Institut Laue-Langevin

An experimental search for an electric dipole moment (EDM) of the neutron has been carried out at the Institut Laue-Langevin, Grenoble. Spurious signals from magnetic-field fluctuations were reduced to insignificance by the use of a cohabiting atomic-mercury magnetometer. Systematic uncertainties, including geometric-phase-induced false EDMs, have been carefully studied. The results may be interpreted as an upper limit on the neutron EDM of |d(n)|< 2.9 x 10(-26)e cm (90% C.L.).

A universal description of the low-energy properties of one-dimensional quantum fluids, based on a harmonic theory of long-wavelength density fluctuations with use of renormalized parameters, is outlined. The structure of long-distance correlations of a spinless fluid is obtained, showing the essential similarity of one-dimensional Bose and Fermi fluids. The results are illustrated by application to the one-dimensional Bose fluid with $\ensuremath{\delta}$-function interaction.

Abstract Magnetization, magnetic susceptibility, optical microscopy, X-ray diffraction and neutron diffraction measurements have been made on a quenched sample of Ni2MnGa. Room-temperature neutron diffraction patterns indicate a highly ordered Heusler alloy, L21 type, structure. The alloy is ferromagnetic with a Curie temperature of 376 K, and a magnetic moment of 4·17μB largely confined to the Mn sites, but probably with a small moment <07·3μB associated with the Ni sites. A martensitic phase transition to a complex tetragonal structure occurs on cooling below 202 K. Neutron diffraction oscillation photographs taken using the D12 facility at Institut Laue Langevin, Grenoble, and low-temperature powder neutron diffraction data enabled details of the low temperature crystallographic superlattice and magnetic structure to be determined. Reasons for the structural transformation are discussed.

We assume the time-averaged structure of a macroion solution to be determined only by the repulsive screened Coulomb pair potential between finite macroions. The counter ions and solvent are treated as a uniform neutralizing background which determines the screening of the potential. We slove the Ornstein-Zernike equation in the mean spherical approximation to obtain closed analytic forms for the direct correlation function c(r) and the structure factor S(Q). In the zero charge limit, the Percus-Yevick hard sphere solution is recovered. As charge is added to the macroions at given volume fraction, the isothermal compressibility decreases and S(Q) shows increasing structure, eventually exhibiting solid-like behaviour. The results provide a useful model basis for studying interacting colloidal systems, and finite ion screened one component plasmas in general.

Abstract This review attempts to present the most salient developments of research on organic conductors and superconductors during the past 10 years. A theoretical introduction treats instabilities of quasi-one-dimensional electron systems and associated precursor effects which are relevant to the experimental results on organic conductors. We then describe the characterization of quasi-one-dimensional organic conductors by their transport, optical and magnetic properties. Finally, two sections are devoted to the experimental investigation of the low temperature instabilities: lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF)2X series. The importance of one-dimensional fluctuations is emphasized in both lattice and superconducting instabilities.

Starting from the most general form of the Anderson hamiltonian, the behaviour of magnetic impurities in real metals is considered, taking into account the orbital structure of the local impurity electrons, crystal field and spin orbit splittings. The analysis is carried out in an atomic limit, in which the impurity has a well defined integer valency (a Schrieffer Wolff transformation is then valid). The main steps of a scaling procedure are described in detail. As the temperature goes down, the excited states of the ground state configuration decouple one after the other. The hierarchy of these decouplings, and their interplay with Kondo singularities are analyzed. When a Fermi liquid picture applies as T → 0, the number of independent parameters may be reduced considerably using symmetry and universality arguments which bypass the numerical description of the crossover region. That first part sets a language in which to describe specific problems. We apply that language to the case where the atomic ground state is an orbital singlet. In the absence of anisotropies, the only parameters are the impurity spin S and the number of orbital channels n. We show that an anomalous fixed point occurs at finite coupling when n > 2 S. That fixed point is unstable with respect to anisotropies. The scaling trajectories are discussed for a cubic crystal field for several choices of valencies. The universality of the low temperature behaviour is clarified. A similar analysis is carried out when the atomic ground state only has one electron (or hole). The influence of crystal field and spin orbit interactions is analyzed — and their relevance to the Kondo crossover and to universality is ascertained.

The structure of the protein-solvent interface is the subject of controversy in theoretical studies and requires direct experimental characterization. Three proteins with known atomic resolution crystal structure (lysozyme, Escherichia coli thioredoxin reductase, and protein R1 of E. coli ribonucleotide reductase) were investigated in parallel by x-ray and neutron scattering in H2O and D2O solutions. The analysis of the protein-solvent interface is based on the significantly different contrasts for the protein and for the hydration shell. The results point to the existence of a first hydration shell with an average density approximately 10% larger than that of the bulk solvent in the conditions studied. Comparisons with the results of other studies suggest that this may be a general property of aqueous interfaces.

