Complementarity Between Neutron and Synchrotron X-Ray Scattering

The following sections are included:

• FOREWORD

• ACKNOWLEDGEMENTS

• EDITORIAL

• ORGANISATION OF THE SCHOOL

• CONTENTS

We give an introduction to the principles of neutron- and X-ray scattering. After a short layout of the kinematical complementarity, we discuss in some detail the elementary interaction of these probes with a sample. Based on the different nature of the coupling of these probes to the various degrees of freedom in the sample we then emphasize the dynamical complementarity.

The advantages of synchrotron radiation for high quality powder X-ray diffraction studies are discussed. An area where this has had a major impact is in the use of powders for crystallographic investigations, in particular the solution of crystal structures when a suitable single crystal is not available. The procedures involved are described and illustrated by examples where structures are solved from powder data, with minimal prior knowledge, or where the known molecular structure is exploited. Finally, synchrotron radiation makes it possible to study very small crystals. This can be an alternative approach.

We extend the discussion of the concepts, techniques and methods of structure determination presented in the companion paper to include aspects of imperfect crystals, with emphasis on neutron powder diffraction. Particular attention is payed to the cases of atomic short-range order (a deviation from randomness) and dislocations (a systematic perturbation of perfect crystallinity). The means for accommodating these structural features in structure analysis procedures are discussed. Generic treatments of particle-size broadening and strain are also mentioned, for both the fixed-wavelength and time-of-flight diffraction techniques.

The convenor of the seminar, K. Yvon, gave a short summary on major limitations of diffraction methods for crystal structure analysis, and in particular on error estimates of structural parameters (e.s.d.'s) as obtained from neutron and synchrotron X-ray data. In order to be useful for a discussion of physical and chemical properties, interatomic distances (bond lengths) should be known to an accuracy of better than 0.1%, while occupancy factors and atomic displacement parameters should be known to an accuracy of typically 1% and 10%, respectively…

A comparison is made of neutron and synchrotron x-ray techniques for the study of magnetic structures and phase transitions. The principal features of neutron magnetic diffraction and x-ray magnetic scattering are presented. These are illustrated with reference to investigations of the antiferromagnetic structures of holmium and USb_0.8Te_0.2. Critical magnetic scattering at the paramagnetic to antiferromagnetic transition is described for the latter material.

The neutron tells how the atoms move. In magnetic systems it shows how the spins oscillate, how they tell us the electronic ground state, and how they interact with the damping effects of the electron liquid. Even in insulators spins in quantum antiferromagnets behave as if they were part of a spin liquid.

We review excitations in low dimensional magnetic systems with emphasis on nonlinear, in particular soliton exciations. First, we discuss soliton band formation in Ising-type systems, making contact with recent neutron scattering experiments. Furthermore we demonstrate how chirality emerges as a new degree of freedom of a soliton. The xy-model is discussed with emphasis on the soliton character of the elementary excitations and the emergence of incommensurate modes in external fields. We discuss spin-1 chains and their representation as a chain of rotors or as a nonlinear σ-model. We then go on to derive classical soliton solutions in magnetic chains and soliton bound states, so called breathers. Semiclassical quantization of solitons in an easy-plane situation is discussed and how the band structure closely resembles that of solitons in Ising systems with transverse exchange. The quantization of breathers remarkably leads to the same bound state energies as the spin-½ xyz-model. The interplay between orbital motion and quantum spin phases is not restricted to one-dimensional systems. We show that in two-dimensional doped antiferromagnets, such phases lead to a quasiparticle dispersion that is in agreement with the observed doping dependence in high T_c compounds without having to invoke next nearest neighbor hopping.

During the course of this seminar we had 4 presentations by people who had participated in the earlier poster session. The speakers and titles were:

(1) U. Gasser (PSI, Switzerland): “Dimer splitting in R_xY_1−xNi₂B₂C”. High resolution inelastic work done on polycrystalline samples at the IRIS spectrometer at ISIS and determining the exchange interactions in these superconducting materials.

(2) B. Roessli (PSI, Switzerland): “Enhancement of magnetic fluctuations in UPd₂Al₃ below T_c”. High resolution work done on single crystals at the cold source IN14 triple axis spectrometer at the ILL and also involving polarisation analysis.

(3) P. Wisniewski (Wroclaw, Poland): “Magnetic structures in U₃X₄-type uranium pnictides - neutron diffraction studies”. Elastic scattering experiments done at Saclay on single crystals with both polarised and unpolarised neutrons.

(4) A. Schneidewind (TU Dresden): “Investigation of magnetic structures of NdCu₂ by synchrotron x-ray scattering". Work done at the ID20 diffractometer at the ESRF and involving resonant and non-resonant scattering, as well as polarisation analysis…

The purpose of this lecture is to discuss the contribution of x-ray scattering techniques to the investigation of some of the intriguing properties of strongly correlated electron systems, such as magnetism, superconductivity, orbital order and the metal-insulator transition. A theoretical introduction to the techniques of resonant and non-resonant elastic and inelastic scattering, as presently used in modern synchrotron radiation sources is presented, together with a look into possible future developments.

