Middle Valley, TN, United States
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Leu L.-C.,Boise State University | Bian J.,Shanghai University | Gout D.,Oak Ridge National Laboratory | Gout D.,Juelich Center for Neutron Science | And 2 more authors.
RSC Advances | Year: 2012

The order-disorder phase transition of magnesium lithium titanate solid-solution (1-x)Li 2TiO 3-xMgO (0 ≤ x ≤ 0.5) ceramics prepared by conventional solid-state processing has been examined. The phase and structural analysis was carried out using electron diffraction, neutron diffraction and high-resolution transmission electron microscopy. Both electron and neutron diffraction results revealed the onset of an order-to-disorder transition at 0.3 < x < 0.4. Superlattice reflections found in certain regions of x = 0.2 samples and most areas of x = 0.3 samples were caused by a twin structure stabilized by Mg incorporation. Rietveld refinements of neutron diffraction data suggested a random distribution of Mg on the Li 4e sites and equal distribution of Mg on the two Ti 4e sites for x ≤ 0.3. As the Mg content continues to increase, the crystal symmetry transforms from monoclinic to cubic rocksalt. Consequently, the cation ordering on the 8f and 4d sites of the C2/c structure became corrupted and turned into short-range ordering on the 4a sites of a cubic structure with symmetry, resulting in diffuse scattering in electron diffraction patterns. © 2012 The Royal Society of Chemistry.

Baum M.,University of Cologne | Leist J.,University of Gottingen | Finger T.,University of Cologne | Schmalzl K.,Juelich Center for Neutron Science | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

The time dependence of switching multiferroic domains in MnWO4 has been studied by time-resolved polarized neutron diffraction. Inverting an external electric field inverts the chiral magnetic component within rise times ranging between a few and some tens of milliseconds in perfect agreement with macroscopic techniques. There is no evidence for any faster process in the inversion of the chiral magnetic structure. The time dependence is well described by a temperature-dependent rise time suggesting a well-defined process of domain reversion. As expected, the rise times decrease when heating towards the upper boundary of the ferroelectric phase. However, switching also becomes faster upon cooling towards the lower boundary, which is associated with a first-order phase transition. © 2014 American Physical Society.

Recarte V.,Public University of Navarra | Perez-Landazabal J.I.,Public University of Navarra | Sanchez-Alarcos V.,Public University of Navarra | Cesari E.,University of the Balearic Islands | And 4 more authors.
Applied Physics Letters | Year: 2013

The martensitic transformation in stoichiometric Ni2MnGa alloys is preceded by a weakly first order transformation from a high temperature cubic phase to a near-cubic modulated intermediate phase related to the presence of a soft phonon mode. This transformation has been proposed to appear as a consequence of the magnetoelastic coupling. Inelastic neutron scattering experiment performed under external magnetic field shows a temperature shift of the characteristic energy dip at ζ ≈ 0.33. Furthermore, an enhancement of the long-wavelength limit (C ′) of this branch with the applied magnetic field has been observed. Both results evidence a strong magnetoelastic interaction at the intermediate transition. © 2013 AIP Publishing LLC.

Wang Q.,Fudan University | Shen Y.,Fudan University | Pan B.,Fudan University | Hao Y.,Fudan University | And 14 more authors.
Nature Materials | Year: 2016

In iron-based superconductors the interactions driving the nematic order (that breaks four-fold rotational symmetry in the iron plane) may also mediate the Cooper pairing. The experimental determination of these interactions, which are believed to depend on the orbital or the spin degrees of freedom, is challenging because nematic order occurs at, or slightly above, the ordering temperature of a stripe magnetic phase. Here, we study FeSe (ref.) - which exhibits a nematic (orthorhombic) phase transition at Ts = 90 K without antiferromagnetic ordering - by neutron scattering, finding substantial stripe spin fluctuations coupled with the nematicity that are enhanced abruptly on cooling through Ts. A sharp spin resonance develops in the superconducting state, whose energy (∼4 meV) is consistent with an electron-boson coupling mode revealed by scanning tunnelling spectroscopy. The magnetic spectral weight in FeSe is found to be comparable to that of the iron arsenides. Our results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations. © 2016 Macmillan Publishers Limited. All rights reserved.

