.6%. This value is consistent with the transmission of 88.9% that they also measured for a single LGML at HASYLAB beamline D4. The bandpass was tunable in the range 1.1% to 2.2%
Upstream sources of the Denmark Strait Overflow : observations from a high-resolution mooring array
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 112 (2016): 94-112, doi:10.1016/j.dsr.2016.02.007.We present the first results from a densely instrumented mooring array upstream of the Denmark Strait
sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September
2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kgm-3 approaching
the sill. The mean transport of overflow water over the length of the deployment was 3.54
± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in
Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the
sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique
based on the hydrographic properties of the water, the transports of these two sources are found to be
2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source
into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of
the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport is more consistent than the transport in either branch; compensation takes place among the pathways that
maintains a stable total overflow transport. This is especially true for the two East Greenland Current
branches whose transports vary out of phase with each other on weekly and longer time scales. We argue
that wind forcing plays a role in this partitioning.The mooring and analysis work was supported by NSF OCE research grants
OCE-0959381 and OCE-1433958, by the European Union 7th Framework Programme (FP7 2007-2013)
under grant agreement n. 308299 NACLIM, and and by the Research Council of Norway through the
Fram Centre Flaggship project 6606-299.2017-03-2
Upstream sources of the Denmark Strait Overflow : observations from a high-resolution mooring array
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 112 (2016): 94-112, doi:10.1016/j.dsr.2016.02.007.We present the first results from a densely instrumented mooring array upstream of the Denmark Strait
sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September
2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kgm-3 approaching
the sill. The mean transport of overflow water over the length of the deployment was 3.54
± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in
Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the
sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique
based on the hydrographic properties of the water, the transports of these two sources are found to be
2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source
into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of
the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport is more consistent than the transport in either branch; compensation takes place among the pathways that
maintains a stable total overflow transport. This is especially true for the two East Greenland Current
branches whose transports vary out of phase with each other on weekly and longer time scales. We argue
that wind forcing plays a role in this partitioning.The mooring and analysis work was supported by NSF OCE research grants
OCE-0959381 and OCE-1433958, by the European Union 7th Framework Programme (FP7 2007-2013)
under grant agreement n. 308299 NACLIM, and and by the Research Council of Norway through the
Fram Centre Flaggship project 6606-299.2017-03-2
Achieving hard X-ray nanofocusing using a wedged multilayer Laue lens
We report on the fabrication and the characterization of a wedged multilayer Laue lens for x-ray nanofocusing. The lens was fabricated using a sputtering deposition technique, in which a specially designed mask was employed to introduce a thickness gradient in the lateral direction of the multilayer. X-ray characterization shows an efficiency of 27% and a focus size of 26 nm at 14.6 keV, in a good agreement with theoretical calculations. These results indicate that the desired wedging is achieved in the fabricated structure. We anticipate that continuous development on wedged MLLs will advance x-ray nanofocusing optics to new frontiers and enrich capabilities and opportunities for hard X-ray microscopy
Variability in the Deep Western Boundary Current : local versus remote forcing
Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 117 (2012): C12022, doi:10.1029/2012JC008369.Horizontal velocity, temperature and salinity measurements from the Line W array for the period 2004â2008 show large changes in the water mass structure and circulation of the Deep Western Boundary Current (DWBC). Fluctuations in the flow with periods from 10 to 60 days are bottom intensified: signals most likely associated with topographic Rossby waves (TRW). A fraction (âŒ15%) of the DWBC transport variability is caused by Gulf Stream rings and meanders. These flow anomalies are surface intensified and fluctuate at frequencies lower than the TRW. Interannual variability in the velocity field appears to be related to changes in the hydrographic properties. The dominant mode of variability is characterized by an overall freshening, cooling, a potential vorticity (PV) increase in the deep Labrador Sea Water (dLSW) and a PV decrease in the Overflow Water (OW). The variability in the flow associated with these property changes is not spatially homogeneous. Offshore (water depths larger than 3500 m) changes in the velocity are in phase with PV changes in the OW: a decrease in the OW PV is accompanied by an increase in the southward (negative) transport. Conversely, variations of the inshore flow are in phase with changes in the dLSW PV (increasing PV and decreasing transport). This trend, true for most of the record, reverses after the winter of 2007â2008. A sudden decrease of the dLSW PV is observed, with a corresponding intensification of the flow in the inner DWBC as well as a northward shift in the Gulf Stream axis.Financial support for the Line W program
(2004â2008) was provided by the U.S. National Science Foundation (grants
OCE-0241354 and OCE-0726720) as well as funding from the WHOIâs
Ocean and Climate Change Institute.2013-06-2
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