30,821 research outputs found
Monte Carlo simulations of interfaces in polymer blends
We review recent simulation studies of interfaces between immiscible
homopolymer phases. Special emphasis is given to the presentation of efficient
simulation techniques and powerful methods of data analysis, such as the
analysis of capillary wave spectra. Possible reasons for polymer
incompatibility and ways to relate model dependent interaction parameters to an
effective Flory Huggins parameter are discussed. Various interfaces are then
considered and characterised with respect to their microscopic structure and
thermodynamic properties. In particular, interfaces between homopolymers of
equal or disparate stiffness are studied, interfaces containing diblock
copolymers, and interfaces confined in thin films. The results are related to
the phase behaviour of ternary homopolymer/copolymer systems, and to wetting
transitions in thin films.Comment: To appear in Annual Reviews of Computational Physics, edt. D.
Stauffe
Combined Interpolation Scheme for Transition and Noble Metals
Combined interpolation scheme for calculating energy bands of transition and noble metal
The secondary structure of RNA under tension
We study the force-induced unfolding of random disordered RNA or
single-stranded DNA polymers. The system undergoes a second order phase
transition from a collapsed globular phase at low forces to an extensive
necklace phase with a macroscopic end-to-end distance at high forces. At low
temperatures, the sequence inhomogeneities modify the critical behaviour. We
provide numerical evidence for the universality of the critical exponents
which, by extrapolation of the scaling laws to zero force, contain useful
information on the ground state (f=0) properties. This provides a good method
for quantitative studies of scaling exponents characterizing the collapsed
globule. In order to get rid of the blurring effect of thermal fluctuations we
restrict ourselves to the groundstate at fixed external force. We analyze the
statistics of rearrangements, in particular below the critical force, and point
out its implications for force-extension experiments on single molecules.Comment: to be published in Europhys. J.
Quantum Google in a Complex Network
We investigate the behavior of the recently proposed quantum Google
algorithm, or quantum PageRank, in large complex networks. Applying the quantum
algorithm to a part of the real World Wide Web, we find that the algorithm is
able to univocally reveal the underlying scale-free topology of the network and
to clearly identify and order the most relevant nodes (hubs) of the graph
according to their importance in the network structure. Moreover, our results
show that the quantum PageRank algorithm generically leads to changes in the
hierarchy of nodes. In addition, as compared to its classical counterpart, the
quantum algorithm is capable to clearly highlight the structure of secondary
hubs of the network, and to partially resolve the degeneracy in importance of
the low lying part of the list of rankings, which represents a typical
shortcoming of the classical PageRank algorithm. Complementary to this study,
our analysis shows that the algorithm is able to clearly distinguish scale-free
networks from other widespread and important classes of complex networks, such
as Erd\H{o}s-R\'enyi networks and hierarchical graphs. We show that the ranking
capabilities of the quantum PageRank algorithm are related to an increased
stability with respect to a variation of the damping parameter that
appears in the Google algorithm, and to a more clearly pronounced power-law
behavior in the distribution of importance among the nodes, as compared to the
classical algorithm. Finally, we study to which extent the increased
sensitivity of the quantum algorithm persists under coordinated attacks of the
most important nodes in scale-free and Erd\H{o}s-R\'enyi random graphs
Gender Matters! Analyzing Global Cultural Gender Preferences for Venues Using Social Sensing
Gender differences is a phenomenon around the world actively researched by
social scientists. Traditionally, the data used to support such studies is
manually obtained, often through surveys with volunteers. However, due to their
inherent high costs because of manual steps, such traditional methods do not
quickly scale to large-size studies. We here investigate a particular aspect of
gender differences: preferences for venues. To that end we explore the use of
check-in data collected from Foursquare to estimate cultural gender preferences
for venues in the physical world. For that, we first demonstrate that by
analyzing the check-in data in various regions of the world we can find
significant differences in preferences for specific venues between gender
groups. Some of these significant differences reflect well-known cultural
patterns. Moreover, we also gathered evidence that our methodology offers
useful information about gender preference for venues in a given region in the
real world. This suggests that gender and venue preferences observed may not be
independent. Our results suggests that our proposed methodology could be a
promising tool to support studies on gender preferences for venues at different
spatial granularities around the world, being faster and cheaper than
traditional methods, besides quickly capturing changes in the real world
A gradient-forming MipZ protein mediating the control of cell division in the magnetotactic bacterium Magnetospirillum gryphiswaldense
Cell division needs to be tightly regulated and closely coordinated with other cellular processes to ensure the generation of fully viable offspring. Here, we investigate division site placement by the cell division regulator MipZ in the alphaproteobacterium Magnetospirillum gryphiswaldense, a species that forms linear chains of magnetosomes to navigate within the geomagnetic field. We show that M. gryphiswaldense contains two MipZ homologs, termed MipZ1 and MipZ2. MipZ2 localizes to the division site, but its absence does not cause any obvious phenotype. MipZ1, by contrast, forms a dynamic bipolar gradient, and its deletion or overproduction cause cell filamentation, suggesting an important role in cell division. The monomeric form of MipZ1 interacts with the chromosome partitioning protein ParB, whereas its ATP-dependent dimeric form shows non-specific DNA-binding activity. Notably, both the dimeric and, to a lesser extent, the monomeric form inhibit FtsZ polymerization in vitro. MipZ1 thus represents a canonical gradient-forming MipZ homolog that critically contributes to the spatiotemporal control of FtsZ ring formation. Collectively, our findings add to the view that the regulatory role of MipZ proteins in cell division is conserved among many alphaproteobacteria. However, their number and biochemical properties may have adapted to the specific needs of the host organism
The degradation of MgB2 under ambient environment
The superconductivities of samples prepared by several procedures were found
to degrade under ambient environment. The degradation mechanism was studied by
measuring the change of surface chemical composition of dense MgB2 pellets
(prepared by hot isostatic pressure, HIPed) under atmospheric exposure using
X-ray Photoelectron Spectroscopy (XPS). Results showed that samples with poor
connectivity between grains and with smaller grain sizes degrade with time when
exposed to ambient conditions. In these samples, the Tc did not change with
time, but the superconducting transition became broader and the Meissner
fraction decreased. In contrast, our well-sintered and the HIPed samples
remained stable for several months under ambient condition. The degradation was
found to be related to surface decomposition as observed by XPS. We observed
the formation of oxidized Mg, primarily in the form of a Mg hydroxide, the
increase of C and O contents, and the reduction of B concentration in the
surface layer of MgB2 samples.Comment: 15 pages, 3 figure
- …