15 research outputs found
Relaxation of dynamically disordered tetragonal platelets in the relaxor ferroelectric
The local dynamics of the lead-free relaxor
(NBT-3.6BT) have been investigated by a combination of quasielastic neutron
scattering (QENS) and ab initio molecular dynamics simulations. In a previous
paper, we were able to show that the tetragonal platelets in the microstructure
are crucial for understanding the dielectric properties of NBT-3.6BT [F. Pforr
et al., Phys. Rev. B 94, 014105 (2016)]. To investigate their dynamics, ab
initio molecular dynamics simulations were carried out using
with 001 cation order as a
simple model system for the tetragonal platelets in NBT-3.6BT. Similarly,
111-ordered was used as a
model for the rhombohedral matrix. The measured single crystal QENS spectra
could be reproduced by a linear combination of calculated spectra. We find that
the relaxational dynamics of NBT-3.6BT are concentrated in the tetragonal
platelets. Chaotic stages, during which the local tilt order changes
incessantly on the timescale of several picoseconds, cause the most significant
contribution to the quasielastic intensity. They can be regarded as an excited
state of tetragonal platelets, whose relaxation back into a quasistable state
might explain the frequency dependence of the dielectric properties of
NBT-3.6BT in the 100 GHz to THz range. This substantiates the assumption that
the relaxor properties of NBT-3.6BT originate from the tetragonal platelets.Comment: 27 pages, 9 figure
Log-Gaussian processes for AI-assisted TAS experiments
To understand the origins of materials properties, neutron scattering
experiments at three-axes spectrometers (TAS) investigate magnetic and lattice
excitations in a sample by measuring intensity distributions in its momentum
(Q) and energy (E) space. The high demand and limited availability of beam time
for TAS experiments however raise the natural question whether we can improve
their efficiency or make better use of the experimenter's time. In fact, using
TAS, there are a number of scientific questions that require searching for
signals of interest in a particular region of Q-E space, but when done
manually, it is time consuming and inefficient since the measurement points may
be placed in uninformative regions such as the background. Active learning is a
promising general machine learning approach that allows to iteratively detect
informative regions of signal autonomously, i.e., without human interference,
thus avoiding unnecessary measurements and speeding up the experiment. In
addition, the autonomous mode allows experimenters to focus on other relevant
tasks in the meantime. The approach that we describe in this article exploits
log-Gaussian processes which, due to the log transformation, have the largest
approximation uncertainties in regions of signal. Maximizing uncertainty as an
acquisition function hence directly yields locations for informative
measurements. We demonstrate the benefits of our approach on outcomes of a real
neutron experiment at the thermal TAS EIGER (PSI) as well as on results of a
benchmark in a synthetic setting including numerous different excitations.Comment: Main: 22 pages, 5 figures | Extended Data: 8 figures | Supplementary
Information: 5 pages, 2 figure
Momentum-dependent magnon lifetime in the metallic non-collinear triangular antiferromagnet CrB2
Non-collinear magnetic order arises for various reasons in several magnetic
systems and exhibits interesting spin dynamics. Despite its ubiquitous
presence, little is known of how magnons, otherwise stable quasiparticles,
decay in these systems, particularly in metallic magnets. Using inelastic
neutron scattering, we examine the magnetic excitation spectra in a metallic
non-collinear antiferromagnet CrB, in which Cr atoms form a triangular
lattice and display incommensurate magnetic order. Our data show intrinsic
magnon damping and continuum-like excitations that cannot be explained by
linear spin wave theory. The intrinsic magnon linewidth shows
very unusual momentum dependence, which our analysis shows to originate from
the combination of two-magnon decay and the Stoner continuum. By comparing the
theoretical predictions with the experiments, we identify where in the momentum
and energy space one of the two factors becomes more dominant. Our work
constitutes a rare comprehensive study of the spin dynamics in metallic
non-collinear antiferromagnets. It reveals, for the first time, definite
experimental evidence of the higher-order effects in metallic antiferromagnets.Comment: 6 pages, 4 figures, accepted for publication in PR
Classical spin liquid or extended critical range in h-YMnO?
Inelastic neutron experiments on the classical triangular-lattice
geometrically frustrated antiferromagnet h-YMnO reveal diffuse, gapless
magnetic excitations present both below and far above the ordering temperature,
. The correlation length of the excitations increases as the temperature
approaches zero, bearing strong resemblance to critical scattering. We model
the scattering as critical spin-spin correlations in a two-dimensional magnetic
ground state, and we speculate that this may provide a general framework to
understand features typically attributed to classical spin liquids
Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession : First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations
Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes-summarized by the term ocean acidification (OA)-could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (similar to 380 mu atm pCO(2)), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (mu 760 mu atm pCO(2)). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a "long-term mesocosm" approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.Peer reviewe
Neutron diffraction analysis of load transfer in DP 600 steel during in situ tensile tests
The load transfer among ferrite orientations and between ferrite and martensite was analysed in DP 600 steel by means of neutron diffraction during in situ tensile tests on the multiple pulse overlap time-of-flight strain scanner POLDI. The material had 0.07 wt% C and a martensite volume fraction of 15%. In situ tests were done in “Young” as well as in “Poisson setup”. The martensite phase could not be probed due to its low tetragonality. The curves of the lattice plane strains as a function of the externally applied macroscopic stress reveal (1) plastic relaxations of
transformation and intergranular stresses in the compliant oriented grains, and (2) a second inflection point in the fully plastic part indicating the onset of plastic deformation of the hard phase.status: publishe
Evaluation of field-induced magnetic moments in the spin-1/2 antiferromagnetic trimerized chain compound Cu3(P2O6OD)2
Patients' acceptance and psychometric properties of the EORTC QLQ-CX24 after surgery.
OBJECTIVE: The aim of this study was to evaluate patients' acceptance and the reliability and validity of a recently developed instrument to assess quality of life among cervical cancer patients, the European Organization for Research and Treatment of Cancer Cervical Cancer Module (EORTC QLQ-CX24), in surgically treated patients after primary surgery. METHODS: The EORTC QLQ-CX24 was administered to 134 cervical cancer patients who had undergone pelvic surgery. Additional questionnaires completed were the EORTC QLQ-C30, and the Hospital Anxiety and Depression Scale. Sociodemographic and medical data were recorded. RESULTS: Missing values occurred in 4.5% of all non-optional items. Internal consistencies (Cronbach's alpha coefficients) for the three multi-item scales ranged from 0.70 to 0.87 (Symptom Experience 0.70, Body Image 0.87, Sexual/Vaginal Functioning 0.76), whereas scaling errors occurred in 6.8%, 0.0%, and 6.3% of the cases. In all domains, the entire range of the scale was used by patients. The scales were able to discriminate between different subgroups of patients. CONCLUSION: These psychometric analyses confirm that the EORTC QLQ-CX24 is a useful tool for the assessment of quality of life in cervical cancer patients after surgery