249 research outputs found
Physical space description of decorated quasicrystals
In this paper the systematic method of dealing with the arbitrary decorations
of quasicrystals is presented. The method is founded on the average unit cell
formalism and operates in the physical space only, where each decorating atom
manifests itself just by an additional component of the displacement density
function in the average unit cell. Such approach allows us to use almost all
classical crystallography algorithms for structure refining based on
experimental data and may meaningly decrease the number of parameters which
have to be fit. Further help for such analysis may be the use of proposed
recently average Patterson function, here applied to decorated sets. As an
example we present a description of a class of decorated quasicrystals based on
Sturmian sequence of two interatomic spacings: we calculate explicitly
structure factor, the shape of average Patterson function and give an algorithm
for pattern analysis.Comment: 17 pages, 5 figure
An extinction rule for a class of 1D quasicrystals
We study decorated one-dimensional quasicrystal obtained by a non-standard
projection of a part of two-dimensional lattice. We focus on the impact of
varying relative positions of decorated sites. First, we give general
expression for the structure factor. Subsequently we analyze an example of
extinction rule.Comment: 5 pages, 2 figures, LaTex2e, to appear in ICQ9 Proceeding
(Philosophical Magazine
Correlations, disorder, and multi-magnon processes in terahertz spin dynamics of magnetic nanostructures: A first-principles investigation
Understanding the profound impact of correlation effects and crystal
imperfections is essential for an accurate description of solids. Here we study
the role of correlation, disorder, and multi-magnon processes in THz magnons.
Our findings reveal that a significant part of the electron self-energy, which
goes beyond the adiabatic local spin density approximation, arises from the
interaction between electrons and a virtual magnon gas. This interaction leads
to a substantial modification of the exchange splitting and a renormalization
of magnon energies, in agreement with the experimental data. We establish a
quantitative hierarchy of magnon relaxation processes based on first
principles
Non-local correlation effects due to virtual spin-flip processes in itinerant electron ferromagnets
We present an ab initio method for eletcronic structure calculations, which
accounts for the interaction of electrons and magnons in ferromagnets. While it
is based on a many body perturbation theory we approximate numerically complex
quantities with quantities from time dependent density functional theory. This
results in a simple and affordable algorithm which allows us to consider more
complex materials than those usually studied in this context (
ferromagnets) while still being able to account for the non-locality of the
self energy. Furthermore, our approach allows for a relatively simple way to
incorporate self-consistency. Our results are in a good agreement with
experimental and theoretical findings for iron and nickel. Especially the
experimental exchange splitting of nickel is predicted accurately within our
theory. Additionally, we study the halfmetallic ferromagnet NiMnSb concerning
its non-qusiparticle states appearing in the bandgap due to spin-flip
excitations
Nonlinear Decay of Quantum Confined Magnons in Itinerant Ferromagnets
Quantum confinement leads to the emergence of several magnon modes in
ultrathin layered magnetic structures. We probe the lifetime of these quantum
confined modes in a model system composed of three atomic layers of Co grown on
different surfaces. We demonstrate that the quantum confined magnons exhibit
nonlinear decay rates, which strongly depend on the mode number, in sharp
contrast to what is assumed in the classical dynamics. Combining the
experimental results with those of linear-response density functional
calculations we provide a quantitative explanation for this nonlinear damping
effect. The results provide new insights into the decay mechanism of spin
excitations in ultrathin films and multilayers and pave the way for tuning the
dynamical properties of such structures
Fibre-optic delivery of time and frequency to VLBI station
The quality of Very Long Baseline Interferometry (VLBI) radio observations
predominantly relies on precise and ultra-stable time and frequency (T&F)
standards, usually hydrogen masers (HM), maintained locally at each VLBI
station. Here, we present an operational solution in which the VLBI
observations are routinely carried out without use of a local HM, but using
remote synchronization via a stabilized, long-distance fibre-optic link. The
T&F reference signals, traceable to international atomic timescale (TAI), are
delivered to the VLBI station from a dedicated timekeeping laboratory.
Moreover, we describe a proof-of-concept experiment where the VLBI station is
synchronized to a remote strontium optical lattice clock during the
observation.Comment: 8 pages, 8 figures, matches the version published in A&A, section
Astronomical instrumentatio
Tilings, tiling spaces and topology
To understand an aperiodic tiling (or a quasicrystal modeled on an aperiodic
tiling), we construct a space of similar tilings, on which the group of
translations acts naturally. This space is then an (abstract) dynamical system.
Dynamical properties of the space (such as mixing, or the spectrum of the
translation operator) are closely related to bulk properties of the individual
tilings (such as the diffraction pattern). The topology of the space of
tilings, particularly the Cech cohomology, gives information on how the
original tiling can be deformed. Tiling spaces can be constructed as inverse
limits of branched manifolds.Comment: 8 pages, including 2 figures, talk given at ICQ
Structure and magnetism of EuS on Bi<sub>2</sub>Se<sub>3</sub>(0001)
The rocksalt-type ferromagnetic (FM) insulator EuS (bulk TCâ=â17âK) grown on Bi2Se3 with well-matched (111) plane of the film and (0001) plane of the substrate is studied. The system may feature magnetic proximity effect breaking the time-reversal symmetry and opening a bandgap in the metallic topologically protected surface state of Bi2Se3. The experimental X-ray diffraction studies are combined with ab initio calculations to resolve contradictory results concerning the enhancement of the TC up to 300âK and the degree of induced magnetization in the system. It is concluded that previous studies relied on idealized and unconfirmed structure models. Herein, it is shown by surface X-ray diffraction (SXRD) with ab initio calculations that a two double layer-thick EuS film grows with a sharp interface and without chemical intermixing in a single domain state in an FCC-type stacking on the Bi2Se3(0001) surface in which the topmost layer is metallic, thereby lifting polarity. A large pz-orbital-derived top-layer sulfur magnetic moment of 0.6 ÎŒB is found, whereas for europium, ÎŒEuâ=â6.9 ÎŒB throughout the film is found. No magnetization within the first Bi2Se3 quintuple layer is found. The calculation of the exchange parameters Jij indicates a complex FM and antiferromagnetic ordering between europium and sulfur with a maximum NĂ©el temperature of 226âK
- âŠ