Neutron Scattering in Novel Materials

The following sections are included:

• EDITORIAL

• ORGANISATION OF THE SCHOOL

• CONTENTS

Neutron scattering is a powerful and direct method for investigating the static and dynamic properties of materials in many fields of science. The weak interaction between neutron and sample allows in most cases the use of the first Born approximation for the calculation of the scattering cross sections. In the following introduction we discuss some basic properties of neutrons, in particular its nuclear and magnetic interaction with matter. We demonstrate that the measured scattering function S(Q,ω) is related to the pair correlation function G(r,t) by a Fourier transform in space and time. This is exemplified for the case of neutron diffraction from periodic structures.

The observation of macroscopic samples by neutron beams represents an important tool for nondestructive testing. Due to the properties of neutrons in interaction with matter, some light nuclei give high contrasts and heavy elements can relatively easily be penetrated. Based on practical experiences from the experimental work at the PSI radiography stations (NEUTRA and NCR), the application range of neutron imaging will be demonstrated with selected examples, also in comparison to the competing techniques of gamma and X-ray methods. This article describes the present state of the art in neutron radiography and tries to extrapolate to future developments.

Tomography is the process of reconstructing a three-dimensional distribution of an attenuation coefficient from a multitude of two-dimensional projections of the sample, taken from different angles. In principle, tomography can be performed with any non-dispersive radiation. With the availability of large areas digital neutron detectors and more powerful computers, neutron tomography is rapidly gaining importance throughout research facilities in the world. This paper provides an introduction to the tomography principle and stresses the importance of neutron beam design for high quality of neutron tomographic images. Several examples taken at the Paul Scherrer Institute and at the Technical University Munich are presented.

Neutron powder diffraction was the first application of neutron scattering, and won the 1994 Nobel prize for Shull. It obtained a new lease of life with Rietveld's 1967 ‘profile refinement method’, and since then has been among the essential techniques applied to all new inorganic materials. In particular, it has proved essential for the study of mineral and magnetic structures, structural transitions, ferro-electrics, solid liquids and gases, ice structures, hydrogen in metals, zeolites, solid electrolytes, quasi-crystals, heavy metal oxides, mixed valence, superconductors, GMR materials, and many others. New kinds of detectors are rapidly extending the power of the technique In this article, we will introduce the experimental techniques, and illustrate their use for new materials.

Neutron scattering methods are powerful tools to study service related changes in the microstructure which results in the deterioration of the mechanical properties. The high penetration depth offers the possibility of non-destructive investigations. Texture analysis gives information about the history of the microstructure in the material. The estimation of the strain distribution in a component detects plastic deformed location and critical positions for crack initiation and crack growth. Investigations of precipitates increase the knowledge about the structural reasons of some hardening and embrittlement processes.

Elastic coherent scattering from real crystals is not confined to Bragg reflections that originate from the average lattice. Diffuse scattering is due to any breaking of the translationl symmetry of the lattice. While the occupation of lattice sites and the static atomic emplacements of solid solutions is obtained from wide-angle scattering, small-angle scattering reveals properties of larger objects such as precipitates. The various scattering contributions are introduced and the present possibilities to identify structural aspects of real crystals will be illustrated for binary and ternary cubic alloys.

Optically active materials are required for applications in photonics, optical communiction, computing and high-density holographic storage techniques. Valuable information on optically active states, optical structurization and its kinetics can be drawn from experiments with neutrons. In particular, slow relaxation processes in the glass phase of photopolymers can be investigated with HOLONS, a new neutron facility at the GKSS research center in Geesthacht The relationship between photons and neutrons is a sysnergetic one, since neutron physics likewise profits from concepts developed in light (nonlinear) optics. Exemplarily, we discuss the electro-neutron-optic effect and holographic grating interferometers for cold neutrons.

The overwhelming part of what is currently considered as soft condensed matter systems consists of organic molecules and sometimes water. Systems range from polymer melts to biomolecules in aqueous solution, they comprise complex fluids like microemulsions, membranes and vesicles, aggregating and non-aggregating polymers in solutions and melts etc. The main elements present in the constituents are H, C, N, O yielding often only poor X-ray contrast. Therfore neutrons turn out to be a unique probe for which contrast is readily at the disposition by H/D replacement. In addition motions on a molecular level are accessible by inelastic neutron spectroscopy. First some general principles are explained which are illustrated by the results of neutron investigation on block-copolymer aggregates in solution and the dynamics of long polymer chains in a melt.

Small Angle Neutron Scattering investigations into the microscopic deformation of chains in uniaxially strained polymer networks are presented. The structure factor S(q) of a single chain inside the network topology is a perfect descriptor to discriminate between current theories of rubber elasticity. The experimental data are succesfully interpreted in terms of a mean-field tube model which describes the confinement of chains due to the chemical crosslinking and chain entanglements by an effective tube which is anisotropic and depends on strain non-affinely as . Different length scales in the rubber are studied and a transition from affine to phantom-like behaviour of chain deformation is found for decreasing lengths of the investigated part of the chain. Applications to block copolymers with different architectures as well to the field of filled elastomers are summarized.

