55 research outputs found
PANDA: Cold three axes spectrometer
The cold three axes spectrometer PANDA, operated by JCNS, Forschungszentrum Jülich, offers high neutron flux over a large dynamic range keeping the instrumental background comparably low
Analyse magnetischer Strukturen an Seltenerd-Cu2-Verbindungen mittels magnetischer Röntgen- und Neutronenbeugung
Die intermetallischen Verbindungen der RCu2-Serie (R = Seltenerd-Elemente) zeigen eine ungewöhnliche Vielfalt von magnetischen Strukturen in Abhängigkeit von Temperatur und äußerem Magnetfeld. Diese Vielfalt ist verursacht durch das Wechselspiel von indirekter Austauschwechselwirkung und anisotropem kristallelektrischen Feld. Die RCu2-Verbindungen kristallisieren in der CeCu2-Struktur, welche als orthorhombische Verzerrung der hexagonalen AlB2-Struktur verstanden werden kann. Ziel der vorliegenden Arbeit ist es, RCu2-Verbindungen mit magnetischer Röntgenbeugung bzw. resonanter magnetischer Röntgenstreuung zu untersuchen, teilweise ergänzt durch Neutronenbeugungsexperimente. Dem zur Neutronenbeugung komplementären Charakter der Röntgenmethoden entspricht es, dass dabei spezielle Details der magnetischen Strukturen untersucht werden. Zusätzlich zur Untersuchung der magnetischen Eigenschaften und Strukturen und zur Suche nach den Ursachen für die auftretenden Phasenübergänge werden magnetoelastische Kopplungen in der pseudohexagonalen Substanzgruppe betrachtet (R = Nd, Gd, Tb, Dy). Der Zusammenhang von kristalliner und magnetischer Struktur wird auch unter Variation und Mischung der Elemente auf dem Seltenerd-Platz erforscht (Tb(1-x)DyxCu2, Tb(1-x)PrxCu2, Dy(1-x)YxCu2). Für die Untersuchung des elementspezifischen Magnetismus bei Vorhandensein mehrerer magnetischer Atomsorten in einem Kristall ist dabei die resonante magnetische Röntgenstreuung die einzig verfügbare Methode. Im Tb0.5Dy0.5Cu2 wird dabei ein unerwartetes Verhalten im Temperaturverlauf der magnetischen Strukturen beobachtet. Ergebnisse aus ergänzenden Neutronenbeugungsexperimenten werden ebenfalls vorgestellt und mittels Rietveld-Verfeinerung der kristallinen und magnetischen Strukturen ausgewertet. Im Ergebnis der Arbeit sind die untersuchten magnetischen Strukturen näher bekannt. Magnetoelastische Wechselwirkungen werden unter Verwendung von Beugungsmethoden neuartig gemessen.The intermetallic compounds of the RCu2 series (R = rare earths) show a large variety of magnetic structures depending on temperature and external field, mainly caused by the interplay of an indirect exchange interaction and the anisotropy of the crystalline electric field. The RCu2 compounds crystallize in the CeCu2 structure, which can be described as an orthorhombic distortion of the hexagonal AlB2 structure. The aim of the present work is the investigation of RCu2 compounds by using resonant and nonresonant magnetic x-ray scattering, supplemented by neutron scattering. Because of the complementarity of magnetic x-ray and neutron scattering this investigation reveals new details of the magnetic structures of the studied compounds. Magnetic properties and magnetic structures are investigated as well as magnetic phase transitions and magneto-elastic coupling in the pseudohexagonal compounds. The correlation between the crystallographic structure and the magnetic structures is studied for different rare earths (R = Nd, Gd, Tb, Dy) but also for partial substitution of magnetic rare earths by magnetic or nonmagnetic ions on the rare earth site (Tb(1-x)DyxCu2, Tb(1-x)PrxCu2, Dy(1-x)YxCu2). Resonant magnetic x-ray scattering is the only method available to investigate the element specific magnetism in crystals with different magnetic ions. By the study of the Tb resonance and the Dy resonance on Tb0.5Dy0.5Cu2 an unexpected developement of the magnetic structures with temperature is observed. The results of supplementary neutron scattering experiments are presented and analyzed by Rietveld refinement of the crystallographic and magnetic structures, mainly focussed on TbCu2. In summary, new insights into the different magnetic structures of RCu2 compounds are gained. The strong magneto-elastic coupling is studied by different scattering methods applied to this problem for the first time
Intertwined dipolar and multipolar order in the triangular-lattice magnet TmMgGaO
A phase transition is often accompanied by the appearance of an order
parameter and symmetry breaking. Certain magnetic materials exhibit exotic
hidden-order phases, in which the order parameters are not directly accessible
to conventional magnetic measurements. Thus, experimental identification and
theoretical understanding of a hidden order are difficult. Here we combine