Dhiman I.,Bhabha Atomic Research Center | Das A.,Bhabha Atomic Research Center | Mittal R.,Bhabha Atomic Research Center | Su Y.,Juelich Center for Neutron Science | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The short-range-ordered magnetic correlations have been studied in half-doped La0.5 Ca0.5-x Srx MnO3 (x=0.1, 0.3, and 0.4) compounds by polarized neutron scattering technique. On doping Sr2+ for Ca2+ ion, these compounds with x=0.1, 0.3, and 0.4 exhibit CE-type, mixture of CE-type and A -type, and A -type antiferromagnetic ordering, respectively. Magnetic diffuse scattering is observed in all the compounds above and below their respective magnetic ordering temperatures and is attributed to magnetic polarons. The correlations are primarily ferromagnetic in nature above TN, although a small antiferromagnetic contribution is also evident. Additionally, in samples x=0.1 and 0.3 with CE-type antiferromagnetic ordering, superlattice diffuse reflections are observed indicating correlations between magnetic polarons. On lowering temperature below TN, the diffuse scattering corresponding to ferromagnetic correlations is suppressed and the long-range-ordered antiferromagnetic state is established. However, the short-range-ordered correlations indicated by enhanced spin-flip scattering at low Q coexist with long-range-ordered state down to 3 K. In x=0.4 sample with A -type antiferromagnetic ordering, superlattice diffuse reflections are absent. Additionally, in comparison to x=0.1 and 0.3 sample, the enhanced spin-flip scattering at low Q is reduced at 310 K, and as temperature is reduced below 200 K, it becomes negligibly low. The variation in radial correlation function, g (r) with temperature indicates rapid suppression of ferromagnetic correlations at the first nearest neighbor on approaching TN. Sample x=0.4 exhibits growth of ferromagnetic phase at intermediate temperatures (∼200 K). This has been further explored using small-angle neutron scattering and neutron depolarization techniques. © 2010 The American Physical Society.

Stock C.,Center for Neutron Research | Stock C.,Indiana University | Chapon L.C.,Rutherford Appleton Laboratory | Schneidewind A.,TU Dresden | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We have investigated spin fluctuations in the langasite compound Ba 3NbFe3Si2O14 both in the ordered state and as a function of temperature. The low-temperature magnetic structure is defined by a spiral phase characterized by magnetic Bragg peaks at q =(0,0,τ1/7) onset at TN=27 K as previously reported by Marty. The nature of the fluctuations and temperature dependence of the order parameter is consistent with a classical second-order phase transition for a two-dimensional triangular antiferromagnet. We show that the physical properties and energy scales including the ordering wave vector, Curie-Weiss temperature, and spin waves can be explained through the use of only symmetric exchange constants without the need for a dominant Dzyaloshinskii-Moriya interaction. This is accomplished through a set of "helical" exchange pathways along the c direction imposed by the chiral crystal structure and naturally explains the magnetic diffuse scattering, which displays a strong vector chirality up to high temperatures, well above the ordering temperature. This illustrates a strong coupling between magnetic and crystalline chirality in this compound. © 2011 American Physical Society.

