611 research outputs found
Trends in the magnetic properties of Fe, Co and Ni clusters and monolayers on Ir(111), Pt(111) and Au(111)
We present a detailed theoretical investigation on the magnetic properties of
small single-layered Fe, Co and Ni clusters deposited on Ir(111), Pt(111) and
Au(111). For this a fully relativistic {\em ab-initio} scheme based on density
functional theory has been used. We analyse the element, size and geometry
specific variations of the atomic magnetic moments and their mutual exchange
interactions as well as the magnetic anisotropy energy in these systems. Our
results show that the atomic spin magnetic moments in the Fe and Co clusters
decrease almost linearly with coordination on all three substrates, while the
corresponding orbital magnetic moments appear to be much more sensitive to the
local atomic environment. The isotropic exchange interaction among the cluster
atoms is always very strong for Fe and Co exceeding the values for bulk bcc Fe
and hcp Co, whereas the anisotropic Dzyaloshinski-Moriya interaction is in
general one or two orders of magnitude smaller when compared to the isotropic
one. For the magnetic properties of Ni clusters the magnetic properties can
show quite a different behaviour and we find in this case a strong tendency
towards noncollinear magnetism
Hanbury Brown and Twiss interferometry at a free-electron laser
We present measurements of second- and higher-order intensity correlation
functions (so-called Hanbury Brown and Twiss experiment) performed at the
free-electron laser (FEL) FLASH in the non-linear regime of its operation. We
demonstrate the high transverse coherence properties of the FEL beam with a
degree of transverse coherence of about 80% and degeneracy parameter of the
order 10^9 that makes it similar to laser sources. Intensity correlation
measurements in spatial and frequency domain gave an estimate of the FEL
average pulse duration of 50 fs. Our measurements of the higher-order
correlation functions indicate that FEL radiation obeys Gaussian statistics,
which is characteristic to chaotic sources.Comment: 19 pages, 6 figures, 1 table, 40 reference
Statistical properties of a free-electron laser revealed by the Hanbury Brown and Twiss interferometry
We present a comprehensive experimental analysis of statistical properties of
the self-amplified spontaneous emission (SASE) free-electron laser (FEL) FLASH
at DESY in Hamburg by means of Hanbury Brown and Twiss (HBT) interferometry.
The experiments were performed at the FEL wavelengths of 5.5 nm, 13.4 nm, and
20.8 nm. We determined the 2-nd order intensity correlation function for all
wavelengths and different operation conditions of FLASH. In all experiments a
high degree of spatial coherence (above 50%) was obtained. Our analysis
performed in spatial and spectral domains provided us with the independent
measurements of an average pulse duration of the FEL that were below 60 fs. To
explain complicated behaviour of the 2-nd order intensity correlation function
we developed advanced theoretical model that includes the presence of multiple
beams and external positional jitter of the FEL pulses. By this analysis we
determined that in most experiments several beams were present in radiating
field and in one of the experiments external positional jitter was about 25% of
the beam size. We envision that methods developed in our study will be used
widely for analysis and diagnostics of the FEL radiation.Comment: 29 pages, 14 figures, 3 table
Seeded x-ray free-electron laser generating radiation with laser statistical properties
The invention of optical lasers led to a revolution in the field of optics
and even to the creation of completely new fields of research such as quantum
optics. The reason was their unique statistical and coherence properties. The
newly emerging, short-wavelength free-electron lasers (FELs) are sources of
very bright coherent extreme-ultraviolet (XUV) and x-ray radiation with pulse
durations on the order of femtoseconds, and are presently considered to be
laser sources at these energies. Most existing FELs are highly spatially
coherent but in spite of their name, they behave statistically as chaotic
sources. Here, we demonstrate experimentally, by combining Hanbury Brown and
Twiss (HBT) interferometry with spectral measurements that the seeded XUV FERMI
FEL-2 source does indeed behave statistically as a laser. The first steps have
been taken towards exploiting the first-order coherence of FELs, and the
present work opens the way to quantum optics experiments that strongly rely on
high-order statistical properties of the radiation.Comment: 24 pages, 10 figures, 37 reference
Soft landing of size selected clusters in rare gas matrices
Soft landing of mass selected clusters in rare gas matrices is a technique used to preserve mass selection in cluster deposition. To prevent fragmentation upon deposition, the substrate is covered with rare gas matrices to dissipate the cluster kinetic energy upon impact. Theoretical and experimental studies demonstrate the power of this technique. Besides STM, optical absorption, excitation, and fluorescence experiments, x-ray absorption at core levels can be used as a tool to study soft landing conditions, as will be shown here. X-ray absorption spectroscopy is also well suited to follow diffusion and agglomeration of clusters on surfaces via energy shifts in core level absorption
