0000000000260922
AUTHOR
Hans J. Lauter
The commensurate-incommensurate transition of hydrogen monolayers physisorbed on graphite
We present a neutron diffraction study of the commensurate-incommensurate (C-IC) transition of hydrogen monolayers physisorbed on the basal planes of exfoliated graphite. It is shown that the solid intermediate phase detected by specific-heat measurements can be described in terms of a striped domain-wall phase with superheavy walls. Comparison with simple models with fixed domain sizes is made. The results are in agreement with recent LEED experiments of Cui and Fain and provide an experimental realization of C-IC transition theories.
Observation of a superfluid component within solid helium.
We demonstrate by neutron scattering that a localized superfluid component exists at high pressures within solid helium in aerogel. Its existence is deduced from the observation of two sharp phonon-roton spectra which are clearly distinguishable from modes in bulk superfluid helium. These roton excitations exhibit different roton gap parameters than the roton observed in the bulk fluid at freezing pressure. One of the roton modes disappears after annealing the samples. Comparison with theoretical calculations suggests that the model that reproduces the observed data best is that of superfluid double layers within the solid and at the helium-substrate interface.
Coarsening of Antiferromagnetic Domains in Multilayers: The Key Role of Magnetocrystalline Anisotropy
The domain structure of an antiferromagnetic superlattice is studied. Synchrotron Mössbauer and polarized neutron reflectometric maps show micrometer-size primary domain formation as the external field decreases from saturation to remanence. A secondary domain state consisting mainly of at least 1 order of magnitude larger domains is created when a small field along the layer magnetizations induces a bulk-spin-flop transition. The domain-size distribution is reproducibly dependent on the magnetic prehistory. The condition for domain coarsening is shown to be the equilibrium of the external field energy with the anisotropy energy.
Heat-capacity and neutron-diffraction studies of HD monolayers physisorbed on graphite
Abstract The phase diagram of HD monolayers physisorbed on graphite is studied in detail by using heat-capacity and neutron-diffraction measurements. The following features are observed: At low coverages the system forms a commensurate (C) (√3 × √3)R30° phase and at coverages near monolayer completion an incommensurate (IC) two-dimensional solid phase. The order-disorder transition of the C phase belongs to the three-state Potts universality class. The C-IC transition occurs via two intermediate phases, i.e., the α phase and a novel phase, which is interpreted as an α-IC coexistence region. Though the molecule HD is a strongly incoherent neutron scatterer, for the first time elastic neutron…