Surface Strain Causes Domino Effect in Decomposition Processes
Molecules decomposing on the surface of a single crystalline substrate may follow a specific direction along the surface, following the path that reduces the strain rising from the opposition of the rigid substrate to its modification. In other words, molecules may follow the path of least resistance provided by the directionality of the crystal. A theoretical investigation of ammonia decomposition on the Si(100)-2x1 surface (represented in the figure by four Si-Si surface dimers occupied with ammonia fragments) explains a long-standing divorce between theoretical and experimental understanding of this reaction. Although ammonia fragments are experimentally detected to migrate subsurface during decomposition, as shown for the silicon surface dimer on the left, theory usually regarded this process as kinetically forbidden due to surface straining effects. However, once this migration process occurs in one dimer, the neighboring fragments can find a direction of insertion (shown with arrows) that reduces the surface strain substantially, creating an effect similar to the falling domino tiles. Thus, the occurrence of this domino-like insertion process can be expected on the surface, possibly producing a template of inserted fragments along the row of surface silicon dimers.
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EP10468
DNA ‘Goes With the Flow’ and Adapts to a Stressful Situation
Physicists have now shown, using a new biomolecular dynamics computational method, that tiny DNA molecular fragments, only 10 billionths of a meter long, can adapt to the changing stresses of a flowing physiological liquid environment by adopting an array of complex shapes. The bonds along a DNA molecule were found to react to any imbalance in force created by passing liquid molecules and nearby nanowalls, resulting in the generation of an array of complex loops, twists, and figure eight shapes. The geometric patterns change as the passing environment changes, creating a dynamic mosaic as the tiny DNA molecules move through the flow. The new computational results indicate that DNA may be far more flexible than thought based on existing experimental data for DNA molecules 100 to 1000 times longer moving through channels. This ultra flexibility may be useful in designing a new generation of environmentally adaptable nanosurfaces, nanosensors, nanoactuators, and nanomotors.
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ET10471
Pulsating active gels: size matters
Polymer gels are ideal candidates for creating active materials that can perform sustained mechanical work. In our paper, we develop the first 3D computational model for chemo-responsive polymer gels. Chemo-responsive gels are so-called because they “respond” to a change in the concentration of specific chemicals by a mechanical action. We focus on the chemo-responsive gels undergoing the Belousov-Zhabotinsky (BZ) reaction. Discovered in the 1950’s, the BZ reaction became a cornerstone of the field of non-linear dynamical phenomena. In our paper, we perform the first simulations of the 3D mechanical oscillations and shape changes of pieces of BZ gels. These mechanical oscillations somewhat resemble the beating of a heart and the BZ gel will continue to beat autonomously until all the reagents in the solution are consumed; the addition of new reagents, however, allows the sample to continue pulsating. Our simulations illustrate that for given reaction parameters, whether or not a BZ gel oscillates actually depends on the size of the sample. Moreover, the size also dictates whether the oscillations are regular or irregular. Our model opens up the possibility of uncovering new three-dimensional, dynamical patterns within chemo-responsive gels and could be a useful tool in designing a variety of active gel-based devices.
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LR11358
Combine big and small physics in every day life!
Sensors have become our "electronic eyes" in virtually any technical
process. Simple physical properties such as pressure and temperature
have traditionally been monitored by homogeneous, brittle metal oxides
(spinels). In our study, we present a completely new solution for the
development of low-cost and heterogeneous sensing nanomaterials. In
contrast to the currently applied materials, we have found that large
arrays of core/shell nanoparticles rapidly change their electrical
conductivity with altered pressure or temperature. These metamaterials
are composed of a highly conductive metal nano-core covered by ultra
thin carbon or ceramic insulating layers. An electrical current through
the sensor has to tunnel through the insulating barrier of about 1 nm
thickness. Since the quantum phenomenon tunneling is highly sensitive,
the here described array of core/shell nanoparticles offer a technically
feasible solution to using a most fundamental physical principle in an
every day application. The sensitivity can be engineered by the system's
architecture and the shell. The flexibility and the easy access to such
nanoparticle assemblies suggest applications as new sensitive piezo- and
thermoresistive sensors.
Figure: Big and small physics! A quantum effect like tunneling of an
electron (right) is a nanometer-range phenomenon. Coupling of million
tunneling gaps in a core/shell nanoparticles assembly (middle) allows
making a macroscopic object (pill, right) with quantum-effect controlled
electrical conductivity.
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BTR1067B
Micro-Brillouin Scattering on Relaxor Ferroelectric 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3
Relaxations inside a polar nanoregion (PNR), nanoscopic structure in relaxor ferroelectric materials, are manifested by the broadband micro-Brillouin scattering on 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 (PZN-0.07PT). The relaxor ferroelectric materials have giant piezoelectric properties and its mechanism of the ferroelectric phase transition is still in controversy. The ferroelectric ordering is, generally, induced by softening of a lattice vibration or by slowing of a dipole relaxation; therefore, it is interesting to study ferroelectric materials by measuring the inelastic light scattering which can access both lattice vibrations and lattice relaxations. In this study, the acoustic phonons and the relaxations in PZN-0.07PT are observed. By considering the nanoscopic piezoelectric coupling inside a PNR, the authors suggest that 180‹ and non-180‹ (71‹ and 109‹) dipole flippings inside a PNR in a paraelectric (ergodic) phase (see a figure below) show slowing to a ferroelectric (non-ergodic) phase. This fact gives new insights into the physics on three dimensional relaxor ferroelectrics, not only PZN-0.07PT but also other Pb-based perovskite relaxor ferroelectrics.
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BH9239
Infering the critical properties of a magnet by its local structure
This recent article by Faraggi and Robb describe how to find the
temperature at which a magnet will lose its permanent magnetization
based on the local properties of its structure. This work is very
exciting in the field of magnetism, describing a new method for
automatically finding the critical temperature (for the transition from
permanent to non-permanent magnet) using only the local characteristics
of the magnetic system. It describes fundamental physics research but
can have wide ranging applicative value, among other things to
spintronics (a possible new age for computers) and magnetic storage.
Following previous work (Phys. Rev. B 74, 014407 (2006)) the distinction
between a paramagnet and a rarefied ferromagnet is illustrated and the
location of these phases in the temperature-density phase diagram is
indicated.
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AV10263
Light probing leaves an atom in surprisingly complex motion.
The confinement and isolation of a single atom (or ion) at a particular
location by electromagnetic fields -- in a “trap” -- has become a
standard technique for investigating the subtleties of the interaction
of light and matter. Light usually serves as the probe for the atomic
dynamics. The probing light, however, inevitably modifies the motion of
the atom via recoil, as was observed already in the early thirties by
Otto Frisch in Hamburg. So far, these effects have been taken into
account in experiments on trapped ions by the application of simple
heuristic models that assumed smooth, bell-shaped distributions of the
recoil velocities. A novel ab-initio calculation has disproved these
naïve assumptions. For the first time, the phenomenon of resonance
fluorescence, and a manipulation of the atom’s internal states known as
“optical pumping” -- for which Alfred Kastler won the Nobel prize in
1966 -- are modelled on equal footing. It turns out that the
distribution of recoil velocities is, in general, sharply peaked and the
corresponding phase-space distribution is anisotropic. Rules have been
derived that allow to minimize motional perturbation, or to prepare a
particular motional state of the atom. The results provide intimate
insight in the dynamics of the interplay of light and matter.
