102 research outputs found
PUMA: Thermal three axes spectrometer
Three axes spectrometers allow the direct measurement of the scattering function S(Q, ω) in single crystals at well defined points of the reciprocal lattice vector Q and frequency ω and thus represent the most general instrument type. PUMA, which is jointly operated by the Institute of Physical Chemistry, Georg-August-Universität Göttingen and the Technische Universität München, is characterised by a very high neutron flux as a result of the efficient use of focussing techniques
Hydration of Na+, Ni2+, and Sm3+ in the Interlayer of Hectorite: A Quasielastic Neutron Scattering Study
International audienceQuasielastic neutron scattering experiments were performed with Na−hectorite, Ni−hectorite, and Sm−hectorite samples in order to find out whether Sm3+ is present in the clay interlayer as a fully hydrated cation (outer-sphere complex), or, as it follows from neutron diffraction data analysis, it is dehydrated and bound to the clay surface (inner-sphere complex). The results obtained for the Sm−hectorite were compared with other interlayer cations: strongly hydrated Ni2+ and relatively weakly hydrated Na+. It was found that water mobility in the Sm−hectorite sample is very close to the water mobility in Ni−hectorite. This is only possible if the Sm3+ ion is fully hydrated. It was shown that water molecules hydrating Ni2+ and Sm3+ exhibit diffusion mobility measurable with backscattering spectrometers. The diffusion coefficients of the exchangeable cations were found using the slow exchange approximations DNi = (0.05 − 0.14) × 10−9 m2/s and DSm = (0.04 − 0.18) × 10−9 m2/s
Magnetoelastic hybrid excitations in CeAuAl
The interactions between elementary excitations such as phonons, plasmons,
magnons, or particle-hole pairs, drive emergent functionalities and electronic
instabilities such as multiferroic behaviour, anomalous thermoelectric
properties, polar order, or superconductivity. Whereas various hybrid
excitations have been studied extensively, the feed-back of prototypical
elementary excitations on the crystal electric fields (CEF), defining the
environment in which the elementary excitations arise, has been explored for
very strong coupling only. We report high-resolution neutron spectroscopy and
ab-initio phonon calculations of {\ceaual}, an archetypal fluctuating valence
compound. The high resolution of our data allows us to quantify the energy
scales of three coupling mechanisms between phonons, CEF-split localized 4f
electron states, and conduction electrons. Although these interactions do not
appear to be atypically strong for this class of materials, we resolve, for the
first time, a profound renormalization of low-energy quasiparticle excitations
on all levels. The key anomalies of the spectrum we observe comprise (1) the
formation of a CEF-phonon bound state with a comparatively low density of
acoustic phonons reminiscent of vibronic modes observed in other materials,
where they require a pronounced abundance of optical phonons, (2) an
anti-crossing of CEF states and acoustic phonons, and (3) a strong broadening
of CEF states due to the hybridization with more itinerant excitations. The
fact that all of these features are well resolved in CeAuAl suggests that
similar hybrid excitations should also be dominant in a large family of related
materials. This promises a predictive understanding towards the discovery of
new magneto-elastic functionalities and instabilities.Comment: 9 pages, 4 figure
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Real-space refinement in PHENIX for cryo-EM and crystallography.
This article describes the implementation of real-space refinement in the phenix.real_space_refine program from the PHENIX suite. The use of a simplified refinement target function enables very fast calculation, which in turn makes it possible to identify optimal data-restraint weights as part of routine refinements with little runtime cost. Refinement of atomic models against low-resolution data benefits from the inclusion of as much additional information as is available. In addition to standard restraints on covalent geometry, phenix.real_space_refine makes use of extra information such as secondary-structure and rotamer-specific restraints, as well as restraints or constraints on internal molecular symmetry. The re-refinement of 385 cryo-EM-derived models available in the Protein Data Bank at resolutions of 6 Å or better shows significant improvement of the models and of the fit of these models to the target maps
Distinguishing and electron pairing symmetries by neutron spin resonance in superconducting NaFeCoAs
A determination of the superconducting (SC) electron pairing symmetry forms
the basis for establishing a microscopic mechansim for superconductivity. For
iron pnictide superconductors, the -pairing symmetry theory predicts the
presence of a sharp neutron spin resonance at an energy below the sum of hole
and electron SC gap energies () below . On the other hand,
the -pairing symmetry expects a broad spin excitation enhancement at an
energy above below . Although the resonance has been observed in
iron pnictide superconductors at an energy below consistent with the
-pairing symmetry, the mode has also be interpreted as arising from the
-pairing symmetry with due to its broad energy width and
the large uncertainty in determining the SC gaps. Here we use inelastic neutron
scattering to reveal a sharp resonance at E=7 meV in SC
NaFeCoAs ( K). On warming towards , the mode
