115 research outputs found
Elastic Green's Function of Icosahedral Quasicrystals
The elastic theory of quasicrystals considers, in addition to the normal
displacement field, three phason degrees of freedom. We present an
approximative solution for the elastic Green's function of icosahedral
quasicrystals, assuming that the coupling between the phonons and phasons is
small.Comment: 8 pages, 4 figures included, latex. To be published in The European
Physical Journal
Tiling models for metadislocations in AlPdMn approximants
The AlPdMn quasicrystal approximants xi, xi', and xi'_n of the 1.6 nm
decagonal phase and R, T, and T_n of the 1.2 nm decagonal phase can be viewed
as arrangements of cluster columns on two-dimensional tilings. We substitute
the tiles by Penrose rhombs and show, that alternative tilings can be
constructed by a simple cut and projection formalism in three dimensional
hyperspace. It follows that in the approximants there is a phasonic degree of
freedom, whose excitation results in the reshuffling of the clusters. We apply
the tiling model for metadislocations, which are special textures of partial
dislocations.Comment: 7 pages, 2 figures, Proceedings of International Conference on
Quasicrystals
Dynamic fracture of icosahedral model quasicrystals: A molecular dynamics study
Ebert et al. [Phys. Rev. Lett. 77, 3827 (1996)] have fractured icosahedral
Al-Mn-Pd single crystals in ultrahigh vacuum and have investigated the cleavage
planes in-situ by scanning tunneling microscopy (STM). Globular patterns in the
STM-images were interpreted as clusters of atoms. These are significant
structural units of quasicrystals. The experiments of Ebert et al. imply that
they are also stable physical entities, a property controversially discussed
currently. For a clarification we performed the first large scale fracture
simulations on three-dimensional complex binary systems. We studied the
propagation of mode I cracks in an icosahedral model quasicrystal by molecular
dynamics techniques at low temperature. In particular we examined how the shape
of the cleavage plane is influenced by the clusters inherent in the model and
how it depends on the plane structure. Brittle fracture with no indication of
dislocation activity is observed. The crack surfaces are rough on the scale of
the clusters, but exhibit constant average heights for orientations
perpendicular to high symmetry axes. From detailed analyses of the fractured
samples we conclude that both, the plane structure and the clusters, strongly
influence dynamic fracture in quasicrystals and that the clusters therefore
have to be regarded as physical entities.Comment: 10 pages, 12 figures, for associated avi files, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/emitted_soundwaves.avi
and
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/dynamic_fracture.av
Topological defects in spinor condensates
We investigate the structure of topological defects in the ground states of
spinor Bose-Einstein condensates with spin F=1 or F=2. The type and number of
defects are determined by calculating the first and second homotopy groups of
the order-parameter space. The order-parameter space is identified with a set
of degenerate ground state spinors. Because the structure of the ground state
depends on whether or not there is an external magnetic field applied to the
system, defects are sensitive to the magnetic field. We study both cases and
find that the defects in zero and non-zero field are different.Comment: 10 pages, 1 figure. Published versio
Anomalous Rashba spin splitting in two-dimensional hole systems
It has long been assumed that the inversion asymmetry-induced Rashba spin
splitting in two-dimensional (2D) systems at zero magnetic field is
proportional to the electric field that characterizes the inversion asymmetry
of the confining potential. Here we demonstrate, both theoretically and
experimentally, that 2D heavy hole systems in accumulation layer-like single
heterostructures show the opposite behavior, i.e., a decreasing, but nonzero
electric field results in an increasing Rashba coefficient.Comment: 4 pages, 3 figure
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