The phonon dispersions of the bcc and fcc phases of pure iron ( alpha -Fe, gamma -Fe, and delta -Fe) at ambient pressure were investigated close to the respective phase transition temperatures. In ...the open bcc structure the transverse phonons along T sub(1)xixi0 and T sub(1) xixi2xi are of particularly low energy. The eigenvectors of these phonons correspond to displacements needed for the transformation to the fcc gamma phase. Especially these phonons, but also all other phonons, soften considerably with increasing temperature. Comparing thermodynamic properties of the fcc and the two bcc phases it is shown that the high-temperature bcc phase is stabilized predominantly by vibrational entropy, whereas for the stabilization of the fcc phase electronic entropy provides an equal contribution.
The FlexiProb project is a joint effort of three soft matter groups at the Universities of Bielefeld, Darmstadt, and Munich with scientific support from the European Spallation Source (ESS), the ...small-K advanced diffractometer (SKADI) beamline development group of the Jülich Centre for Neutron Science (JCNS), and the Heinz Maier-Leibnitz Zentrum (MLZ). Within this framework, a flexible and quickly interchangeable sample carrier system for small-angle neutron scattering (SANS) at the ESS was developed. In the present contribution, the development of a sample environment for the investigation of soft matter thin films with grazing-incidence small-angle neutron scattering (GISANS) is introduced. Therefore, components were assembled on an optical breadboard for the measurement of thin film samples under controlled ambient conditions, with adjustable temperature and humidity, as well as the optional in situ recording of the film thickness via spectral reflectance. Samples were placed in a 3D-printed spherical humidity metal chamber, which enabled the accurate control of experimental conditions via water-heated channels within its walls. A separately heated gas flow stream supplied an adjustable flow of dry or saturated solvent vapor. First test experiments proved the concept of the setup and respective component functionality.
The European Spallation Source (ESS), which is under construction in Lund (Sweden), will be the leading and most brilliant neutron source and aims at starting user operation at the end of 2023. Among ...others, two small angle neutron scattering (SANS) machines will be operated. Due to the high brilliance of the source, it is important to minimize the downtime of the instruments. For this, a collaboration between three German universities and the ESS was initialized to develop and construct a unified sample environment (SE) system. The main focus was set on the use of a robust carrier system for the different SEs, which allows setting up experiments and first prealignment outside the SANS instruments. This article covers the development and construction of a SE for SANS experiments with foams, which allows measuring foams at different drainage states and the control of the rate of foam formation, temperature, and measurement position. The functionality under ESS conditions was tested and neutron test measurement were carried out.
As part of the development of the new European Spallation Source (ESS) in Lund (Sweden), which will provide the most brilliant neutron beams worldwide, it is necessary to provide different sample ...environments with which the potential of the new source can be exploited as soon as possible from the start of operation. The overarching goal of the project is to reduce the downtimes of the instruments related to changing the sample environment by developing plug and play sample environments for different soft matter samples using the same general carrier platform and also providing full software integration and control by just using unified connectors. In the present article, as a part of this endeavor, the sample environment for in situ SANS and dynamic light scattering measurements is introduced.
Polarized inelastic neutron scattering under a magnetic field is used to get a microscopic insight into the spin resonance of the heavy fermion superconductor CeCoIn(5). The resonance line shape is ...found to depend on the neutron polarization: Some of the spectral weight is common to the two polarization channels while the remaining part is distributed equally between them. This is evidence for the spin resonance being a degenerate mode with three fluctuation channels: A Zeeman split contribution and an additional longitudinal mode.
Inelastic neutron scattering was used to study the effect of 5 and 40 mol% cholesterol on the lateral nanoscale dynamics of phospholipid membranes. By measuring the excitation spectrum at several ...lateral
q
||
values (up to
q
||
= 3 Å
−1
), complete dispersion curves were determined of gel, fluid and liquid-ordered phase bilayers. The inclusion of cholesterol had a distinct effect on the collective dynamics of the bilayer’s hydrocarbon chains; specifically, we observed a pronounced stiffening of the membranes on the nanometer length scale in both gel and fluid bilayers, even though they were experiencing a higher degree of molecular disorder. Also, for the first time we determined the nanoscale dynamics in the high-cholesterol liquid-ordered phase of bilayers containing cholesterol. Namely, this phase appears to be “softer” than fluid bilayers, but better ordered than bilayers in the gel phase.
A longstanding problem in facility based experiments is the time one must wait whilst changing sample temperature. This issue is most often confronted at cryogenic temperatures above 100 K, where ...thermal response times can be very slow. These can be drastically reduced by minimising the amount of material being controlled to as little as possible beyond the sample itself. Peltier elements provide an effective way to achieve this, but are mostly used to regulate with respect to room temperature. We report on the Huginn sub-cryostat, a cryostat insert with dual Peltier regulation, useable down to 100 K, that enables large and rapid temperature jumps of up of 10 K to 20 K in minutes or less. Moreover, temperature stability is improved by an order of magnitude in the presence of a variable background temperature.
Inelastic neutron scattering (INS) experiments and random phase approximation calculations have been used to investigate the low-energy spin-wave excitations in PrNi2Si2. The modulated magnitude of ...the ordered magnetic moments of Pr3+ ions implies that the associate, longitudinally polarized magnetic excitations are more intense and dispersive than the usual transverse spin waves. Within the random phase approximation the results are in good overall agreement with the predictions made by the model determined previously from the paramagnetic excitations. The most unusual observation is the well-defined amplitude mode detected close to the magnetic Bragg point existing simultaneously with the phason mode. At low energies, an extra mode is observed to hybridize with the magnetic phasons in the neighborhood of the magnetic Brillouin zone center. A magnetoelastic interaction between the magnetic excitations and the longitudinal phonons is able to explain part of the disturbances, but it is concluded that the extra mode must be of some other, unknown origin.
We here present inelastic neutron scattering results on the strongly correlated cubic superconductor UBe13 (T-c = 0.85 K) obtained on a large single crystal by high-resolution cold neutron three-axis ...spectroscopy. We observed spin dynamics at a unique momentum space position building up below T similar to 50 K and changing significantly on entering the superconducting state. The observed short-range longitudinal character of the correlations can be understood as a result of competing magnetic interactions. The energy dependence in the normal state reflects the energy scales determined from specific heat, whereas the low-temperature data suggest the opening of a superconducting gap. Our findings are consistent with a superconducting order parameter exhibiting s +/- or d-wave symmetry and placing pure UBe13 in the strong coupling regime.
More than eighty scientists gathered together for a workshop on the perspectives in Single Crystal Neutron Spectroscopy (SCNS) from December 12-14, 2002 at the Institut Laue-Langevin, Grenoble, ...France. Experts in both triple-axis spectroscopy (TAS) and time-of-flight (TOF) instrumentation, as well as condensed matter and material scientists, contributed to the meeting, which was organized jointly by the ILL and its collaborating research groups of the CEA/Grenoble and the FZ Jülich with support from the Neutron Round Table and the City of Grenoble.
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