energy hardly softens while its energy width increases rapidly. By comparing
with calculated spin-excitations spectra within the and
-pairing symmetries, we conclude that the ground-state resonance in
NaFeCoAs is only consistent with the -pairing, and
is inconsistent with the -pairing symmetry.Comment: 9 pages, 8 figures. submitted to PR
Magnetoelectric metglas/bidomain y + 140°-cut lithium niobate composite for sensing fT magnetic fields
We investigated the magnetoelectric properties of a new laminate composite material based on y+140°-cut congruent lithium niobate piezoelectric plates with an antiparallel polarized “head-to-head” bidomain structure and metglas used as a magnetostrictive layer. A series of bidomain lithium niobate crystals were prepared by annealing under conditions of Li2O outdiffusion from LiNbO3 with a resultant growth of an inversion domain. The measured quasi-static magnetoelectric coupling coefficient achieved |αE31| = 1.9 V·(cm·Oe)–1. At a bending resonance frequency of 6862 Hz, we found a giant |αE31| value up to 1704 V·(cm·Oe)–1. Furthermore, the equivalent magnetic noise spectral density of the investigated composite material was only 92 fT/Hz1/2, a record value for such a low operation frequency. The magnetic-field detection limit of the laminated composite was found to be as low as 200 fT in direct measurements without any additional shielding from external noises.publishe
Radiation hydrodynamics of SN 1987A: I. Global analysis of the light curve for the first 4 months
The optical/UV light curves of SN 1987A are analyzed with the multi-energy
group radiation hydrodynamics code STELLA. The calculated monochromatic and
bolometric light curves are compared with observations shortly after shock
breakout, during the early plateau, through the broad second maximum, and
during the earliest phase of the radioactive tail. We have concentrated on a
progenitor model calculated by Nomoto & Hashimoto and Saio, Nomoto, & Kato,
which assumes that 14 solar masses of the stellar mass is ejected. Using this
model, we have updated constraints on the explosion energy and the extent of
mixing in the ejecta. In particular, we determine the most likely range of E/M
(explosion energy over ejecta mass) and R_0 (radius of the progenitor). In
general, our best models have energies in the range E = (1.1 +/- 0.3) x 10^{51}
ergs, and the agreement is better than in earlier, flux-limited diffusion
calculations for the same explosion energy. Our modeled B and V fluxes compare
well with observations, while the flux in U undershoots after about 10 days by
a factor of a few, presumably due to NLTE and line transfer effects. We also
compare our results with IUE observations, and a very good quantitative
agreement is found for the first days, and for one IUE band (2500-3000 A) as
long as for 3 months. We point out that the V flux estimated by McNaught &
Zoltowski should probably be revised to a lower value.Comment: 27 pages AASTeX v.4.0 + 35 postscript figures. ApJ, accepte
Study of exclusive one-pion and one-eta production using hadron and dielectron channels in pp reactions at kinetic beam energies of 1.25 GeV and 2.2 GeV with HADES
We present measurements of exclusive ensuremathπ+,0 and η production in pp reactions at 1.25GeV and 2.2GeV beam kinetic energy in hadron and dielectron channels. In the case of π+ and π0 , high-statistics invariant-mass and angular distributions are obtained within the HADES acceptance as well as acceptance-corrected distributions, which are compared to a resonance model. The sensitivity of the data to the yield and production angular distribution of Δ (1232) and higher-lying baryon resonances is shown, and an improved parameterization is proposed. The extracted cross-sections are of special interest in the case of pp → pp η , since controversial data exist at 2.0GeV; we find \ensuremathσ=0.142±0.022 mb. Using the dielectron channels, the π0 and η Dalitz decay signals are reconstructed with yields fully consistent with the hadronic channels. The electron invariant masses and acceptance-corrected helicity angle distributions are found in good agreement with model predictions
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Macromolecular structure determination using X-rays, neutrons and electrons: recent developments in Phenix.
Diffraction (X-ray, neutron and electron) and electron cryo-microscopy are powerful methods to determine three-dimensional macromolecular structures, which are required to understand biological processes and to develop new therapeutics against diseases. The overall structure-solution workflow is similar for these techniques, but nuances exist because the properties of the reduced experimental data are different. Software tools for structure determination should therefore be tailored for each method. Phenix is a comprehensive software package for macromolecular structure determination that handles data from any of these techniques. Tasks performed with Phenix include data-quality assessment, map improvement, model building, the validation/rebuilding/refinement cycle and deposition. Each tool caters to the type of experimental data. The design of Phenix emphasizes the automation of procedures, where possible, to minimize repetitive and time-consuming manual tasks, while default parameters are chosen to encourage best practice. A graphical user interface provides access to many command-line features of Phenix and streamlines the transition between programs, project tracking and re-running of previous tasks
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