neutron scattering and thermodynamic probes to study the newly discovered
rare-earth triangular-lattice magnet TmMgGaO. Clear magnetic Bragg peaks at
K points are observed in the elastic neutron diffraction measurements. More
interesting, however, is the observation of sharp and highly dispersive spin
excitations that cannot be explained by a magnetic dipolar order, but instead
is the direct consequence of the underlying multipolar order that is "hidden"
in the neutron diffraction experiments. We demonstrate that the observed
unusual spin correlations and thermodynamics can be accurately described by a
transverse field Ising model on the triangular lattice with an intertwined
dipolar and ferro-multipolar order.Comment: Published versio
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
Electron-doping evolution of the low-energy spin excitations in the iron arsenide BaFeNiAs superconductors
We use elastic and inelastic neutron scattering to systematically investigate
the evolution of the low-energy spin excitations of the iron arsenide
superconductor BaFe2-xNixAs2 as a function of nickel doping x. In the undoped
state, BaFe2As2 exhibits a tetragonal-to-orthorhombic structural phase
transition and simultaneously develops a collinear antiferromagnetic (AF) order
below TN = 143 K. Upon electron-doping of x = 0.075 to induce bulk
superconductivity with Tc = 12.3 K, the AF ordering temperature reduces to TN =
58 K.We show that the appearance of bulk superconductivity in
BaFe1.925Ni0.075As2 coincides with a dispersive neutron spin resonance in the
spin excitation spectra, and a reduction in the static ordered moment. For
optimally doped BaFe1.9Ni0.1As2 (Tc = 20 K) and overdoped BaFe1.85Ni0.15As2 (Tc
= 15 K) superconductors, the static AF long-range order is completely
suppressed and the spin excitation spectra are dominated by a resonance and
spin-gap at lower energies. We determine the electron-doping dependence of the
neutron spin resonance and spin gap energies, and demonstrate that the
three-dimensional nature of the resonance survives into the overdoped regime.
If spin excitations are important for superconductivity, these results would
suggest that the three-dimensional character of the electronic superconducting
gaps are prevalent throughout the phase diagram, and may be critical for
superconductivity in these materials
Three-dimensional Resonance in superconducting BaFeNiAs
We use inelastic neutron scattering to study magnetic excitations of the
FeAs-based superconductor BaFeNiAs above and below its
superconducting transition temperature K. In addition to gradually
open a spin gap at the in-plane antiferromagnetic ordering wavevector
, the effect of superconductivity is to form a three dimensional
resonance with clear dispersion along the c-axis direction. The intensity of
the resonance develops like a superconducting order parameter, and the mode
occurs at distinctively different energies at and . If the
resonance energy is directly associated with the superconducting gap energy
, then is dependent on the wavevector transfers along the
c-axis. These results suggest that one must be careful in interpreting the
superconducting gap energies obtained by surface sensitive probes such as
scanning tunneling microscopy and angle resolved photoemission.Comment: 5 pages, 4 figure
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
Low-temperature antiferromagnetic order in orthorhombic CePdAl
We report the magnetization, ac susceptibility, and specific heat of
optically float-zoned single crystals of CePdAl. In comparison to the
properties of polycrystalline CePdAl reported in the literature, which
displays a tetragonal crystal structure and no long-range magnetic order, our
single crystals exhibit an orthorhombic structure () and order
antiferromagnetically below a N\'eel temperature = 5.6 K. The
specific heat at zero-field shows a clear -type anomaly with a broad
shoulder at . A conservative estimate of the Sommerfeld coefficient
of the electronic specific heat, ,
indicates a moderately enhanced heavy-fermion ground state. A twin
microstructure evolves in the family of planes spanned by the basal plane
lattice vectors and , with the magnetic hard axis
common to all twins. The antiferromagnetic state is characterized
by a strong magnetic anisotropy and a spin-flop transition induced under
magnetic field along the easy direction, resulting in a complex magnetic phase
diagram. Taken together our results reveal a high sensitivity of the magnetic
and electronic properties of CePdAl to its structural modifications
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