Chua Y.Z.,University of Rostock | Schulz G.,University of Rostock | Shoifet E.,University of Rostock | Huth H.,University of Rostock | And 3 more authors.
Colloid and Polymer Science | Year: 2014

Molecular dynamics is often studied by broad band dielectric spectroscopy (BDS) because of the wide dynamic range available and the large number of processes resulting in electrical dipole fluctuations and with that in a dielectrically detectable relaxation process. Calorimetry on the other hand is an effective analytical tool to characterize phase and glass transitions by its signatures in heat capacity. In the linear response scheme, heat capacity is considered as entropy compliance. Consequently, only processes significantly contributing to entropy fluctuations appear in calorimetric curves. The glass relaxation is a prominent example for such a process. Here, we present complex heat capacity at the dynamic glass transition (segmental relaxation) of polystyrene (PS) and poly(methyl methacrylate) (PMMA) in a dynamic range of 11 orders of magnitude, which is comparable to BDS. As one of the results, we determined the characteristic length scale of the corresponding fluctuations. The dynamic glass transition measured by calorimetry is finally compared to the cooling rate dependence of fictive temperature and BDS data. For PS, dielectric and calorimetric data are similar but for PMMA with its very strong secondary relaxation process some peculiarities are observed. © 2014 Springer-Verlag.

Lancor B.,University of Wisconsin - Madison | Babcock E.,Juelich Center for Neutron Science | Wyllie R.,University of Wisconsin - Madison | Walker T.G.,University of Wisconsin - Madison
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

We present measurements of the circular dichroism of optically pumped Rb vapor near the D1 resonance line. Collisions with the buffer gases He3 and N2 reduce the transparency of the vapor, even when fully polarized. We use two methods to measure this effect, show that the He results can be understood from RbHe potential curves, and show how this effect conspires with the spectral profile of the optical pumping light to increase the laser power demands for optical pumping of very optically thick samples. © 2010 The American Physical Society.

Lancor B.,University of Wisconsin - Madison | Babcock E.,Juelich Center for Neutron Science | Wyllie R.,University of Wisconsin - Madison | Walker T.G.,University of Wisconsin - Madison
Physical Review Letters | Year: 2010

We present measurements, by using two complementary methods, of the breakdown of atomic angular momentum selection rules in He-broadened Rb vapor. Atomic dark states are rendered weakly absorbing due to fine-structure mixing during Rb-He collisions. The effect substantially increases the photon demand for optical pumping of dense vapors. © 2010 The American Physical Society.

Babcock E.,Juelich Center for Neutron Science | Salhi Z.,Juelich Center for Neutron Science | Appavou M.-S.,Juelich Center for Neutron Science | Feoktystov A.,Juelich Center for Neutron Science | And 5 more authors.
Physics Procedia | Year: 2013

In soft matter small angle neutron scattering (SANS) studies at large Q values, incoherent scattering becomes the dominant signal. In the Q-range of interest to this work, from 0.2 Å-1 to about 1.0 Å-1, the coherent scattering from the typical protein or polymer in a D2O buffer solution inevitably drops one to two orders of magnitude or more below the total scattering. Even after careful and accurate subtraction of the measured D2O buffer scattering, the remaining corrected, i.e. sample-only, signal will still be dominated by diffuse incoherent scattering from hydrogen in the sample itself. This is the exact region of interest when one wishes to probe the structural changes in "living" proteins caused by interactions and motions related to function. To further complicate the problem, there is strong motivation to measure this Q-regime at very low concentrations because it has been shown with wide angle X-ray scattering that proteins can undergo concentration-dependent structural changes that rapidly increase below concentrations of about 5% [1] motivating the study of protein solutions at ever lower concentrations. In this case the signal from the protein will inevitably become much less than the scattering of the D2O buffer solution it is contained in. Polarization analysis offers the opportunity to separate the weak coherent signal from the larger incoherent signal and perhaps enable measurements under the conditions described above. This paper will address the issues associated with the correct separation of coherent and incoherent scattering for soft matter samples. We have performed tests measurements on KWS2 which show the viability of the method on a protonated α-lactalbumin solution at 2.5% (1 mm thick) and 0.25% (2 mm thick) concentrations in a D2O buffer solution. Additionally describe a the method of implementation using 3He spin filters, some practical considerations, and future plans for a dedicated device at the JCNS. © 2013 The Authors.

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