63 research outputs found
Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites
The improper ferroelectricity in YMnO and other related multiferroic
hexagonal manganites are known to cause topologically protected ferroelectric
domains that give rise to rich and diverse physical phenomena. The local
structure and structural coherence across the ferroelectric transition,
however, were previously not well understood. Here we reveal the evolution of
the local structure with temperature in YMnO using neutron total scattering
techniques, and interpret them with the help of first-principles calculations.
The results show that, at room temperature, the local and average structures
are consistent with the established ferroelectric symmetry. On
heating, both local and average structural analyses show striking anomalies
from K up to the Curie temperature consistent with increasing
fluctuations of the order parameter angle. These fluctuations result in an
unusual local symmetry lowering into a \textit{continuum of structures} on
heating. This local symmetry breaking persists into the high-symmetry non-polar
phase, constituting an unconventional type of order-disorder transition.Comment: 10 pages, 5 figure
Thermal evolution of antiferromagnetic correlations and tetrahedral bond angles in superconducting FeTeSe
It has recently been demonstrated that dynamical magnetic correlations
measured by neutron scattering in iron chalcogenides can be described with
models of short-range correlations characterized by particular {choices of
four-spin plaquettes, where the appropriate choice changes as the} parent
material is doped towards superconductivity. Here we apply such models to
describe measured maps of magnetic scattering as a function of two-dimensional
wave vectors obtained for optimally superconducting crystals of
FeTeSe. We show that the characteristic antiferromagnetic wave
vector evolves from that of the bicollinear structure found in underdoped
chalcogenides (at high temperature) to that associated with the stripe
structure of antiferromagnetic iron arsenides (at low temperature); {these can
both be described with the same local plaquette, but with different
inter-plaquette correlations}. While the magnitude of the low-energy magnetic
spectral weight is substantial at all temperatures, it actually weakens
somewhat at low temperature, where the charge carriers become more itinerant.
The observed change in spin correlations is correlated with the dramatic drop
in the electronic scattering rate and the growth of the bulk nematic response
on cooling. Finally, we also present powder neutron diffraction results for
lattice parameters in FeTeSe indicating that the tetrahedral bond
angle tends to increase towards the ideal value on cooling, in agreement with
the increased screening of the crystal field by more itinerant electrons and
the correspondingly smaller splitting of the Fe orbitals
Liquid-like thermal conduction in a crystalline solid
A solid conducts heat through both transverse and longitudinal acoustic
phonons, but a liquid employs only longitudinal vibrations. Here, we report
that the crystalline solid AgCrSe2 has liquid-like thermal conduction. In this
compound, Ag atoms exhibit a dynamic duality that they are exclusively involved
in intense low-lying transverse acoustic phonons while they also undergo local
fluctuations inherent in an order-to-disorder transition occurring at 450 K. As
a consequence of this extreme disorder-phonon coupling, transverse acoustic
phonons become damped as approaching the transition temperature, above which
they are not defined anymore because their lifetime is shorter than the
relaxation time of local fluctuations. Nevertheless, the damped longitudinal
acoustic phonon survives for thermal transport. This microscopic insight might
reshape the fundamental idea on thermal transport properties of matter and
facilitates the optimization of thermoelectrics.Comment: four figures, supplemental informatio
Incommensurate magnetism near quantum criticality in CeNiAsO
Two phase transitions in the tetragonal strongly correlated electron system
CeNiAsO were probed by neutron scattering and zero field muon spin rotation.
