63 research outputs found
Lattice models and Landau theory for type II incommensurate crystals
Ground state properties and phonon dispersion curves of a classical linear
chain model describing a crystal with an incommensurate phase are studied. This
model is the DIFFOUR (discrete frustrated phi4) model with an extra
fourth-order term added to it. The incommensurability in these models may arise
if there is frustration between nearest-neighbor and next-nearest-neighbor
interactions. We discuss the effect of the additional term on the phonon
branches and phase diagram of the DIFFOUR model. We find some features not
present in the DIFFOUR model such as the renormalization of the
nearest-neighbor coupling. Furthermore the ratio between the slopes of the soft
phonon mode in the ferroelectric and paraelectric phase can take on values
different from -2. Temperature dependences of the parameters in the model are
different above and below the paraelectric transition, in contrast with the
assumptions made in Landau theory. In the continuum limit this model reduces to
the Landau free energy expansion for type II incommensurate crystals and it can
be seen as the lowest-order generalization of the simplest Lifshitz-point
model. Part of the numerical calculations have been done by an adaption of the
Effective Potential Method, orginally used for models with nearest-neighbor
interaction, to models with also next-nearest-neighbor interactions.Comment: 33 pages, 7 figures, RevTex, submitted to Phys. Rev.
Electronic coupling of colloidal CdSe nanocrystals monitored by thin-film positron-electron momentum density methods
The effect of temperature controlled annealing on the confined valence
electron states in CdSe nanocrystal arrays, deposited as thin films, was
studied using two-dimensional angular correlation of annihilation radiation
(2D-ACAR). A reduction in the intensity by ~35% was observed in a feature of
the positron annihilation spectrum upon removal of the pyridine capping
molecules above 200 degrees Celsius in a vacuum. This reduction is explained by
an increased electronic interaction of the valence orbitals of neighboring
nanocrystals, induced by the formation of inorganic interfaces. Partial
evaporation of the nanoporous CdSe layer and additional sintering into a
polycrystalline thin film was observed at a relatively low temperature of ~486
degrees Celsius.Comment: The article has been accepted by Applied Physics Letters. After it is
published, it will be found at http://apl.aip.or
Hydrogen vacancies facilitate hydrogen transport kinetics in sodium hydride nanocrystallites
Electronic structure and orientation relationship of Li nanoclusters embedded in MgO studied by depth-selective positron annihilation two-dimensional angular correlation
Een wereld van verschil. Ervaringen met traditie en veranderingen binnen zustercongregaties
Contains fulltext :
240528.pdf (publisher's version ) (Open Access
Religieuze vrouwen: bruid, moeder, zuster. Geschiedenis van twee Nederlandse zustercongregaties, 1820-1940 Hilversum:Verloren ,1996 90-6550-520-2
Sterven voor de wereld. Een religieus ideaal in meervoud
Contains fulltext :
240686.pdf (publisher's version ) (Open Access)141 p
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