282 research outputs found
Stratification of canopy magnetic fields in a plage region. Constraints from a spatially-regularized weak-field approximation method
The role of magnetic fields in the chromospheric heating problem remains
greatly unconstrained. Most theoretical predictions from numerical models rely
on a magnetic configuration, field strength and connectivity whose details have
not been well established with observational studies. High-resolution studies
of chromospheric magnetic fields in plage are very scarce or non-existent in
general. Our aim is to study the stratification of the magnetic field vector in
plage regions. We use high-spatial resolution full-Stokes observations acquired
with CRISP instrument at the Swedish 1-m Solar Telescope in the Mg I
5173, Na I 5896 and Ca II 8542 lines. We have
developed a spatially-regularized weak-field approximation (WFA) method based
on the idea of spatial regularization. This method allows for a fast
computation of magnetic field maps for an extended field of view. The fidelity
of this new technique has been assessed using a snapshot from a realistic 3D
magnetohydrodynamics simulation. We have derived the depth-stratification of
the line-of-sight component of the magnetic field from the photosphere to the
chromosphere in a plage region. The magnetic fields are concentrated in the
intergranular lanes in the photosphere and expand horizontally toward the
chromosphere, filling all the space and forming a canopy. Our results suggest
that the lower boundary of this canopy must be located around 400-600 km from
the photosphere. The mean canopy total magnetic field strength in the lower
chromosphere ( km) is 658 G. At km we estimate
G. We propose a modification to the WFA that
improves its applicability to data with worse signal-to-noise ratio. These
methods provide a quick and reliable way of studying multi-layer magnetic field
observations without the many difficulties inherent to other inversion methods.Comment: Accepted for publication on 2020-08-2
Superconductivity in CoSr2(Y1-xCax)Cu2O7+d
The roles of aliovalent Ca(II)-for-Y(III) substitution and
high-pressure-oxygen annealing in the process of "superconducterizing" the
Co-based layered copper oxide, CoSr2(Y1-xCax)Cu2O7+d (Co-1212), were
investigated. The as-air-synthesized samples up to x = 0.4 were found
essentially oxygen stoichiometric (-0.03 <= d <= 0.00). These samples, however,
were not superconductive, suggesting that the holes created by the
divalent-for-trivalent cation substitution are trapped on Co in the charge
reservoir. Ultra-high-pressure heat treatment carried out at 5 GPa and 500C for
30 min in the presence of Ag2O2 as an excess oxygen source induced bulk
superconductivity in these samples. The highest Tc was obtained for the
high-oxygen-pressure treated x = 0.3 sample at ~40 K.Comment: 16 pages, 6 figures, submitted to Solid State Communication
Atomic structure and vibrational properties of icosahedral BC boron carbide
The atomic structure of icosahedral BC boron carbide is determined by
comparing existing infra-red absorption and Raman diffusion measurements with
the predictions of accurate {\it ab initio} lattice-dynamical calculations
performed for different structural models. This allows us to unambiguously
determine the location of the carbon atom within the boron icosahedron, a task
presently beyond X-ray and neutron diffraction ability. By examining the inter-
and intra-icosahedral contributions to the stiffness we show that, contrary to
recent conjectures, intra-icosahedral bonds are harder.Comment: 9 pages including 3 figures, accepted in Physical Review Letter
Recommended from our members
XRAYL: a program for producing idealized powder diffraction line profiles from overlapped powder patterns
The X-ray diffraction patterns of samples of polycrystalline materials are used to identify and characterize phases. Very often the total (or composite) profile consists of a series of overlapping profiles. In many applications it is necessary to separate the component profiles from the total profile. (In this document the terms {ital profile, line}, and {ital peak} are used interchangeably to represent these features of X-ray or neutron diffraction patterns.) A computer program, XRAYL, first developed in the 1980s and subsequently enlarged and improved, allows the fitting of analytical functions to powder diffraction lines. The fitting process produces parameters of chosen profile functions, diffraction line by diffraction line. The resulting function parameters may then be used to generate ``idealized`` powder diffraction lines as counts at steps in 2{Omega}. The generated lines are effectively free of statistical noise and contributions from overlapping lines. Each separated line extends to background on both sides of the generated profile. XRAYL may, therefore, be used in X-ray powder diffraction profile analysis as a preprocessor program that is, separating peaks and feeding the ``resolved`` data to subsequent analysis programs. This self- contained document includes: (1) a description of the fitting functions coded into XRAYL, (2) an outline of the least-squares algorithm used in fitting the profile function, (3) the file formats and contents utilized by the computer code, (4) the user options and their presentation requirements for execution of the program, (5) an example of input and output for a test case, and (6) source code listings on a diskette
X-ray diffraction measurements of the c-axis Debye-Waller factors of YBa2Cu3O7 and HgBa2CaCu2O6
We report the first application of x-rays to the measurement of the
temperature dependent Bragg peak intensities to obtain Debye-Waller factors on
high-temperature superconductors. Intensities of (0,0,l) peaks of YBa2Cu3O7 and
HgBa2CaCu2O6 thin films are measured to obtain the c-axis Debye-Waller factors.
