Hans J. Lauter

4He films on graphite studied by neutron scattering

The properties of 4He films adsorbed on graphite have been studied by neutron scattering. In particular excitations of the commensurate phase of the monolayer are discussed. The first two adsorbed layers are solid and the next ones stay liquid. At the boundaries of the superfluid film excitations could be studied. Also the phonons, maxon and rotons of the film are investigated. An explanation of the lower density of the very thin films compared to bulk 4He is given.

Towards 3D polarization analysis in neutron reflectometry

General equations for spin-flip and non-spin-flip specular reflection and off-specular scattering are analyzed for the case of multidomain states of magnetic multilayers. The results are illustrated by numerical calculations performed for Fe/Cr multilayers using an original software developed on the basis of the supermatrix formalism. This routine allows to calculate the reflectivities and scattering cross sections from the layered structures of any complexity for any directions of the incident polarization vector and directions of the polarization analysis.

3He-Layers on Graphite

Abstract Neutron diffraction studies show that in the monolayer of 3He on graphite the commensurate phase is separated from the incommensurate phase by a domain wall phase. Under the pressure of the second adsorbed layer the first layer is driven into an 8*8 commensurability with the substrate. This coincides probably with events seen in NMR (1) and heat capacity measurements (2) in the same coverage region.

Morphology of thin nanocomposite films of asymmetric diblock copolymer and magnetite nanoparticles

Thin self-assembled nanocomposite films of an asymmetric diblock copolymer and nanoparticles are fabricated. The morphologies of the films of the diblock copolymer poly(styrene-block-n-butyl methacrylate), P(Sd-b-BMA), with different volume fractions of large magnetite Fe(3)O(4) nanoparticles are studied before and after annealing. Neutron reflectometry reveals remarkable evidence that confining asymmetric copolymer to a limit of two layers forces the film, after the annealing, to form a mixed cylindrical-lamellar two-layer structure. This complex morphology is very stable and is preserved after the incorporation of nanoparticles up to 10% volume fraction. The other striking result is that the monodispersed nanoparticles with affinity to the polystyrene (PS) domain and with a size of 10 nm, which is close to the size of the PS chains, are assembled by the diblock copolymer matrix, so the distribution of the nanoparticles is reduced solely to the PS domain of the film. Our studies demonstrate that for asymmetric block copolymers in thin film geometry the self-assembly is strongly influenced by the interfacial and surface energies of the blocks and substrate.

Theoretical Analysis of Neutron Scattering Spectra of 4He Films on Graphite

We compare microscopic calculations of the dynamic structure function of helium films adsorbed to a graphite substrate with neutron scattering data. Starting from a generalized Feynman theory of collective excitations, we include successively, three-phonon interactions, self-energy corrections, and experimental broadening and thereby improve the agreement between theoretical predictions and experiments. The inclusion of three-phonon vertices allows high-lying excited states to decay into lesser energetic ones and thus leads to a natural linewidth. It is shown that the theoretical linewidth of the excitations is typically smaller than the experimental resolution, and that much structure of the spectrum in the neutron scattering data is obscured under a broad plateau between the ripplon and the phonon excitations. This analysis leaves little doubt of the existence of layer-phonons and that the observed shoulder in the dynamic structure function, near the roton minimum, is actually a 2D roton propagating in the inner-most liquid layer.