Nowadays life would be difficult to imagine without the ubiquitous presence of polymers. They are very important from both, the technical/practical and the basic point of view. As chains made of simply connected objects, polymers show typical soft matter properties, whereas glass-forming system features prevail at local scales. These long objects present an intriguing dynamical complexity.

Energy-resolving neutron scattering techniques are capable of providing unique spatiotemporal information on the rich dynamics of biological macromolecules and their building blocks. Following a summary of basic aspects and an outline of the principal effects observable, quasielastic and inelastic scattering studies of globular and fibrous proteins, polypeptides, and peptide models are reviewed. The aim is to provide a concise, informative overview for non-specialist readers.

This lecture intends to give a didactic introduction to the method of spin contrast variation for macromolecular research. The dependence of the coherent and incoherent scattering cross sections on nuclear polarization is discussed first The next section describes the experimental prerequisites to create a high nuclear polarization in hydrogenous materials. Spin contrast variation, which has become a powerful tool to introduce large and changeable contrast in a large class of samples, is described next. Covering a field with many facets on very limited space, the material is presented selecting a few simple pictures rather than aiming at completeness. Present developments of time resolved neutron scattering experiments with potential new applications will be briefly outlined at the end.

An outline of the basic optics, on the instrumental aspects and the two general measurement modes to perform a reflectometric experiment is presented. Aspects of polarized neutron reflectivity, visible in off-specular intensity maps are discussed. Subsequently the paper describes the case of supported biomembranes as a model system of complex lipid structures that show structural processes in different environments. Experimental data and interpretation are compared.

Nanocrystalline materials can exhibit properties which are considerably different from their coarse-grained counterparts, making them unique for basic or applied research and also very promising for potential applications. The topics which are addressed in the present paper are magnetic properties and magnetic correlations on the nanometer scale, mechanical properties, in particular the influence of grain boundaries on the elastic and plastic behaviour, and vibration modes of grain boundary atoms in nanostructured materials.

Some examples of magnetic ordering phenomena in rare-earth compounds that include ferromagnetism, commensurate antiferromagnetism as well as incommensurate magnetic structures are given. The basic concepts concerning magnetic structures and their investigation are discussed. The determination of magnetic moment arrangements by neutron scattering is illustrated with special emphasis on neutron powder diffraction.

The basic formalism describing the crystalline electric field (CEF) interaction in rare-earth compounds is presented as a perturbation of the spectroscopic state of a free ion in the form of the Stevens operator equivalents. We then emphasize the relation between the CEF effects and the inelastic neutron scattering technique which is a unique tool to study the CEF excitations, especially in optically opaque metallic systems. Using a single-ion approximation as a starting point, we discuss how the study of the CEF effects helps to understand fundamental interactions in different materials, including weak and hard magnets, intermediate valence compounds, and high temperature copper-oxide superconductors.

Collective excitations in magnetic materials can be investigated by means of inelastic neutron scattering. We show that this experimental method gives access to the complete spectrum of magnetic fluctuations through the energy- and momentum-dependence of the dynamical susceptibility. We focus on the dynamical properties of magnets with localized spin densities and of metals. From such studies, microscopic parameters like exchange integrals, spin-wave stiffness and relaxation times can be determined. This is of great help to test current theories in magnetism.

Cu²⁺ magnetic insulators KCuCI₃ and TICuCI₃ are unconventional antiferromagnets of dimer nature. Both assume a quantum disordered singlet ground state, and show a finite spin excitation gap to triplet states. Despite the lack of classical Néel order, true three-dimensional magnetic interactions are realized in the above compounds. Recent inelastic neutron scattering experiments are presented, which elucidate the nature of triplet modes under this circumstance. Model predictions around the dimer limit are derived and successfully compared to the observed dynamic properties. The latter are representative of a large class of gapped quantum systems, for which a general phase diagram is discussed.

The relevance of magnetism for the mechanism responsible for high-temperature superconductivity remains an open and still interesting issue. The observation by inelastic neutron scattering of strong antiferromagnetic dynamical correlations in superconducting cuprates is discussed in relation to the unusual physical properties of the cuprates as well as in relation to the superconducting pairing.

The behaviour of vortices in the mixed state of type II superconductors is a rich and diverse field to which neutron and muon techniques have contributed greatly. A brief introduction is given to the use of these two complementary methods, and the successful application of this approach is illustrated by selected examples covering a range of problems.

We discuss the possible connection existing between the magnetic and electronic excitations measured in the high-temperature superconductors. Both angle resolved photoemission (ARPES) and inelastic neutron scattering (INS) experiments are reviewed, with an introduction to the ARPES technique. In particular, it is shown that the shape of the spectral function below T_c as measured by ARPES can be explained by the interaction of the electrons with a collective mode whose energy matches that of the magnetic resonance as obtained by INS.

The basic properties of superfluid ⁴He and the simple model used to explain these properties are outlined. Its importance as a test-bed for theories of weakly interacting bose liquids is commented upon. The experimental problems involved in carrying out neutron inelastic scattering studies of this material are discussed and examples given of recent work in the field.

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The following sections are included:

• LIST OF PARTICIPANTS