Networks, shaken and stirred!
Can one really slow down an epidemic by disinfecting those airplanes that
fly between international hubs? Many networks are notoriously difficult to
break as they are practically immune to random failure and, like internet,
even resistant to intentional attack. How can one devise a strategy for
manipulating the robustness or fragility of a network, which is useful even
when we don't know its structure?
A robust network can be rendered fragile by a procedure akin to shaking.
When links connecting hubs are selectively removed, the network is prone to
"heart failure" as its central structure deteriorates. Adopting a
depreciation strategy we call "central bias" we predict, and verify by
simulation, that a dynamic state can be reached in which the network becomes
fragile to random failure, although its static characteristics qualify it as
being robust.
Conversely, a fragile network can be rendered robust by gentle stirring.
Employing a failure scenario we call "peripheral bias" we find that
selectively removing links between poorly connected nodes can boost the
network resilience. In both procedures, the depreciation process acts in a
stealth manner. The network agents, or nodes, are left unsuspecting. Only
links are affected and all intelligence required are the connectivities of
the adjacent nodes.
***
LU11307
3D X-ray imaging of human chromosome
- Nano X-ray CT Scanner for Cell Organelles -
We observed three-dimensional structure of an unstained human chromosome
by using coherent X-ray diffraction. The observed images reveal an axial
structure with high electron-density in the chromosome, which other
microscopic methods have been unable to visualize under unstained
condition. Such mesoscopic-scale structures as cell organelle have been
difficult to be observed, although molecular structures of their
components like DNA and histone proteins could be studied by X-ray
crystallography or electron microscopy. In observing mesoscopic-scale
structures, high penetration power and high imaging contrast are
essential. In optical microscope, it is well-recognized that phase
contrast microscope (Frits Zernike: Nobel prize in physics in 1953)
provides better image contrast for unstained biological samples than
bright-field microscope. In the X-ray regime also, high image contrast
can be obtained in phase contrast imaging. Our lensless coherent X-ray
diffraction microscopy is an ideal form of X-ray phase contrast imaging,
since there is no contrast degradation due to lenses. Our result
experimentally demonstrates the high imaging ability of coherent X-ray
diffraction for unstained biological specimens, which is transparent to
X-rays, opening a novel and strong mean of exploring cellular structures.
***
BV10739
Giant Bond Length Contraction in Cobalt Nanoislands
In a bulk crystal, atoms are surrounded by neighbouring atoms arranged in
three dimensions leading to a large number of neighbouring atoms (n) such
as n=12 in the case of most metals.
By contrast, atoms in a two-dimensional nanocrystal consisting of only 10
to 40 atoms are surrounded by a significantly reduced number of nearest
neighbours (n=2 to 6). This leads to strongly modified chemical and
physical properties, one of them is the rearrangement of the atoms.
Despite its fundamental importance, the knowledge of the atomic geometry
in nanostructures is quite scarce where the interatomic distance
represents the most important parameter.
In this study, Cobalt atoms were deposited on a Copper surface to form
two-dimensional nanoislands and x-ray diffraction experiments were carried
out. They provide a quantitative experimental proof of a dramatic
reduction of the (average) interatomic distance from d=2.51 Å in bulk
Cobalt to values in the 2.35-2.45 Å range. Our study [1] is based on the
analysis of the positions of the Cobalt atoms relative to the surface
Copper atoms involving a statistical disorder approach in analogy to the
Debye-Waller type treatment of thermal vibrations.
***
LN11102
Origin of Nanorods Diameter Discovered
Ever wonder why the diameter of nanorods is on the order of 100 nm?
The origin has been discovered and reported in Physical Review Letters XX,
xxx (2008). In retrospect, synthesis of nanorods has been in practice for
decades, without knowing the origin. Surface steps are effective in slowing
down the mass transport of surface atoms, and aggregated surface steps are
even more effective. This extra effectiveness makes the diameter of nanorods
100 nm or so; without it the diameter would go up to 10 microns. This extra
effectiveness has escaped attention in scientific literatures. This
discovery provokes a revisit of surface processing theories. Beyond
scientific understanding, the discovery of the origin paves the way to
controlling the diameter of nanorods. Nanorods have numerous applications
as, for example, photonic materials and catalysts in fuel cells.
***
LX11641
Fluctuations away from equilibrium
When forced, many complex natural systems away from thermal equilibrium
respond discontinuously. Avalanche of atoms causing propagation of
fracture, jerky movements of continental plates during earthquakes,
evolution or extinction of species, or even stock market fluctuations have
the commonness in jerky response when subjected to an external force, and
often display a universal scale-free size distribution of the bursts. The
scalelessness is believed to be a manifestation of critical behavior, such
as that observed at second order phase transitions, but it was debated for
many years whether it occurs at a definite driving force, or if the
complexity and long-range interactions enforce the critical state over a
wide drive magnitude. In this work, we have developed, and demonstrated, a
scheme to identify such a drive magnitude, where the system is expected to
behave in a coherent manner over very long distances. Using a prototype
system based on Nickel-Titanium shape memory alloys, which itself is a
technologically well-known material used widely as mechanical actuators to
deployable structures in satellites, we show that fluctuations in
electrical resistance act as an excellent probe to criticality. Our
experiments not only propose a "noise calorimetric" technique to study
phase changes in non-equilibrium systems, but may also constitute a new
non-invasive approach to hazard prediction which is portable to different
fields of research.