Scattering of neutrons on the magnetic flux lines is extremely useful tool to study the “vortex matter”. The disapearence of the vortex lattice observed by SANS in High Temperature Superconductors provides the direct evidence for the melting transition of the Abrikosov lattice. The observation of the square lattice in Sr₂RuO₄ shed light on the symmetry of the order parameter in this unconventional superconductor.

The convenor of the seminar, J. Mesot (Argonne National Laboratory, USA) gave first an introduction to the Angle Resolved Photoemission Spectroscopy (ARPES) technique applied to the high-temperature superconductors (HTSC). Since the photoemission intensity is directly proportional to the spectral function, which is itself related to the self-energy function, relevant information about the electronic structure of the HTSC materials can be obtained…

The use of x-ray and neutron diffraction techniques in the study of thin films and superlattices is discussed with reference to specific examples. Advances in x-ray scattering, and in particular the ability to perform x-ray magnetic scattering experiments, means that it is timely to assess the relative merits of the two techniques. The examples are chosen to illustrate the type of detailed information that can be obtained on systems ranging from thin films up to superlattices. It is shown in particular how high-resolution x-ray scattering is particularly well suited to the study of these artificial structures.

The main issues for discussion were presented. They included topics such as the morphology and roughness of multilayer interfaces and their correlation with the performance of multilayers as devices; the characterization of interface roughness and morphology into specular and off-specular neutron and X-ray scattering; the issue of magnetic roughness at magnetic interfaces and its determination by neutron and X-ray scattering; the study of epitaxy and crystallinity of multilayers by neutron and X-ray diffraction; growth of multilayers for optimum performances; the study of novel physical properties of artificially structural materials created by juxtaposing magnetic, superconducting or semiconducting layers; and applications of multilayers as monchromators and supermirrors, in soft X-ray astronomy and as devices…

In these lecture notes we discuss chain motion of long linear homopolymers in the melt. We first present pertinent results on the mechanical dynamic properties as well as on chain diffusion. We then turn to the microscopic motions which are behind the macroscopic phenomena and introduce neutron spin echo spectroscopy as a unique tool to investigate such motions. Polymers are systems with a large number of internal degrees of freedom. Therefore, unlike in common liquids the conformational entropy is an important driving force for chain motion giving rise e.g. to rubber elasticity. We present NSE investigations on such motions which are commonly described in the so called Rouse model. A comparative study employing NSE and molecular dynamics simulations explores the limits of this model description. At larger length scales and correspondingly longer times the topological hindrance for motion imposed by the surrounding chains bears on the freedom of chain motion which is gradually restricted to motion along the chain profile. Thus, the chains move like snakes or reptiles, a mechanism called reptation. Again we present NSE data on the space time evaluation of chain motion. They display (i) the existence of defined length scale beyond which topological constraints become important and (ii) show that beyond this length scale the reptation mechanism prevails.

We review the basic theoretical methods used to treat small-angle scattering from porous materials, treated as general two-phase systems, and also the basic experimental techniques for carrying out such experiments. We discuss the special forms of the scattering when the materials exhibit mass or surface fractal behavior, and review the results of recent experiments on several types of porous media, and also SANS experiments probing the phase behavior of binary fluid mixtures or polymer solutions confined in porous materials. Finally, we discuss the analogous technique of off-specular scattering from surfaces and interfaces which is used to study surface roughness of various kinds.

Some topics of condensed matter physics of high current interest are discussed with respect to their importance in science and, where appropriate, towards applications.

The seminar concentrated on inelastic scattering of neutrons and x-rays with high energy resolution. As introduction the convenor of the seminar, B. Dorner (Institute Laue-Langevin, Grenoble), presented the high resolution technique of backscattering (Bragg angles very near to 90°)…

The performance of instruments using neutron beams can be significantly improved by optimizing all the neutron optical components that are responsible for the transport of the “good” neutrons from the source to the sample and to the detector. The principal interaction of neutrons with matter is of nuclear and magnetic origin. Therefore, most devices use these interactions for defining the phase space of the neutrons (energy, divergence), either by diffraction from bent crystal monochromators or by reflection from artificial multilayers at grazing incidence. In order to define the spin degrees of freedom, polarizing mirrors and monochromators, or absorbers (³He) are used. In this contribution we discuss neutron optical devices based on crystal monochromators and artificial multilayers.

Both fundamental aspects and specific recent performances of reflective, diffractive and refractive X-ray optics for synchrotron beamlines are presented with emphasis on the special requirements due to the low emittance and the high brilliance of third generation storage ring sources.

One aim for a school is to encourage young people to realize their potential. Naturally, the young are particularly responsive to encouragement if the current and forecast weather of life is bright. Is the weather bright for scientists who plan to utilize beams of neutrons and beams of X-rays in their scientific research…

The following sections are included:

• LIST OF PARTICIPANTS