H. Dosch

Glancing-angle X-ray scattering studies of the premelting of ice surfaces

Abstract The onset of surface melting at [lcub]00.1[rcub] (“basal”), [lcub]10.0[rcub] and [lcub]11.0[rcub] (“nonbasal”) surfaces of hexagonal ice is studied experimentally by glancing-angle X-ray scattering. The preparation of mirror-like single crystal ice surfaces and a special in situ X-ray chamber is described in detail. The X-ray scattering experiments using synchrotron radiation give clear evidence that all the investigated high-symmetry surfaces exhibit surface melting with onset temperatures T s ≅ − 13.5°C for the basal and T s ≅ − 12.5°C for the nonbasal surfaces. The temperature dependence of the thickness of the quasiliquid is presented and discussed in view of various theoretical predictions. In addition we found thermal faceting at nonbasal surfaces as well as a diffuse (“rough”) solid-quasiliquid interface. The thermal properties of the observed surface disorder implies that the correlation length within the socalled “surface melt” is much larger than the correlation length in the bulk liquid indicating the presence of significant “substrate” interactions.

The evanescent neutron wave diffractometer: On the way to surface sensitive neutron scattering

A novel experimental technique to observe the Bragg scattering of evanescent neutron waves is presented. The so‐called EVA diffractometer, installed at the high‐flux reactor of the Institut Laue‐Langevin, allows the identification of neutron scattering from single crystal surfaces. We discuss the experimental setup, intensity, and resolution considerations and some first Bragg scattering signals from evanescent neutron waves excited at CaF2, InP, and MnF2 single crystal surfaces. The experimental results are discussed within the framework of the so‐called distorted wave Born approximation and within a dynamical scattering theory. The limitations of both theoretical approaches to describe the Bragg scattering of evanescent neutron waves are indicated.

Surface-sensitive magnetic neutron Bragg scattering: Perspective and limits

Abstract In the application of neutron scattering to problems related with surface and interfacial magnetism three different experimental techniques may be envisaged: reflectivity measurements, glancing angle neutron scattering and truncation rod scattering. While neutron reflectivity measurements have become a routine experimental tool nowadays, the two other techniques are virtually unexplored. In this article I focus onto glancing angle neutron Bragg scattering and discuss some present attempts and applications, future prospects and the limitations of this new technique in the field of surface-related magnetism. A special emphasis is thereby put on the performance of the evanescent neutron wave diffractometer EVA at the Institut Laue-Langevin in Grenoble.

Critical phenomena at the Fe3Al(11̄0) surface: a glancing angle X-ray scattering study

Abstract We describe the details of a glancing angle X-ray scattering study of the critical behaviour of the order parameter and the order parameter fluctuations at the Fe 3 Al(110) surface when the alloy approaches the bulk critical temperature. The temperature dependence of the X-ray scattering at and around the (111) superlattice reflection has been measured in a surface sensitive scattering geometry and analyzed in view of the predictions of modern renormalization theories which account for the presence of a surface. It is found that the equilibrium surface displays two different regimes: the top three layers are strongly affected by surface segregation phenomena and display a complicated thermodynamic behaviour, the observed critical phenomena in the subsequent layers are governed by the new surface exponents β 1 = 0.75 ± 0.06, γ 11 = −0.33 ± 0.06, η ∥ = 1.52 ± 0.04 and ν = 0.60± 0.15. It is demonstrated by this study that on the one hand the presence of the surface alters the character of the order-disord transition near the surface in a significant manner, and that on the other hand the observed surface critical scattering and, consequently, the surface free energy and the surface-dominated pair correlation function still show scaling behaviour. In particular the scaling laws 2 β 1 = ν (1 + η ∥ ) and γ 11 = ν (1 − η ∥ ) are confirmed by the experiment.

Depth controlled grazing angle neutron diffraction

Abstract We present first experimental results from evanescent neutron scattering on single crystal surfaces. The experiments have been performed at the Institut Laue Langevin on the new instrument EVA which is dedicated to surface sensitive neutron scattering work. By a careful of both the grazing incidence angle and the grazing exit angle we were for the first time able to identify unambigously neutron Bragg scattering which originates from the surface of the crystal. We present the experimental set-up and discuss some results which we observed from the surface of CaF2 and InP single crystals.

Observation of neutron truncation rod scattering

We report measurements of neutron truncation rod scattering. This surface-induced neutron scattering has been observed at the (111) surface of a perfect Si single crystal by a high-resolution three-crystal scattering experiment. We describe the details of the experiment and discuss the experimental findings within a kinematic approach. Possible applications of this new neutron scattering method lie in the field of surface magnetism and the study of surfaces of light-element systems.

Attenuation of phonon scattering intensities by point defects

The cross section of the coherent, inelastic neutron scattering is characteristically decreased in the presence of defect-induced lattice distortions. This effect is in analogy to the attenuation of Bragg intensities due to a static Debye-Waller factor. The integrated scattering intensities from transversal acoustic (TA-) phonons of the system NbN0.014 have been measured and are shown to be attenuated with respect to the scattering intensities from the phonons of a pure Nb crystal. We discuss the obtained results through comparison with various model calculations.

Neutron Diffraction under Grazing Incidence: Recent Results from the Evanescent Wave Diffractometer

We report the first exit angle resolved magnetic neutron diffraction experiment under grazing incidence. The antiferromagnetic (100) Bragg reflection was excited under grazing incidence on a (001) MnF2 single crystal surface. By a careful control of the exit angle we were able to identify unambiguously the surface scattering. We present the experimental set up and compare some results with those of a CaF2 single crystal.

Location of interstitial deuterium in Niobium by Huang diffuse scattering

Huang diffuse scattering experiments were performed on NbD single crystals using neutrons. So far such experiments were used to determine the strength and symmetry of the distortion field around the deuteriuml. In the present study the interstitial site occupancy has also been determined and it is found that deuterium occupies tetrahedral sites in Niobium.