The evolution of the structural properties of ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ was determined as a function of temperature, average $A$-site radius $〈{r}_{A}〉,$ and applied pressure for the ``optimal'' doping range $x=0.25,$ 0.30, by using high-resolution neutron powder diffraction. The metal-insulator transition, which can be induced both as a function of temperature and of $〈{r}_{A}〉,$ was found to be accompanied by significant structural changes. Both the paramagnetic charge-localized phase, which exists at high temperatures for all values of $〈{r}_{A}〉,$ and the spin-canted ferromagnetic charge-ordered phase, which is found at low temperatures for low values of $〈{r}_{A}〉,$ are characterized by large metric distortions of the ${\mathrm{MnO}}_{6}$ octahedra. These structural distortions are mainly incoherent with respect to the space-group symmetry, with a significant coherent component only at low $〈{r}_{A}〉.$ These distortions decrease abruptly at the transition into the ferromagnetic metal phase. These observations are consistent with the hypothesis that, in the insulating phases, lattice distortions of the Jahn-Teller type, in addition to spin scattering, provide a charge-localization mechanism. The evolution of the average structural parameters indicates that the variation of the electronic bandwidth is the driving force for the evolution of the insulator-to-metal transition at ${T}_{C}$ as a function of ``chemical'' and applied pressure.

The unusual magnetic properties of ${\mathrm{La}}_{0.5}$ ${\mathrm{Ca}}_{0.5}$ ${\mathrm{MnO}}_{3}$ were found to be associated with structural and magnetic ordering phenomena, resulting from the close interplay between charge, orbital, and magnetic ordering. Analysis of synchrotron x-ray and neutron powder diffraction data indicates that the anomalous and hysteretic behavior of the lattice parameters occurring between ${\mathrm{T}}_{\mathrm{C}}$ \ensuremath{\sim}225 K and ${\mathrm{T}}_{\mathrm{N}}$ \ensuremath{\sim}155 K is due to the development of a Jahn-Teller (J-T) distortion of the ${\mathrm{MnO}}_{6}$ octahedra, the ${\mathrm{d}}_{\mathrm{z}}^{2&gt;}$ orbitals being oriented perpendicular to the orthorhombic b axis. We observed an unusual broadening of the x-ray Bragg reflections throughout this temperature region, suggesting that this process occurs in stages. Below ${\mathrm{T}}_{\mathrm{N}}$ , the development of well-defined satellite peaks in the x-ray patterns, associated with a transverse modulation with q=[1/2-\ensuremath{\varepsilon},0,0], indicates that quasicommensurate (\ensuremath{\varepsilon}\ensuremath{\sim}0) orbital ordering occurs within the a-c plane as well. The basic structural features of the charge-ordered low-temperature phase were determined from these satellite peaks. The low-temperature magnetic structure is characterized by systematic broadening of the magnetic peaks associated with the ``${\mathrm{Mn}}^{+3}$ '' magnetic sublattice. This phenomenon can be explained by the presence of magnetic domain boundaries, which break the coherence of the spin ordering on the ${\mathrm{Mn}}^{+3}$ sites while preserving the coherence of the spin ordering on the ${\mathrm{Mn}}^{+4}$ sublattice as well as the identity of the two sublattices. The striking resemblance between these structures and the structural ``charge ordering'' and ``discommensuration'' domain boundaries, which were recently observed by electron diffraction and real-space imaging, strongly suggests that these two types of structures are the same and implies that, in this system, commensurate long-range charge ordering coexists with quasicommensurate orbital ordering.

The structure of a dispersion of charged colloidal particles interacting through a screened Coulomb potential is well described at moderate to high densities by calculating the correlations in the mean spherical approximation (MSA). In this paper we extend the validity of the MSA calculation to arbitrarily low densities, using a physical rescaling argument which preserves the analytic form of the MSA solution. Our results are in excellent agreement with other numerical calculations and with experimental low-density light scattering data. The method may be viewed as a generalization of Gillan's prescription for one component plasmas to systems interacting through a Yukawa potential. An advantage of the present prescription is that it allows a smooth transition from strong to weak coupling, and it implies no functional relation between the experimentally independent parameters.

A relation between the spectrum and correlation exponents of the Luttinger model is argued to be a general property of a universality class called "Luttinger liquids." The spinless fermion model equivalent to the $S=\frac{1}{2}$ Heisenberg-Ising -$\mathrm{XY}$ chain in a field is argued to belong to this class, allowing for the first time the systematic calculation of its correlation exponents.

An effective environmental force constant is introduced to quantify the molecular resilience (or its opposite, "softness") of a protein structure and relate it to biological function and activity. Specific resilience-function relations were found in neutron-scattering experiments on purple membranes containing bacteriorhodopsin, the light-activated proton pump of halobacteria; the connection between resilience and stability is illustrated by a study of myoglobin in different environments. Important advantages of the neutron method are that it can characterize the dynamics of any type of biological sample-which need not be crystalline or monodisperse-and that it enables researchers to focus on the dynamics of specific parts of a complex structure with deuterium labeling.

