23 research outputs found
Desynchronizing effect of high-frequency stimulation in a generic cortical network model
Transcranial Electrical Stimulation (TCES) and Deep Brain Stimulation (DBS)
are two different applications of electrical current to the brain used in
different areas of medicine. Both have a similar frequency dependence of their
efficiency, with the most pronounced effects around 100Hz. We apply
superthreshold electrical stimulation, specifically depolarizing DC current,
interrupted at different frequencies, to a simple model of a population of
cortical neurons which uses phenomenological descriptions of neurons by
Izhikevich and synaptic connections on a similar level of sophistication. With
this model, we are able to reproduce the optimal desynchronization around
100Hz, as well as to predict the full frequency dependence of the efficiency of
desynchronization, and thereby to give a possible explanation for the action
mechanism of TCES.Comment: 9 pages, figs included. Accepted for publication in Cognitive
Neurodynamic
Random walk on disordered networks
Random walks are studied on disordered cellular networks in 2-and
3-dimensional spaces with arbitrary curvature. The coefficients of the
evolution equation are calculated in term of the structural properties of the
cellular system. The effects of disorder and space-curvature on the diffusion
phenomena are investigated. In disordered systems the mean square displacement
displays an enhancement at short time and a lowering at long ones, with respect
to the ordered case. The asymptotic expression for the diffusion equation on
hyperbolic cellular systems relates random walk on curved lattices to
hyperbolic Brownian motion.Comment: 10 Pages, 3 Postscript figure
Nonlinear Diffusion Through Large Complex Networks Containing Regular Subgraphs
Transport through generalized trees is considered. Trees contain the simple
nodes and supernodes, either well-structured regular subgraphs or those with
many triangles. We observe a superdiffusion for the highly connected nodes
while it is Brownian for the rest of the nodes. Transport within a supernode is
affected by the finite size effects vanishing as For the even
dimensions of space, , the finite size effects break down the
perturbation theory at small scales and can be regularized by using the
heat-kernel expansion.Comment: 21 pages, 2 figures include
Fe and N self-diffusion in amorphous FeN: A SIMS and neutron reflectivity study
Simultaneous measurement of self-diffusion of iron and nitrogen in amorphous
iron nitride (Fe86N14) using secondary ion mass spectroscopy (SIMS) technique
has been done. In addition neutron reflectivity (NR) technique was employed to
study the Fe diffusion in the same compound. The broadening of a tracer layer
of 57Fe8615N14 sandwiched between Fe86N14 layers was observed after isothermal
vacuum annealing of the films at different temperatures in SIMS measurements.
And a decay of the Bragg peak intensity after isothermal annealing was observed
in [Fe86N14/57Fe86N14]10 multilayers in NR. Strong structural relaxation of
diffusion coefficient was observed below the crystallization temperature of the
amorphous phase in both measurements. It was observed from the SIMS
measurements that Fe diffusion was about 2 orders of magnitude smaller compared
to nitrogen at a given temperature. The NR measurements reveal that the
mechanism of Fe self-diffusion is very similar to that in metal-metal type
metallic glasses. The structural relaxation time for Fe and N diffusion was
found comparable indicating that the obtained relaxation time essentially
pertain to the structural relaxation of the amorphous phase.Comment: 10 pages 12 figure
Reactive phase formation in the diffusion zone between Si3N4 and Ni-Cr alloys
The interaction between dense Si3N4 and Ni-Cr-alloys at 1398K was investigated. Reactive phase formation can be explained by assuming a N2-pressure at the interface. To understand the thermodn. and diffusion kinetics in this system direct nitriding of Ni,Cr(Si)-alloys from the N2-gas atm. was performed and a thermodn. description of the nitrogen behavior was attempted by using the Thermo-calc databank system. The \"up hill\" diffusion of nitrogen towards the front of nitride ppts. can be predicted. [on SciFinder (R)
New model for tracer-diffusion in amorphous solid
The tracer-diffusion and structure of polymorphic states of amorphous solid
is studied by mean of the statistic relaxation technique and simplex
analysis. Several different metastable states of amorphous iron have been
constructed based on the model containing 2 × 105 atoms. All models have
almost the same pair radial distribution functions, but they differ in the
potential energy per atom and the density. We found a large number of
vacancy-simplexes which varies according to the relaxation and serves as a
diffusion vehicle. New diffusion mechanism for tracer-diffusion is found of
which the elementary diffusion process likes a collapse of “microscopic
bubble” in amorphous matrix. This includes a jump of diffusing atom and the
collective movement of a large number of neighboring atoms. The diffusion
constant D determined in accordance with considered diffusion mechanism is in
reasonable agreement with experimental data. The decrease in diffusion
constant D upon thermal annealing is explained by the reducing
vacancy-simplex concentration which is caused by both the local atomic
rearrangement and the elimination of excess free volume