While lattice constant and some Debye-Waller factor measurements on high Tc
superconductors show anomalies at the transition temperature, our measurements
by x-ray diffraction show a smooth transition of the c-axis Debye-Waller
factors through T. This suggests that the dynamic displacements of the
heavy elements along the c-axis direction in these compounds do not have
anomalies at Tc. This method in combination with measurements by other
techniques will give more details concerning dynamics of the lattice.Comment: 4 pages, 2 figures. To be published in Physical Review B (Brief
Report
Do rebreathing manoeuvres for non-invasive measurement of cardiac output during maximum exercise test alter the main cardiopulmonary parameters?
Background: Inert gas rebreathing has been recently described as an emergent reliable non-invasive method for cardiac output determination during exercise, allowing a relevant improvement of cardiopulmonary exercise test clinical relevance. For cardiac output measurements by inert gas rebreathing, specific respiratory manoeuvres are needed which might affect pivotal cardiopulmonary exercise test parameters, such as exercise tolerance, oxygen uptake and ventilation vs carbon dioxide output (VE/VCO2) relationship slope. Method: We retrospectively analysed cardiopulmonary exercise testing of 181 heart failure patients who underwent both cardiopulmonary exercise testing and cardiopulmonary exercise test+cardiac output within two months (average 16 \ub1 15 days). All patients were in stable clinical conditions (New York Heart Association I\u2013III) and on optimal medical therapy. Results: The majority of patients were in New York Heart Association Class I and II (78.8%), with a mean left ventricular ejection fraction of 31 \ub1 10%. No difference was found between the two tests in oxygen uptake at peak exercise (1101 (interquartile range 870\u20131418) ml/min at cardiopulmonary exercise test vs 1103 (844\u20131389) at cardiopulmonary exercise test-cardiac output) and at anaerobic threshold. However, anaerobic threshold and peak heart rate, peak workload (75 (58\u2013101) watts and 64 (42\u201390), p < 0.01) and carbon dioxide output were significantly higher at cardiopulmonary exercise testing than at cardiopulmonary exercise test+cardiac output, whereas VE/VCO2 slope was higher at cardiopulmonary exercise test+cardiac output (30 (27\u201335) vs 33 (28\u201337), p < 0.01). Conclusion: The similar anaerobic threshold and peak oxygen uptake in the two tests with a lower peak workload and higher VE/VCO2 slope at cardiopulmonary exercise test+cardiac output suggest a higher respiratory work and consequent demand for respiratory muscle blood flow secondary to the ventilatory manoeuvres. Accordingly, VE/VCO2 slope and peak workload must be evaluated with caution during cardiopulmonary exercise test+cardiac output
Non-LTE inversions of a confined X2.2 flare: I. Vector magnetic field in the photosphere and chromosphere : I. The vector magnetic field in the photosphere and chromosphere
Obtaining the magnetic field vector accurately in the solar atmosphere is essential for studying changes in field topology during flares and to reliably model space weather. We tackle this problem by applying various inversion methods to a confined X2.2 flare in NOAA AR 12673 on September 6, 2017, comparing the photospheric and chromospheric magnetic field vector with those from two numerical models of this event. We obtain the photospheric field from Milne-Eddington (ME) and (non-)local thermal equilibrium (non-LTE) inversions of Hinode SOT/SP Fe I 6301.5Å and 6302.5Å. The chromospheric field is obtained from a spatially-regularised weak field approximation (WFA) and non-LTE inversions of Ca II 8542Å observed with CRISP at the Swedish 1-m Solar Telescope. The LTE- and non-LTE-inferred photospheric field