The structure of cellulose microfibrils in wood is not known in detail, despite the abundance of cellulose in woody biomass and its importance for biology, energy, and engineering. The structure of the microfibrils of spruce wood cellulose was investigated using a range of spectroscopic methods coupled to small-angle neutron and wide-angle X-ray scattering. The scattering data were consistent with 24-chain microfibrils and favored a "rectangular" model with both hydrophobic and hydrophilic surfaces exposed. Disorder in chain packing and hydrogen bonding was shown to increase outwards from the microfibril center. The extent of disorder blurred the distinction between the I alpha and I beta allomorphs. Chains at the surface were distinct in conformation, with high levels of conformational disorder at C-6, less intramolecular hydrogen bonding and more outward-directed hydrogen bonding. Axial disorder could be explained in terms of twisting of the microfibrils, with implications for their biosynthesis.

Stoichiometric RMnO3 perovskites have been prepared in the widest range of R3+ ionic sizes, from PrMnO3 to ErMnO3. Soft-chemistry procedures have been employed; inert-atmosphere annealings were required to synthesize the materials with more basic R cations (R = Pr, Nd), in order to minimize the unwanted presence of Mn4+. On the contrary, annealings in O2 flow were necessary to stabilize the perovskite phases for the last terms of the series, HoMnO3, ErMnO3, and YMnO3, thus avoiding or minimizing the formation of competitive hexagonal phases with the same stoichiometry. The samples have been investigated at room temperature by high-resolution neutron powder diffraction to follow the evolution of the crystal structures along the series. The results are compared with reported data for LaMnO3. The distortion of the orthorhombic perovskite (space group Pbnm), characterized by the tilting angle of the MnO6 octahedra, progressively increases from Pr to Er due to simple steric factors. Additionally, all of the perovskites show a distortion of the MnO6 octahedra due to the orbital ordering characteristic of the Jahn-Teller effect of Mn3+ cations. The degree of orbital ordering slightly increases from La to Tb and then remains almost unchanged for the last terms of the series. The stability of the crystal structure is also discussed in light of bond-valence arguments.

Following the early prediction of the skyrmion lattice (SkL)--a periodic array of spin vortices--it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with Cnv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C3v) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Néel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices.

The magnetic transitions in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0.25, 0.50$) were studied by synchrotron x-ray and neutron powder diffraction. For $x=0.25$ the large $\ensuremath{\Delta}V=0.13%$ at ${T}_{C}$ is due to the simultaneous insulator-to-metal transition. For $x=0.50$ below ${T}_{C}$ we observed a drastic decrease of the $b$ axis and an increase of the $a$ and $c$ axes, due to a Jahn-Teller distortion of Mn${\mathrm{O}}_{6}$ octahedra. The ferromagnetic transition for $x=0.50$ is accompanied by an unusual peak broadening terminating at ${T}_{N}$. This newly observed behavior can be explained assuming that multiple intermediate phases are simultaneously present in the sample.

Extensive and high-quality quasi-elastic incoherent neutron scattering data were obtained for water in the temperature range extending from room temperature down to -20 \ifmmode^\circ\else\textdegree\fi{}C in the supercooled state. The analysis generally confirms findings of our previous experiment [S. H. Chen, J. Teixeira, and R. Nicklow, Phys. Rev. A 26, 3477 (1982)], but in particular three new results have been obtained: (a) two relaxation times are clearly identified, which are related to the short-time and intermediate-time diffusion of water molecules, respectively, and their temperature dependence has been determined; (b) one of these relaxation times is associated with jump diffusion of the protons, and the temperature dependence of the jump length has been qualitatively determined; (c) the Q dependence of the scattering intensity integrated over the quasi-elastic region gives a Debye-Waller factor which is temperature independent.

The crystal structure of LnFeAsO$_{1-y}$ (Ln = La, Nd) has been studied by the powder neutron diffraction technique. The superconducting phase diagram of NdFeAsO$_{1-y}$ is established as a function of oxygen content which is determined by Rietveld refinement. The small As-Fe bond length suggests that As and Fe atoms are connected covalently. FeAs$_4$-tetrahedrons transform toward a regular shape with increasing oxygen deficiency. Superconducting transition temperatures seem to attain maximum values for regular FeAs$_4$-tetrahedrons.

The methylammonium cation in [CH3NH3]PbI3 demonstrates increasing positional disorder on heating from 100 K to 352 K. In the tetragonal phase, stable between 165 K and 327 K, the cation is disordered over four sites directed toward the faces of the distorted cubic [PbI3](-) framework and migrates towards the cavity centre with increasing temperature.

We present the result of an experiment to measure the electric dipole moment (EDM) of the neutron at the Paul Scherrer Institute using Ramsey's method of separated oscillating magnetic fields with ultracold neutrons. Our measurement stands in the long history of EDM experiments probing physics violating time-reversal invariance. The salient features of this experiment were the use of a ^{199}Hg comagnetometer and an array of optically pumped cesium vapor magnetometers to cancel and correct for magnetic-field changes. The statistical analysis was performed on blinded datasets by two separate groups, while the estimation of systematic effects profited from an unprecedented knowledge of the magnetic field. The measured value of the neutron EDM is d_{n}=(0.0±1.1_{stat}±0.2_{sys})×10^{-26} e.cm.