Introduction to Quasielastic Neutron Scattering

This tutorial introduction has been written for people who are not specialized in neutron scattering or in other scattering methods but who are interested and would like to get an impression and learn about the method of Quasielastic Neutron Scattering (QENS). The theoretical (scattering process) as well as the experimental basics (neutron sources, neutron scattering instruments, experimental periphery) are explained in a generally understandable way, with only the most essential formulas. QENS addresses the stochastic dynamics in condensed matter, and it is pointed out for which problems and for which systems in condensed matter research QENS is a powerful method. Thus sufficient information is provided to enable non-experts to think about their own QENS experiment and to understand related literature in this area of researc

Proton diffusivity in the BaZr0.9Y0.1O3−δ proton conductor

The thermally activated proton diffusion in BaZr0.9Y0.1O3−δ was studied with electrochemical impedance spectroscopy (IS) and quasi-elastic neutron scattering (QENS) in the temperature range 300-900K. The diffusivities for the bulk material and the grain boundaries as obtained by IS obey an Arrhenius law with activation energies of 0.46eV and 1.21eV, respectively. The activation energies obtained by IS for the bulk are 0.26eV above 700K and 0.46eV, below 700K. The total diffusivity as obtained by IS is by one order of magnitude lower than the microscopic diffusivity as obtained by QENS. The activation energies obtained by QENS are 0.13eV above 700K and 0.04eV, below 700K. At about 700K, the diffusion constants for IS and QENS have a remarkable crossover, suggesting two processes with different activation energie

Direct observation of phase coherence in 3-<b>k</b> magnetic configurations

International audienceWe report the observation by neutron diffraction of phase coherent Bragg reflections in a multi-k magnetic configuration with a spatial periodicity outside the conventional scattering cross-section. The peaks, which exist in the 3-k state of UAs0.8Se0.2, display long-range order with a wavevector dependence characteristic of a magnetic interaction. The results confirm the long-range order and temperature dependence reported in an earlier study of similar peaks in this material using x-ray resonant scattering by (a) the non-trivial extension to the technique of neutron diffraction, and (b) the observation of similar 3-k phase-coherent reflections in other samples by x-ray resonant scattering. The importance of the neutron diffraction results lies primarily in the fact that magnetic neutron diffraction is well established as a weak probe operating on thermodynamic time scales. This alleviates concern that the rapid (10-15 - 10-14 s), strong interaction, characteristic of the resonant x-ray technique, is imaging a transient or non-equilibrium configuration. Likewise, the extension of the x-ray resonant scattering results to other samples establishes the generality of this effect. The enigma of how to understand the observed diffraction, which appears to lie strictly outside both the conventional neutron and x-ray scattering cross sections, remains

Introduction to Quasielastic Neutron Scattering

ISSN:0942-9352ISSN:0044-3336ISSN:2196-715

Structural and Dynamical Properties of Water Confined in Highly Ordered Mesoporous Silica in the Presence of Electrolytes

International audienc

Ligand Dynamics in Nanocrystal Solids Studied with Quasi-Elastic Neutron Scattering

Nanocrystal surfaces are commonly populated by organic ligands, which play a determining role in the optical, electronic, thermal, and catalytic properties of the individual nanocrystals and their assemblies. Understanding the bonding of ligands to nanocrystal surfaces and their dynamics is therefore important for the optimization of nanocrystals for different applications. In this study, we use temperature-dependent, quasielastic neutron scattering (QENS) to investigate the dynamics of different surface bound alkanethiols in lead sulfide nanocrystal solids. We select alkanethiols with mono- and dithiol terminations, as well as different backbone types and lengths. QENS spectra are collected both on a time-offlight spectrometer and on a backscattering spectrometer, allowing us to investigate ligand dynamics in a time range from a few picoseconds to nanoseconds. Through model-based analysis of the QENS data, we find that ligands can either (1) precess around a central axis, while simultaneously rotating around their own molecular axis, or (2) only undergo uniaxial rotation with no precession. We establish the percentage of ligands undergoing each type of motion, the average relaxation times, and activation energies for these motions. We determine, for example, that dithiols which link facets of neighboring nanocrystals only exhibit uniaxial rotation and that longer ligands have higher activation energies and show smaller opening angles of precession due to stronger ligand−ligand interactions. Generally, this work provides insight into the arrangement and dynamics of ligands in nanocrystal solids, which is key to understanding their mechanical and thermal properties, and, more generally, highlights the potential of QENS for studying ligand behavior.ISSN:1936-0851ISSN:1936-086

Quasielastic and elastic scattering studies of aligned DMPC multilayers at different hydrations

Lipid model membranes such as 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (DMPC) serve as role models for their more complex counterparts in biological systems. Quasielastic neutron scattering (QENS) [1-3], inelastic neutron scattering (INS) [4] and neutron spin echo spectroscopy (NSE) [5] have been employed to study local as well as collective dynamics of these membranes on a ps-ns time scale. Most of these studies lack a systematic investigation of the behavior of the model membranes in dependence on their hydration. We now started a detailed investigation of hydration effect on model membrane systems. The complexity of the dynamics can be further reduced by selective deuteration, which allows to distinguish between dynamics of different part of the lipid molecules. In the here presented work we have used chain deuterated DMPC-d54 to study the dynamics of the lipid head group. To probe both dynamics in the plane of the membrane and perpendicular to it, the samples were prepared on cleaned silicon wafers. The hydration for the two samples was adjusted by hydrating them for pure D2O and from a saturated salt solution respectively, resulting in two different states of hydration (repeat distance d=62.5 Å with 15 water molecules per lipid and d = 54.9 Å with 9 water molecules per lipid, respectively). The alignment and mosaicity were checked prior to the measurements for all samples by neutron diffraction and was found to be below 1°. QENS experiments were performed at the time-of-flight spectrometer TOFTOF at the research reactor FRMII in Munich (energy resolution: 56 μeV FWHM) in the temperature range from 5°C to 30°C to cover the main phase transition from the Pβ gel phase to the liquid crystalline Lα phase of DMPC which occurs around 23°C. Elastic incoherent neutron scattering (EINS) measurements were performed at the high momentum transfer backscattering spectrometer IN13 (energy resolution 8 μeV FWHM) and the cold neutron backscattering spectrometer IN16 (energy resolution 0.9 μeV FWHM) both at the Institut Laue-Langevin (ILL), Grenoble. For the QENS experiment elastic incoherent structure factors (EISF) and diffusion constants were extracted, which indicate that hydration has a clear influence on the mobility of this system [6]. The integrated intensities from the EINS experiments showed a shift of the main phase transition as a function of hydration which coincides with a change of the slopes of the mean square displacements [7]. In addition to earlier QENS [1-3] and backscattering [8] investigations, these experiments extend our knowledge of model membrane systems. References [1] S. König et al., J.Phys.II France, 2 (1992) 1589-1615 [2] M.C. Rheinstädter et al., Phys. Rev. E 75 (2007) 011907 [3] S. Busch et al., JACS 132, (2010), 3232-3233 [4] M.C. Rheinstädter et al., Phys Rev. Lett. 93 (2004) 108107 [5] M.C. Rheinstädter et al., Phys. Rev. Lett. 97 (2006) 048103 [6] M. Trapp et al., in preperation [7] M. Trapp et al., Spectrosc.-int. J., accepted (2010) [8] M.C. Rheinstädter et al., Phys. Rev. E 71 (2005) 061908 (2002