For = 8.7(3) K, a second order phase transition yields an
incommensurate spin density wave with wave vector . For = 7.6(3) K, we find co-planar commensurate order with a
moment of , reduced to of the saturation moment of the
Kramers doublet ground state, which we establish by
inelastic neutron scattering. Muon spin rotation in
shows the commensurate order only exists for x 0.1 so the transition at
= 0.4(1) is from an incommensurate longitudinal spin density wave to a
paramagnetic Fermi liquid
Reprobing the mechanism of negative thermal expansion in siliceous faujasite
Combined Rietveld refinement and pair distribution function analysis of total neutron scattering data unveils the finer details of the negative thermal expansion mechanism of siliceous faujasite.</p
Manifold of spin states and dynamical temperature effects in LaCoO3: Experimental and theoretical insights
The unconventional transport and magnetic properties of perovskitelike lanthanum cobalt oxide LaCoO3 have been studied for more than five decades. This highly correlated electron system exhibits a variety of peculiar properties that are desirable for environmentally friendly energy solutions, fuel cell technologies, novel diesel engines, and oxyfuel power plants. However, the true spin state of the Co3+ ion is an important but still unresolved issue that underlies these applications. Although many theoretical models have been proposed, finding supporting experimental evidence of spin-state transitions is extremely difficult. Not until recently have new advanced scattering methods emerged allowing unprecedented precision in determining the crystal structure of LaCoO3. In this work, we combine high-resolution extended x-ray absorption fine structure, x-ray powder diffraction, and neutron powder and single-crystal diffraction over a broad range of temperatures, from 2 up to 1000 K, as well as quantum mechanical modeling to study the spin-state transition in LaCoO3 and in a reference sample of LaGaO3. Our results suggest that the Co ions are mainly in a low-spin state at temperatures below 150 K, with a minority of ions in a high-spin state. With an increase in the temperature the gradual transition from low- to intermediate-spin state occurs up until 550 K. At the metal-insulator transition at 550 K, the long-range domains of the intermediate-spin states become a dominant contribution. Above 550 K, a transition from intermediate- to high-spin state is observed. It is established that a slight change in the degree of pd hybridization can lead to the appearance of a spin-state transition which might be induced by both temperature and surface effects in powder crystallites. © 2019 American Physical Society.U.S. Department of Energy, USDOERussian Foundation for Basic Research, RFBR: 17-302-50018-molnrOffice of Science, SCThe authors are indebted to V. Efimov for stimulating discussions, to D. Chernyshov (ESRF) for his help with diffraction experiments and data analysis, and to A. Kuzmin for software creation to calculate U ⊥ and U | | from the anisotropic ADP. The results of the theoretical part of the work including DFT + DMFT calculations were obtained within the state assignment of Minobrnauki of Russia (topic Electron No. AAAA-A18-118020190098-5). Calculations were performed using the Supercomputing Center of IMM UrB RAS. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work was based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen. The reported study was funded by the RFBR within research Project No. 17-302-50018-molnr. The EXAFS experiments were performed on beamline BM29 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France
Universal geometric frustration in pyrochlores
Materials with the pyrochlore/fluorite structure have diverse technological applications, from magnetism to nuclear waste disposal. Here we report the observation of structural instability present in the pyrochlores A₂Zr₂O₆Oʹ (A = Pr, La) and Yb₂Ti₂O₆Oʹ, that exists despite ideal stoichiometry, ideal cation-ordering, the absence of lone pair effects, and a lack of magnetic order. Though these materials appear to have good long-range order, local structure probes find displacements, of the order of 0.01 nm, within the pyrochlore framework. The pattern of displacements of the A₂Oʹ sublattice mimics the entropically-driven fluxional motions characteristic of and well-known in the silica mineral β-cristobalite. The universality of such displacements within the pyrochlore structure adds to the known structural diversity and explains the extreme sensitivity to composition found in quantum spin ices and the lack of ferroelectric behavior in pyrochlores
The instrument suite of the European Spallation Source
An overview is provided of the 15 neutron beam instruments making up the initial instrument suite of the
European Spallation Source (ESS), and being made available to the neutron user community. The ESS neutron
source consists of a high-power accelerator and target station, providing a unique long-pulse time structure
of slow neutrons. The design considerations behind the time structure, moderator geometry and instrument
layout are presented.
The 15-instrument suite consists of two small-angle instruments, two reflectometers, an imaging beamline,
two single-crystal diffractometers; one for macromolecular crystallography and one for magnetism, two powder
diffractometers, and an engineering diffractometer, as well as an array of five inelastic instruments comprising
two chopper spectrometers, an inverse-geometry single-crystal excitations spectrometer, an instrument for vibrational
spectroscopy and a high-resolution backscattering spectrometer. The conceptual design, performance
and scientific drivers of each of these instruments are described.
All of the instruments are designed to provide breakthrough new scientific capability, not currently
available at existing facilities, building on the inherent strengths of the ESS long-pulse neutron source of high
flux, flexible resolution and large bandwidth. Each of them is predicted to provide world-leading performance
at an accelerator power of 2 MW. This technical capability translates into a very broad range of scientific
capabilities. The composition of the instrument suite has been chosen to maximise the breadth and depth
of the scientific impact o
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