379 research outputs found
Network theory approach for data evaluation in the dynamic force spectroscopy of biomolecular interactions
Investigations of molecular bonds between single molecules and molecular
complexes by the dynamic force spectroscopy are subject to large fluctuations
at nanoscale and possible other aspecific binding, which mask the experimental
output. Big efforts are devoted to develop methods for effective selection of
the relevant experimental data, before taking the quantitative analysis of bond
parameters. Here we present a methodology which is based on the application of
graph theory. The force-distance curves corresponding to repeated pulling
events are mapped onto their correlation network (mathematical graph). On these
graphs the groups of similar curves appear as topological modules, which are
identified using the spectral analysis of graphs. We demonstrate the approach
by analyzing a large ensemble of the force-distance curves measured on:
ssDNA-ssDNA, peptide-RNA (system from HIV1), and peptide-Au surface. Within our
data sets the methodology systematically separates subgroups of curves which
are related to different intermolecular interactions and to spatial
arrangements in which the molecules are brought together and/or pulling speeds.
This demonstrates the sensitivity of the method to the spatial degrees of
freedom, suggesting potential applications in the case of large molecular
complexes and situations with multiple binding sites
Correlative Microscopy of Morphology and Luminescence of Cu porphyrin aggregates
Transfer of energy and information through molecule aggregates requires as
one important building block anisotropic, cable-like structures. Knowledge on
the spatial correlation of luminescence and morphology represents a
prerequisite in the understanding of internal processes and will be important
for architecting suitable landscapes. In this context we study the morphology,
fluorescence and phosphorescence of molecule aggregate structures on surfaces
in a spatially correlative way. We consider as two morphologies, lengthy
strands and isotropic islands. It turns out that phosphorescence is quite
strong compared to fluorescence and the spatial variation of the observed
intensities is largely in line with the amount of dye. However in proportion,
the strands exhibit more fluorescence than the isotropic islands suggesting
weaker non-radiative channels. The ratio fluorescence to phosphorescence
appears to be correlated with the degree of aggregation or internal order. The
heights at which luminescence saturates is explained in the context of
attenuation and emission multireflection, inside the dye. This is supported by
correlative photoemission electron microscopy which is more sensitive to the
surface region. The lengthy structures exhibit a pronounced polarization
dependence of the luminescence with a relative dichroism up to about 60%,
revealing substantial perpendicular orientation preference of the molecules
with respect to the substrate and parallel with respect to the strands
High resolution characterisation of microstructural evolution in RbFeSe crystals on annealing
The superconducting and magnetic properties of phase-separated
AFeSe compounds are known to depend on post-growth heat
treatments and cooling profiles. This paper focusses on the evolution of
microstructure on annealing, and how this influences the superconducting
properties of RbFeSe crystals. We find that the minority phase in
the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb
content and increased Fe content compared to the matrix. The microstructure is
rather complex, with two-phase mesoscopic plate-shaped features aligned along
{113} habit planes. The minority phase are strongly facetted on the {113}
planes, which we have shown to be driven by minimising the volume strain energy
introduced as a result of the phase transformation. Annealing at 488K results
in coarsening of the mesoscopic plate-shaped features and the formation of a
third distinct phase. The subtle differences in structure and chemistry of the
minority phase(s) in the crystals are thought to be responsible for changes in
the superconducting transition temperature. In addition, scanning photoemission
microscopy has clearly shown that the electronic structure of the minority
phase has a higher occupied density of states of the low binding energy Fe3d
orbitals, characteristic of crystals that exhibit superconductivity. This
demonstrates a clear correlation between the Fe-vacancy-free phase with high
c/a ratio and the electronic structure characteristics of the superconducting
phase.Comment: 6 figures v2 is exactly the same as v1. The typesetting errors in the
abstract have been correcte
Quantum interference structures in the conductance plateaus of gold nanojunctions
The conductance of breaking metallic nanojunctions shows plateaus alternated
with sudden jumps, corresponding to the stretching of stable atomic
configurations and atomic rearrangements, respectively. We investigate the
structure of the conductance plateaus both by measuring the voltage dependence
of the plateaus' slope on individual junctions and by a detailed statistical
analysis on a large amount of contacts. Though the atomic discreteness of the
junction plays a fundamental role in the evolution of the conductance, we find
that the fine structure of the conductance plateaus is determined by quantum
interference phenomenon to a great extent.Comment: 4 pages, 4 figure
Correlations in nano-scale step fluctuations: comparison of simulation and experiments
We analyze correlations in step-edge fluctuations using the
Bortz-Kalos-Lebowitz kinetic Monte Carlo algorithm, with a 2-parameter
expression for energy barriers, and compare with our VT-STM line-scan
experiments on spiral steps on Pb(111). The scaling of the correlation times
gives a dynamic exponent confirming the expected step-edge-diffusion
rate-limiting kinetics both in the MC and in the experiments. We both calculate
and measure the temperature dependence of (mass) transport properties via the
characteristic hopping times and deduce therefrom the notoriously-elusive
effective energy barrier for the edge fluctuations. With a careful analysis we
point out the necessity of a more complex model to mimic the kinetics of a
Pb(111) surface for certain parameter ranges.Comment: 10 pages, 9 figures, submitted to Physical Review
Microstructural analysis of phase separation in iron chalcogenide superconductors
The interplay between superconductivity, magnetism and crystal structure in
iron-based superconductors is a topic of great interest amongst the condensed
matter physics community as it is thought to be the key to understanding the
mechanisms responsible for high temperature superconductivity. Alkali metal
doped iron chalcogenide superconductors exhibit several unique characteristics
which are not found in other iron-based superconducting materials such as
antiferromagnetic ordering at room temperature, the presence of ordered iron
vacancies and high resistivity normal state properties. Detailed
microstructural analysis is essential in order to understand the origin of
these unusual properties. Here we have used a range of complementary scanning
electron microscope based techniques, including high-resolution electron
backscatter di raction mapping, to assess local variations in composition and
lattice parameter with high precision and sub-micron spatial resolution. Phase
separation is observed in the Csx Fe2-ySe2 crystals, with the minor phase
distributed in a plate-like morphology throughout the crystal. Our results are
consistent with superconductivity occurring only in the minority phase.Comment: Accepted for publication in a special edition of Supercond. Sci.
Techno
Transition from tunneling to direct contact in tungsten nanojunctions
We apply the mechanically controllable break junctions technique to
investigate the transition from tunneling to direct contact in tungsten. This
transition is quite different from that of other metals and is determined by
the local electronic properties of the tungsten surface and the relief of the
electrodes at the point of their closest proximity. The conductance traces show
a rich variety of patterns from the avalanche-like jump to a mesoscopic contact
to the completely smooth transition between direct contact and tunneling. Due
to the occasional absence of an adhesive jump the conductance of the contact
can be continuously monitored at ultra-small electrode separations. The
conductance histograms of tungsten are either featureless or show two distinct
peaks related to the sequential opening of spatially separated groups of
conductance channels. The role of surface states of tungsten and their
contribution to the junction conductance at sub-Angstrom electrode separations
are discussed.Comment: 6 pages, 6 figure
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