Excitations in the commensurate phase of H2, D2 and HD on graphite

We measured the excitations of the hydrogen isotopes (Hz, HD. Dz) adsorbed on graphite in the commensurate phase. A simple model of two in-plane springs reproduces well the observed neutron spectra and gives the dispersion relations. The isotopic mass effect alone cannot explain the energy shift of these modes. The behaviour of the quantum gases hydrogen (HZ) and its isotopes, deuterium (D2) and deuterated hydrogen (HD), is of considerable experimental and theoretical interest, in particular for the study of their phase transitions in two dimensions (2D). These gases adsorbed on graphite are a good approach to an ideal 2D system in the incommensurate phase, but lock at lower coverages into a (fix fi) R30” commensurate phase (C-phase) due to the graphite surface potential corrugation. The structure of the different phases has been investigated mainly by neutron scattering [l, 21. The C-phase presents a nearest neighbour distance (a,,) of 4.26A, even though unn in the hcp-plane for the 3D solid hydrogen is nearly 20% smaller [3]. It is the high compressibility due to zero point motion of the absorbed molecules that makes this structure energetically favourable [4]. To understand the influence of the substrate on the different phases of the adsorbate it is necessary to know the substrate-adsorbate and adsorbate-adsorbate interactions. Here we report inelastic neutron scattering (INS) measurements in the commensurate phase: the dependence of the excitations on the different isotopes. The out-of-plane oscillation of the absorbed molecules is given by the variation of the adsorption potential in the direction normal to the graphite surface plane. The in-plane oscillations in the C-phase are given mainly by the corrugation of this potential along the surface. In this work only the in-plane oscillations will be presented. In earlier INS experiments [l, 51 only a dispersionless excitation could be seen and the interpretation was that the molecules oscillate in the wells produced by the substrate as independent oscillators. In recent measurements [6] of D, on graphite realized with a higher energy resolution we were able to measure the coupling between the D, molecules in the C-phase. First principle calculations [7] have been realized as well on these systems. We add here high resolution measurements for HD and H, on graphite and compare them to the mentioned calculations and to the D, results. The INS data were taken with the three axis spectrometer IN3 at the ILL with a fixed final energy (E, = 0.99THz), a Be filter after the sample and a horizontally focusing analyzer. The energy resolution across the elastic line was 0.033 THz (1.59 K) FWHM for D, and HD, and 0.049 THz (2.36 K) for H,. The sample cell was filled with papyex sheets, the graphite basal plane being in the neutron scattering plane. This geometry favours the measurement of the inplane excitation.

Magnetic flux distribution inside an YBa2Cu3O7 superconducting thin film in the mixed state

The stud). with polarized neutron reflection showed the spatially resolved nuclear and magnetic structure in a high-temperature superconducting (HT,) film of YBa2Cu,0, in the mixed state in an external field parallel to its surfam. The obtajned magnetic flux profile is composed of the penetrating Meissner field and the flux-line rows. The Aux-line rows show good linear arrangements. .4 local critical magnetization inside the HT, filnl was determined. The total magnetization exhibits flux-line row transirions and increases in average tvith increasing external field due to the surface barrier and the Aus-line interaction. 1999 Elsevicr Science B.V. All rights reserved.

Superfluid 4He dynamics beyond quasiparticle excitations

The dynamics of superfluid He4 at and above the Landau quasiparticle regime is investigated by high-precision inelastic neutron scattering measurements of the dynamic structure factor. A highly structured response is observed above the familiar phonon-maxon-roton spectrum, characterized by sharp thresholds for phonon-phonon, maxon-roton, and roton-roton coupling processes. The experimental dynamic structure factor is compared to the calculation of the same physical quantity by a dynamic many-body theory including three-phonon processes self-consistently. The theory is found to provide a quantitative description of the dynamics of the correlated bosons for energies up to about three times that of the Landau quasiparticles.

Neutron Scattering Studies of Quantum Films

The phase diagrams of a monolayer of adsorbed gases or light molecules on flat substrates look similar to the phase diagrams in 3-dimensions (3-D). This means that the usual coexistence regions, the triple point and the critical point are present as well in 2-dimensions (2-D) as in 3-D 1. The phase diagrams can be studied by adsorption isotherm or heat capacity measurements, which reveal mainly the coexistence regions or the phase boundaries, respectively. However, the substrate can not always be regarded to be ideally flat. In many cases the adsorbate does see the adsorption sites of the substrate and locks into a commensurate phase (C-phase). In the case of graphite as substrate the (√3 × √3) R 30° overstructure is seen in many cases. This structure is shown in fig.l. The heavier rare gases and the light molecules (N2, CD4) exhibit the C-phase only if the lattice parameter of the dense plane in 3-D is close to the nearest neighbor distance in the C-phase. This is the case because the nearest neighbor distance nearly does not change in the densest plane if the adjacent planes are taken off which means the dimensionality changes from 3-D to 2-D.

Lattice Dynamics of Commensurate Monolayers Adsorbed on Graphite

Abstract A review of the lattice dynamics of gases adsorbed on graphite in the commensurate phase is presented, taking into account their quantum character. The phonon dispersion curves are characterized by an energy gap in the acoustic branch at the zone center. Its magnitude is related to the corrugation of the in-plane adsorption potential. The energy of the zone boundary phonons, on the other hand, is determined by the interaction between adatoms. The measurement of these quantities allows the comparison with theoretical models.