Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites

The improper ferroelectricity in YMnO$_3$ 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$_3$ 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 $P6_3cm$ symmetry. On heating, both local and average structural analyses show striking anomalies from $\sim 800$ 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

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 $T <T_{N1}$ = 8.7(3) K, a second order phase transition yields an incommensurate spin density wave with wave vector $\textbf{k} = (0.44(4), 0, 0)$. For $T < T_{N2}$ = 7.6(3) K, we find co-planar commensurate order with a moment of $0.37(5)~\mu_B$, reduced to $30 \%$ of the saturation moment of the $|\pm\frac{1}{2}\rangle$ Kramers doublet ground state, which we establish by inelastic neutron scattering. Muon spin rotation in $\rm CeNiAs_{1-x}P_xO$ shows the commensurate order only exists for x $\le$ 0.1 so the transition at $x_c$ = 0.4(1) is from an incommensurate longitudinal spin density wave to a paramagnetic Fermi liquid

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