Quantum Monte Carlo simulations of infinitely strongly correlated fermions
Numerical simulations of the two-dimensional t-J model in the limit are performed for rather large systems (up to ) using a
world-line loop-algorithm. It is shown that in the one-hole case with J=0,
where no minus signs appear, very low temperatures () are
necessary in order to reach Nagaoka's state. J/t \ltsim 0.05 leads to the
formation of partially polarized systems, whereas J/t \gtsim 0.05
corresponds to minimal spin. The two-hole case shows enhanced total spin up to
the lowest attainable temperatures ().Comment: 6 pages, 5 figure
The most pathogenic transthyretin variant, L55P, forms amyloid fibrils under acidic conditions and protofilaments under physiological conditions
The L55P transthyretin (TTR) familial amyloid polyneuropathy-associated variant is distinct from the other TTR variants studied to date and the wild-type protein in that the L55P tetramer can dissociate to the monomeric amyloidogenic intermediate and form fibril precursors under physiological conditions (pH 7.0, 37 degrees C). The activation barrier associated with L55P-TTR tetramer dissociation is lower than the barrier for wild-type transthyretin dissociation, which does not form fibrils under physiological conditions. The L55P-TTR tetramer is also very sensitive to acidic conditions, readily dissociating to form the monomeric amyloidogenic intermediate between pH 5.5-5.0 where the wild-type TTR adopts a nonamyloidogenic tetrameric structure. The formation of the L55P monomeric amyloidogenic intermediate involves subtle tertiary structural changes within the beta-sheet rich subunit as discerned from Trp fluorescence, circular dichroism analysis, and ANS binding studies. The assembly of the L55P-TTR amyloidogenic intermediate at physiological pH (pH 7.5) affords protofilaments that elongate with time. TEM studies suggest that the entropic barrier associated with filament assembly (amyloid fibril formation) is high in vitro, amyloid being defined by the laterally assembled four filament structure observed by Blake upon isolation of "fibrils" from the eye of a FAP patient. The L55P-TTR protofilaments formed in vitro bind Congo red and thioflavin T (albeit more weakly than the fibrils produced at acidic pH), suggesting that the structure observed probably represents an amyloid precursor. The structural continuum from misfolded monomer through protofilaments, filaments, and ultimately fibrils must be considered as a possible source of pathology associated with these diseases
Ferromagnetism in the large-U Hubbard model
We study the Hubbard model on a hypercubic lattice with regard to the
possibility of itinerant ferromagnetism. The Dynamical Mean Field theory is
used to map the lattice model on an effective local problem, which is treated
with help of the Non Crossing Approximation. By investigating spin dependent
one-particle Green's functions and the magnetic susceptibility, a region with
nonvanishing ferromagnetic polarization is found in the limit . The
-T-phase diagram as well as thermodynamic quantities are discussed. The
dependence of the Curie temperature on the Coulomb interaction and the
competition between ferromagnetism and antiferromagnetism are studied in the
large limit of the Hubbard model.Comment: 4 pages, 5 figures, accepted for publication in Physical Review B,
Rapid Communication
Modeling Amyloid Beta Peptide Insertion into Lipid Bilayers
Inspired by recent suggestions that the Alzheimer's amyloid beta peptide (A
beta) can insert into cell membranes and form harmful ion channels, we model
insertion of the 40 and 42 residue forms of the peptide into cell membranes
using a Monte Carlo code which is specific at the amino acid level. We examine
insertion of the regular A-beta peptide as well as mutants causing familial
Alzheimer's disease, and find that all but one of the mutants change the
insertion behavior by causing the peptide to spend more simulation steps in
only one leaflet of the bilayer. We also find that A-beta 42, because of the
extra hydrophobic residues relative to A-beta 40, is more likely to adopt this
conformation than A-beta 40 in both wild-type and mutant forms. We argue
qualitatively why these effects happen. Here, we present our results and
develop the hypothesis that this partial insertion increases the probability of
harmful channel formation. This hypothesis can partly explain why these
mutations are neurotoxic simply due to peptide insertion behavior. We further
apply this model to various artificial A-beta mutants which have been examined
experimentally, and offer testable experimental predictions contrasting the
roles of aggregation and insertion with regard to toxicity of A-beta mutants.
These can be used through further experiments to test our hypothesis.Comment: 14 pages; 8 figures; 2nd revisio
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