components are strongly correlated throughout the atmosphere, with stronger field and higher temperatures in the non-LTE inversions. For the chromospheric field, the non-LTE inversions correlate well with the spatially-regularised WFA. We find strong-field patches of over 4.5 kG in the photosphere, co-located with similar concentrations exceeding 3 kG in the chromosphere. The obtained field strengths are up to 2-3 times higher than in the numerical models, with more concentrated and structured photosphere-to-chromosphere shear close to the polarity inversion line. The LTE and non-LTE Fe I inversions yield essentially the same photospheric field, while ME inversions fail to reproduce the field vector orientation where Fe I is in emission. Our inversions confirm the locations of flux rope footpoints that are predicted by numerical models. However, pre-processing and lower spatial resolution lead to weaker and smoother field in the models than what the data indicate. This emphasises the need for higher spatial resolution in the models to better constrain pre-eruptive flux ropes.Peer reviewe
Generalized calculation of magnetic coupling constants for Mott-Hubbard insulators: Application to ferromagnetic Cr compounds
Using a Rayleigh-Schr\"odinger perturbation expansion of multi-band Hubbard
models, we present analytic expressions for the super-exchange coupling
constants between magnetic transition metal ions of arbitrary separation in
Mott-Hubbard insulators. The only restrictions are i) all ligand ions are
closed shell anions and ii) all contributing interaction paths are of equal
length. For short paths, our results essentially confirm the
Goodenough-Kanamori-Anderson rules, yet in general there does not exist any
simple rule to predict the sign of the magnetic coupling constants. The most
favorable situation for ferromagnetic coupling is found for ions with less than
half filled d shells, the (relative) tendency to ferromagnetic coupling
increases with increasing path length. As an application, the magnetic
interactions of the Cr compounds RbCrCl, CrCl, CrBr and CrI
are investigated, all of which except CrCl are ferromagnets.Comment: 13 pages, 6 eps figures, submitted to Phys Rev
Nuclear Inelastic X-Ray Scattering of FeO to 48 GPa
The partial density of vibrational states has been measured for Fe in
compressed FeO (w\"ustite) using nuclear resonant inelastic x-ray scattering.
Substantial changes have been observed in the overall shape of the density of
states close to the magnetic transiton around 20 GPa from the paramagnetic (low
pressure) to the antiferromagnetic (high pressure) state. Our data indicate a
substantial softening of the aggregate sound velocities far below the
transition, starting between 5 and 10 GPa. This is consistent with recent
radial x-ray diffraction measurements of the elastic constants in FeO. The
results indicate that strong magnetoelastic coupling in FeO is the driving
force behind the changes in the phonon spectrum of FeO.Comment: 4 pages, 4 figure
Nuclear Quadrupole Resonance Frequency in Multi-Layered Cuprates
The 63Cu nuclear quadrupole resonance (NQR) frequency, nu_q, in the
multi-layered cuprates is calculated in the cluster model by the exact
diagonalization method. The charge imbalance between the outer CuO_2 plane (OP)
with apical oxygen (OA) and the inner plane (IP) without OA in three-layered
Tl2223 is estimated by comparing our results with the experimental nu_q. In
Tl-based cuprates with more than three layers, we predict a large enhancement
of the splitting of nu_q between OP and IP by taking into account the reduction
of bond length between Cu and OA and a resulting enhancement of the charge
transfer energy. Our results show that the NQR frequency is a useful quantity
to estimate the charge imbalance in the multi-layered cuprates.Comment: 4 pages, 5 figure
- …